tag:blogger.com,1999:blog-78107160348492410252024-03-13T22:20:52.419+05:30Telecom Cellular Technology ConceptsVikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.comBlogger25125tag:blogger.com,1999:blog-7810716034849241025.post-25070331833918404082012-03-28T16:03:00.000+05:302012-03-28T16:03:00.195+05:30Cyclic Delay DiversityCyclic Delay Diversity (CDD) is a simple approach to introduce spatial diversity to an Orthogonal Frequency Division Multiplexing (OFDM) based transmission scheme that itself has no built-in diversity. It also can be regarded as a Space-Time Code (STC).
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But in contrast to that there is no additional effort in the receiver necessary, since the different codewords result in a changed channel impulse response in the receiver. They insert virtual echos and thus increase the frequency selectivity of the channel seen by the receiver.
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Cyclic Delay Diversity (CDD) is a diversity scheme used in OFDM-based telecommunication systems, transforming spatial diversity into frequency diversity avoiding intersymbol interference.
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Cyclic delay diversity is an elegant diversity technique for OFDM based transmission systems, which does not introduce additional effort in the receiver. For OFDMA systems with many users and slow fading channels, cyclic delay diversity cannot provide full diversity for one user. Hence, a new technique, called time-varying cyclic delay diversity, is introduced. With this technique the diversity can be increased, which leads to lower bit and frame error rates during an
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In CDD, the signal on the second or each additional antenna is not delayed but cyclically shifted. Therefore, no inter-symbol interference can occur and thus there are no limits for the cyclic shift. Another advantage of CDD is that there is no additional complexity in the receiver. Also there is no rate loss even for a large number of antennasin contrast to other Space-Time Codes.
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In principle,CDD shifts the TX-signal in time direction and transmit these modified signal copies over separate TX-antennas. The TX-antenna specific signal modifications, i.e. the time shifts, are inserted in cyclically, such that no additional inter symbol interference (ISI) occurs. CDD is capable to offer a larger degree of diversity since they increase the number of resolvable channel propagation paths. This additional diversity has to be exploited by the OFDM system itself by means of techniques, which guarantee a certain amount of Hamming distance for the data bearing signal, i.e. channel coding or spreading.Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-31592248830203932822012-03-27T11:13:00.001+05:302012-03-27T11:13:20.241+05:30Spatial Multiplexing<div dir="ltr" style="text-align: left;" trbidi="on">
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Spatial multiplexing is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals, so-called streams, from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed, more than one time.</div>
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In spatial multiplexing different signals or data bits are transmitted through several independent (spatial) communication channels by multiple antennas and at the same time the receiving side also use multiple antennas for receiving signals-this way increase the date transmission rate which is in direct proportion to the number of antennas used for both transmission and receiving purpose. The higher the number of antennas, the higher the number of data transmission rate.
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Multiple antennas are used to provide diversity gain (receive and transmit diversity )and increase the reliability of wireless links. With channel knowledge at the transmitter, multiple transmit antennas can also provide a power gain via
transmit beam-forming. Multiple transmit antennas are used to induce channel variations, which can then be exploited by opportunistic communication techniques. The scheme can be interpreted as opportunistic beam-forming and provides a power gain as well.
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Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams, creating parallel channels for free. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher Signal to Noise Ratio (SNR). The maximum number of spatial streams is limited by the lesser in the number of antennas at the transmitter or receiver. Spatial multiplexing can be used with or without transmit channel knowledge.
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</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-26502222865787776052011-12-01T12:17:00.000+05:302011-12-01T12:40:24.121+05:30Orthogonal Frequency Division Multiple Access (OFDMA)<div dir="ltr" style="text-align: left;" trbidi="on">
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Orthogonal Frequency Division Multiplexing (OFDM) is a technique for transmitting large amounts of digital data over a radio wave The technology works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. OFDM reduces the amount of crosstalk in signal transmissions.
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OFDMA is a multi-user OFDM that allows multiple access on the same channel (a channel being a group of evenly spaced subcarriers).
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OFDMA distributes subcarriers among users so all users can transmit and receive at the same time within a single channel on what are called subchannels. What’s more, subcarrier-group subchannels can be matched to each user to provide the best performance, meaning the least problems with fading and interference based on the location and propagation characteristics of each user.
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OFDMA is a multiplexing technique that subdivides the bandwidth in to multiple frequency sub-carriers. Here the input data stream is divided in to several parallel sub-streams of reduced data rate and each substream is modulated and transmitted on a separate Orthogonal Subcarrier.
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It is a multi-user version of the popular orthogonal frequency division multiplexing [ OFDM ] digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users.
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The main advantages of OFDMA over TDMA/CDMA stem from the scalability of OFDMA, the uplink orthogonality of OFDMA and the ability of OFDMA to take advantage of the frequency selectivity of the channel. Other advantages of OFDMA include its MIMO-friendliness and ability to provide superior quality of service (QoS).<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhtyWuGi-LfCzdxtac6mHpvSHBdPP0aOmix4LcJ0Ac-JpFbC5tqiqkfvu1nED1v0TRym1CESq17m9Z7kxyk8VDpRl6wz-mRIcWuAi2vXORarDNmvS3dkAyPGQKILuIXgX5cR9K0oa3p-nc/s1600/OFDMA.PNG" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="261" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhtyWuGi-LfCzdxtac6mHpvSHBdPP0aOmix4LcJ0Ac-JpFbC5tqiqkfvu1nED1v0TRym1CESq17m9Z7kxyk8VDpRl6wz-mRIcWuAi2vXORarDNmvS3dkAyPGQKILuIXgX5cR9K0oa3p-nc/s320/OFDMA.PNG" width="320" /></a></div>
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OFDMA Advantages </div>
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<li style="text-align: justify;">Averaging interference's from neighboring cells, by using different basic carrier permutations between users in different cells. </li>
<li style="text-align: justify;">Interference’s within the cell are averaged by using allocation with cyclic permutations. </li>
<li style="text-align: justify;">Enables orthogonality in the uplink by synchronizing users in time and frequency. </li>
<li style="text-align: justify;">Enables Multipath mitigation without using Equalizers and training sequences. </li>
<li style="text-align: justify;">Enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage. </li>
<li style="text-align: justify;">Enables spatial diversity by using antenna diversity at the Base Station and possible at the Subscriber Unit. </li>
<li style="text-align: justify;">Enables adaptive modulation for every user QPSK, 16QAM, 64QAM and 256QAM.
Enables adaptive carrier allocation in multiplication of 23 carriers = nX23 carriers up to 1587 carriers (all data carriers). </li>
<li style="text-align: justify;">Offers Frequency diversity by spreading the carriers all over the used spectrum.
Offers Time diversity by optional interleaving of carrier groups in time.</li>
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</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-25277542114696147432011-11-25T18:04:00.000+05:302011-12-01T12:24:33.207+05:30Advantages & Disadvantages of OFDM<div dir="ltr" style="text-align: left;" trbidi="on">
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<span class="Apple-style-span" style="color: #3d85c6;"><b>Advantages of OFDM </b></span></div>
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Orthogonal frequency division multiplexing is commonly implemented in many emerging communications protocols because it provides several advantages over the traditional FDM approach to communications channels. More specifically, OFDM systems allow for greater spectral efficiency reduced intersymbol interference (ISI), and resilience to multi-path distortion.
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<b>Spectral Efficiency
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In a traditional FDM system, each channel is spaced by about 25% of the channel width. This is done to ensure that adjacent channels do not interfere. This is illustrated in the diagram below, which shows the guard bands between individual channels.
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi2TvjFVNwmVX5hsXgOUwTeCqiVo5vo7LaLN4hqsKV8k6V7WulWJDpnoZaviU-ZCBMS30BAynRuBLNaTz4Uj_ecAeRSupJ-sjqcSsIxGQj0u7dtTawCceJKNMCqSba2JDLghWT3SmWoGmk/s1600/FDM.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="188" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi2TvjFVNwmVX5hsXgOUwTeCqiVo5vo7LaLN4hqsKV8k6V7WulWJDpnoZaviU-ZCBMS30BAynRuBLNaTz4Uj_ecAeRSupJ-sjqcSsIxGQj0u7dtTawCceJKNMCqSba2JDLghWT3SmWoGmk/s400/FDM.gif" width="400" /></a></div>
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Because of the requirement for guard bands, it is required to the symbol rate to allow for guard bands to exist. In general, the allowed channel bandwidth (Bw) is 2/Rs. As a result of this, the channels are able to be separated adequately.
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n an OFDM system, on the other hand, the channels actually overlap. As a result, it is possible to maximize the symbol rate, and thus the throughput, for a given bandwidth. In the image below, we illustrate overlapping sub-carriers in an OFDM system. In this scenario, the channel bandwidth (Bw) approaches 1 / Rs. Thus, as the number of sub-carriers approaches infinity, OFDM systems allow for nearly double the spectral efficiency.
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj4MeXCE1J0lVbjMRHoEXJWu-09wYaDEeA-547jI-z_5KYmrDF3RV5B01g3i7WJerPsAVEN9BbhoHo3L5mnLbUwrcq2lL6QqUPFNaULybFwu7iFBPfTV9266UqaM7bH90LPLmly0nhnzfg/s1600/OFDM-1.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="210" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj4MeXCE1J0lVbjMRHoEXJWu-09wYaDEeA-547jI-z_5KYmrDF3RV5B01g3i7WJerPsAVEN9BbhoHo3L5mnLbUwrcq2lL6QqUPFNaULybFwu7iFBPfTV9266UqaM7bH90LPLmly0nhnzfg/s400/OFDM-1.gif" width="400" /></a></div>
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Note that with an OFDM system, it is still required to have a guard band between each individual channel. However, the effective symbol rate for the combined sub-carriers is greater than if a single carrier were used instead.
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjRca61U7HS70aipPCFYOSlEoO69138_u7hJNxfo_Q_lCaIbplFnXHB14EtnNU-7B8_gxIbH9tbxYsR5IitZN9rJdSgRnPCpnHtEpTq6ce_n-2CzJD4nNT1L-jROtdDlDJwI5ZPGjU9wXE/s1600/OFDM-2.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="143" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjRca61U7HS70aipPCFYOSlEoO69138_u7hJNxfo_Q_lCaIbplFnXHB14EtnNU-7B8_gxIbH9tbxYsR5IitZN9rJdSgRnPCpnHtEpTq6ce_n-2CzJD4nNT1L-jROtdDlDJwI5ZPGjU9wXE/s400/OFDM-2.gif" width="400" /></a></div>
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Note that the effect of using overlapping orthogonal sub-carriers also requires the use of a cyclic prefix to prevent intersymbol interference (ISI). Thus, some of the advantages gained through overlapping sub-carriers are compromised. However, the spectral efficiency advantage is great enough such that greater throughput is available in an OFDM system.
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<b>Reduced Inter Symbol Interference (ISI)
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n mono-carrier systems, intersymbol interference is often caused through the multi-path characteristics of a wireless communications channel. Note that when transmitting an electromagnetic wave over a long distance, the signal passes through a variety of physical mediums. As a result, the actual received signal contains the direct path signal overlaid with signal reflections of smaller amplitudes. The diagram below illustrates how, at high symbol rates, reflected signals can interfere with subsequent symbols.
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In wireless systems, this creates difficulty because the received signal can be slightly distorted. In this scenario, the direct path signal arrives as expected, but slightly attenuated reflections arrive later in time. These reflections create a challenge because they interfere with subsequent symbols transmitted along the direct path. These signal reflections are typically mitigated through a pulse-shaping filter, which attenuates both the starting and ending sections of the symbol period. However, as the figure above illustrates, this problem becomes much more significant at high symbol rates. Because the reflections make up a significant percentage of the symbol period, ISI will also be substantial.
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjPoaycsh6cQySpf1-g1JJv3HiT4KBWUpwV1sy-_X7ZUMzrrUUw1THz0udV0Wc6_P8RVe1uuju_lHczDF5VBcKR9f6GMu-fGbCkgjB25x0eLrSWfHANYa40LtxQMGGTm7ZHhL0oqg_OSkg/s1600/ISI-1.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="205" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjPoaycsh6cQySpf1-g1JJv3HiT4KBWUpwV1sy-_X7ZUMzrrUUw1THz0udV0Wc6_P8RVe1uuju_lHczDF5VBcKR9f6GMu-fGbCkgjB25x0eLrSWfHANYa40LtxQMGGTm7ZHhL0oqg_OSkg/s400/ISI-1.gif" width="400" /></a></div>
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OFDM systems mitigate this problem by utilizing a comparatively long symbol period. In addition, they do this without sacrificing throughput by utilizing multiple sub-carriers per channel. Below, we illustrate the time domain of OFDM symbols. Note that in an OFDM system, the symbol rate can be reduced while still achieving similar or even higher throughput.
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiSzovjajYT4ia1xoSrY4lUnp5blatCSnk8RaM5UPYcoaxcOLC_PCE4Yi0ef7eyHiBgQzJhLCKJAhU_QISTDOargEeAWV1vWv8GP3LX8d7vqx4JN5O-ltOM8YN-se_Qvv4f68029mFHov0/s1600/ISI-2.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="205" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiSzovjajYT4ia1xoSrY4lUnp5blatCSnk8RaM5UPYcoaxcOLC_PCE4Yi0ef7eyHiBgQzJhLCKJAhU_QISTDOargEeAWV1vWv8GP3LX8d7vqx4JN5O-ltOM8YN-se_Qvv4f68029mFHov0/s400/ISI-2.gif" width="400" /></a></div>
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Note from the illustration above that the time required for the reflections to fully attenuate is the same as before. However, by utilizing a smaller symbol rate, the signal reflections make up only a small percentage of the total symbol period. Thus, it is possible to simply add a guard interval to remove interference from reflections without significantly decreasing system throughput.
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<b>Other Advantages</b></div>
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<li style="text-align: justify;">Flexibility of deployment across various frequency bands with little needed modification to the air interface. </li>
<li style="text-align: justify;">Averaging interferences from neighboring cells, by using different basic carrier permutations between users in different cells. </li>
<li style="text-align: justify;">Interferences within the cell are averaged by using allocation with cyclic permutations. </li>
<li style="text-align: justify;">Enables orthogonality in the uplink by synchronizing users in time and frequency. </li>
<li style="text-align: justify;">Enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage. </li>
<li style="text-align: justify;">Enables adaptive carrier allocation in multiplication of 23 carriers = nX23 carriers up to 1587 carriers (all data carriers). </li>
<li style="text-align: justify;">Offers Frequency diversity by spreading the carriers all over the used spectrum. </li>
<li style="text-align: justify;">Offers Time diversity by optional interleaving of carrier groups in time.</li>
<li style="text-align: justify;">Using the cell capacity to the utmost by adaptively using the highest modulation a user can use, this is allowed by the gain added when less carriers are allocated (up to 18dB gain for 23 carrier allocation instead of 1587 carriers), therefore gaining in overall cell capacity.</li>
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<span class="Apple-style-span" style="color: #3d85c6;"><b>Disadvantages of OFDM</b></span>
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<li style="text-align: justify;">Peak to average power ratio (PAPR) is high </li>
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<li style="text-align: justify;"> High power transmitter ampli ers need linearlization </li>
<li style="text-align: justify;"> Low noise receiver ampli ers need large dynamic range</li>
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<li style="text-align: justify;">Capacity and power loss due to guard interval </li>
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<li style="text-align: justify;">Bandwidth and power loss due to the guard interval can be signi cant </li>
<li style="text-align: justify;">The guard interval consumes 20% of the bandidth and transmit power in
IEEE802.11a
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<li style="text-align: justify;">Frequency off sets and phase noise sensitivity </li>
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<li style="text-align: justify;"> Phase noise is especially acute at high carrier frequencies</li>
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<li style="text-align: justify;">The OFDM signal has a noise like amplitude with a very large dynamic range, therefore it
requires RF power amplifiers with a high peak to average power ratio. </li>
<li style="text-align: justify;">It is more sensitive to carrier frequency offset and drift than single carrier systems are due
to leakage of the DFT.</li>
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</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-27479526033759735762011-11-24T15:43:00.000+05:302011-12-01T12:24:53.767+05:30OFDM Basics<div dir="ltr" style="text-align: left;" trbidi="on">
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OFDM stands for Orthogonal Frequency Division Multiplexing. OFDM is a technique that allows a base station to split a chunk of radio spectrum into sub-channels. The signal strength of the sub-channels and the number of channels assigned to different devices can be varied as needed. OFDM allows high data rates, even far from a base station, and it copes well with the type of radio interference that is common in urban areas, where signals reflect off walls to produce confusing echoes.</div>
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This is a method to transmit data using a large number of carriers that are separated on different frequencies to carry one data stream which has been broken up into many signals.</div>
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Speeds that are lower are easier to detect. It was discovered that by using multiple subcarriers the receiver
could detect signals easier in environments with interference. Subcarriers transmit a lower speed signal which is
converted by to its original high-speed signal at the other end. The subcarriers for OFDM are modulated by several methods including QAM and QPSK.
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The basic principle of OFDM is to split a high-rate datastream into a number of lower rate streams that are transmitted simultaneously over a number of subcarriers. Because the symbol duration increases for lower rate parallel subcarriers, the relative amount of dispersion in time caused by multipath delay spread is decreased. Intersymbol interference is eliminated almost completely by introducing a guard time in every OFDM symbol. In the guard time, the symbol is cyclically extended to avoid intercarrier interference.</div>
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In OFDM design, a number of parameters are up for consideration, such as the number of subcarriers, guard time, symbol duration, subcarrier spacing, modulation type per subcarrier. The choice of parameters is influenced by system requirements such as available bandwidth, required bit rate, tolerable delay spread, and Doppler values. Some requirement are conflicting. For instance, to get a good delay spread tolerance, a large number of subcarriers
with small subcarrier spacing is desirable, but the opposite is true for a good tolerance against Doppler spread and phase noise.</div>
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OFDM is present in</div>
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– LTE</div>
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– Mobile WiMax IEEE 802.16e</div>
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– xDSL</div>
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– Wireless LAN IEEE 802.11a,g,</div>
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What is OFDM?</div>
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• Orthogonal FDM – it’s multiplexing</div>
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• It’s more:
– Multi Carrier
– Digital modulation (PSK, QAM)
– Digital processing</div>
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• Demultiplexing</div>
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In a conventional serial data system, the symbols are transmitted sequentially, with the frequency spectrum of each data symbol allowed to occupy the entire available bandwidth. A When the data rate is sufficient high, several adjacent symbols may be completely distorted over frequency selective fading or multipath delay spread channel.</div>
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In OFDM</div>
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The spectrum of an individual data element normally occupies only a small part of available bandwidth.
Because of dividing an entire channel bandwidth into many narrow subbands, the frequency response over each individual subchannel is relatively flat.</div>
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A parallel data transmission system offers possibilities for alleviating this problem encountered with serial systems.</div>
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- Resistance to frequency selective fading.</div>
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The process of mapping the information bits onto the signal constellation plays a fundamental role in determining the
properties of the modulation. An OFDM signal consists of a sum of sub-carriers, each of which contains M-ary phase shift keyed (PSK) or quadrature amplitude modulated (QAM) signals.
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Modulation types over OFDM systems</div>
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- Phase shift keying (PSK)</div>
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- Quadrature amplitude modulation (QAM)</div>
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OFDM communications systems are able to more effectively utilize the frequency spectrum through overlapping sub-carriers. These sub-carriers are able to partially overlap without interfering with adjacent sub-carriers because the maximum power of each sub-carrier corresponds directly with the minimum power of each adjacent channel.
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As an example , figure below shows four subcarriers from one OFDM signal. In this example, all subcarriers have the phase and amplitude, but in practice the amplitudes and phases may be modulated differently for each
subcarrier. Note that each subcarrier has exactly an integer number of cycles in the interval T , and the number of cycles between adjacent subcarries differs by exactly one. This properly accounts for the orthogonality between subcarriers.</div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgSLQ7cnRCaJ9ZLO7YGHT9pt-kypcoDnlI62gUTQmayiUuatxZCygpVoJZ5UytNL69fXh-QbvhPsgEJZhC7hrrLWGlx9Dydu9ts1BIEpGCNoEXanPKUp_BXSoQ5dJfQD9HjKtF5qV7TDZk/s1600/OFDM.PNG" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="171" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgSLQ7cnRCaJ9ZLO7YGHT9pt-kypcoDnlI62gUTQmayiUuatxZCygpVoJZ5UytNL69fXh-QbvhPsgEJZhC7hrrLWGlx9Dydu9ts1BIEpGCNoEXanPKUp_BXSoQ5dJfQD9HjKtF5qV7TDZk/s400/OFDM.PNG" width="400" /></a></div>
<br /></div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-53476387365266762552011-09-07T18:11:00.000+05:302011-09-07T18:11:31.045+05:30Public Land Mobile Network (PLMN)<div dir="ltr" style="text-align: left;" trbidi="on">
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<b>PLMN </b></div>
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A Public Land Mobile Network (PLMN) is a network established and operated by an Administration or RPOA for the specific purpose of providing land mobile communication services to the public. It provides communication possibilities for mobile users. </div>
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For communications between mobile and fixed users, interworking with a fixed network is necessary.
A PLMN may provide service in one, or a combination, of frequency bands. As a rule, the borders of a country limit a PLMN. Depending on national regulations there may be more than one PLMN per country. A relationship exists between each subscriber and his home PLMN (HPLMN). If communications are handled over another PLMN, this PLMN is referred to as the visited PLMN (VPLMN). </div>
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A PLMN is identified by the Mobile Country Code (MCC) and the Mobile Network Code (MNC). Each operator providing mobile services has its own PLMN. PLMNs interconnect with other PLMNs and Public switched telephone networks (PSTN) for telephone communications or with internet service providers for data and internet access of which links are defined as interconnect links between providers. </div>
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<b>PLMN Area</b></div>
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The PLMN area is the geographical area in which a PLMN provides communication services according to the specifications to mobile users. In the PLMN area, the mobile user can set up calls to a user of a terminating network. The terminating network may be a fixed network, the same PLMN, another PLMN or other types of PLMN. </div>
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Terminating network users can also set up calls to the PLMN. </div>
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The PLMN area is allocated to a PLMN. In general the PLMN area is restricted to one country. It can also be determined differently, depending on the different telecommunication services, or type of MS.
If there are several PLMNs in one country, their PLMN areas may overlap. In border areas, the PLMN areas of different countries may overlap. Administrations will have to take precautions to ensure that cross border coverage is minimized in adjacent countries unless otherwise agreed.
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Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-73673244585611282732011-07-18T22:37:00.000+05:302011-08-03T11:06:28.024+05:30Gateway GPRS Support Node [GGSN]<div dir="ltr" style="text-align: left;" trbidi="on">
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The Gateway GPRS Support Node (GGSN) is a main component of the GPRS network. The GGSN is responsible for the interworking between the GPRS network and external packet switched networks, like the Internet and X.25 networks.</div>
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From the external networks’ point of view, the GGSN is a router to a sub-network, because the GGSN ‘hides’ the GPRS infrastructure from the external network. When the GGSN receives data addressed to a specific user, it checks if the user is active. If it is, the GGSN forwards the data to the SGSN serving the mobile user, but if the mobile user is inactive, the data are discarded. On the other hand, mobile-originated packets are routed to the right network by the GGSN.</div>
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To do all this,the GGSN keeps a record of active mobile users and the SGSN the mobile users are attached to. It allocates IP addresses to mobile users and last but not least, the GGSN is responsible for the billing.</div>
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GGSN is found in GPRS, UMTS and HSPA networks. Since GGSN is located at the heart of the mobile network, GGSN serves critical role for session management, routing, interworking with charging and billing system.</div>
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The GGSN is the anchor point that enables the mobility of the user terminal in the GPRS/UMTS networks. In essence, it carries out the role in GPRS equivalent to the Home Agent in Mobile IP. It maintains routing necessary to tunnel the Protocol Data Units (PDUs) to the SGSN that service a particular MS (Mobile Station).</div>
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The GGSN converts the GPRS packets coming from the SGSN into the appropriate packet data protocol (PDP) format (e.g., IP or X.25) and sends them out on the corresponding packet data network. In the other direction, PDP addresses of incoming data packets are converted to the GSM address of the destination user. The readdressed packets are sent to the responsible SGSN. For this purpose, the GGSN stores the current SGSN address of the user and his or her profile in its location register. The GGSN is responsible for IP address assignment and is the default router for the connected user equipment (UE). The GGSN also performs authentication and charging functions.</div>
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Other functions include subscriber screening, IP Pool management and address mapping, QoS and PDP context enforcement.</div>
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Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-21113812061798204092010-12-09T13:29:00.000+05:302010-12-09T13:29:42.945+05:30Basic Concepts of ModulationModulation is the process of facilitating the transfer of information over a medium. Sound transmission in air has limited range for the amount of power your lungs can generate. To extend the range your voice can reach, we need to transmit it through a medium other than air, such as a phone line or radio. The process of converting information (ex. voice) so that it can be successfully sent through a medium (wire or radio waves) is called modulation. There are three basic types of digital modulation techniques.<br />
<br />
<ul><li>Amplitude-Shift Keying (ASK)</li>
<li>Frequency-Shift Keying (FSK)</li>
<li>Phase-Shift Keying (PSK)</li>
</ul><div>All of these techniques vary a parameter of sinusoid to represent the information which we wish to send. A sinusoid has three different parameters that can be varied. These are amplitude, phase and frequency. Modulation is a process of mapping such that it takes your voice (as an example of signal) converts it into some aspect of sine wave and then transmits the sine wave, leaving the actual voice behind. The sine wave on the other side is remapped back to a near copy of your sound.</div><div><br />
</div><div>The medium is the thing through which the sine wave travels. So wire is a medium and so are air, water and space. The sine wave is called carrier. The information to be sent, which can be voice or data is called the information signal. Once the carrier is mapped with the information to be sent, it is no longer a sine wave and we call it the signal. The signal has the unfortunate luck of getting corrupted by noise as it travels.</div><div><br />
</div><div>In ASK, the amplitude of the carrier is changed in response to information and all else is kept fixed. Bit 1 is transmitted by a carrier of one particular amplitude. To transmit 0, we change the amplitude keeping the frequency constant. On-Off Keying (OOK) is a special form of ASK, where one of the amplitude is zero.</div><div><br />
</div><div>In FSK, we change the frequency in response to information, one particular frequency for a 1 and another frequency for a 0.</div><div><br />
</div><div>In PSK, we change the phase of the sinusoidal carrier to indicate information. Phase in this context is the starting angle at which the sinusoid starts. To transmit 0, we shift the phase of the sinusoid by 180°. Phase shift represents the change in the state of the information in this case.</div><div><br />
</div><div>ASK techniques are the most susceptible to the effects of non-linear devices which compress and distort signal amplitude. To avoid such distortion, the system must be operated in the linear range, away from the point of maximum power where most of the non-linear behavior occurs. Despite this problem in high frequency carrier systems, Amplitude Shift Keying is often used in wire-based radio signaling, both with or without a carrier.</div><div><br />
</div><div>ASK is also combined with PSK to create hybrid systems such as a Quadrature Amplitude Modulation (QAM) where both the amplitude and the phase are changed at the same time.</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-40635527758867504732010-11-29T17:13:00.000+05:302010-11-29T17:13:03.481+05:30What is EP NOFor acquiring an AT signal on reverse link (during connection setup), RN needs to know the approximate distance from the antenna. RN uses this distance and a BTS wide search window configurable value to search the signal from AT. This distance is known as Earliest PN Offset (EPNO).Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-6818455156619123972010-11-16T18:13:00.000+05:302010-11-16T18:19:46.217+05:30Walsh CodesWalsh Code is a group of spreading codes having good autocorrelation properties and poor crosscorrelation properties. Walsh codes are the backbone of CDMA systems and are used to develop the individual channels in CDMA. <br />
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For IS-95, 64 codes are available. Code 0 is used as the pilot and code 32 is used for synchronization. Codes 1 though 7 are used for control channels, and the remaining codes are available for traffic channels. Codes 2 through 7 are also available for traffic channels if they are not needed. For cdma2000, there exists a multitude of Walsh codes that vary in length to accommodate the different data rates and Spreading Factors of the different Radio Configurations.<br />
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IS-95 uses 64 Walsh codes and these allow the creation of 64 channels the base station. In other words, a base station can talk to a maximum of 64 (this number is actually only 54 because some codes are used for pilot and synch channels) mobiles at the same time. CDMA 2000 used 256 of these codes.<br />
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Walsh codes are created out of Haddamard matrices and Transform. Haddamard is the matrix type from which Walsh created these codes. Walsh codes have just one outstanding quality. In a family of Walsh codes, all codes are orthogonal to each other and are used to create channelization within the 1.25 MHz band.<br />
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Here are first four Hadamard matrices. The code length is the size of the matrix. Each<br />
row is one Walsh code of size N. The first matrix gives us two codes; 00, 01. The second<br />
matrix gives: 0000, 0101, 0011, 0110 and so on.<br />
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Their main purpose of Walsh codes in CDMA is to provide orthogonality among all the users in a cell. Each user traffic channel is assigned a different Walsh code by the base station. IS-95 has capability to use 64 codes, whereas CDMA 2000 can use up to 256 such codes. They are also used to create an orthogonal modulation on the forward link and are used for modulation and spreading on the reverse channel.<br />
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Orthogonal means that cross correlation between Walsh codes is zero when aligned. However, the auto-correlation of Walsh-Hadamard codewords does not have good characteristics. It can have more than one peak and this makes it difficult for the receiver to detect the beginning of the codeword without an external synchronization. The partial sequence cross correlation can also be non-zero and un-synchronized users can interfere with each other particularly as the multipath environment will differentially delay the sequences. This is why Walsh-Hadamard codes are only used in synchronous CDMA and only by the base station which can maintain orthogonality between signals for its users.<br />
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<div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiau_9OdPUSnKTn8Spt3X9l4BC7Na9dVn7twMfCiyn4-bkjBwLRnFBjGrNTzAZudXEJc3O448SP5nLAb0Sznv_wfXONvgYr7YR1C_V5HxSjlfR3Z01OszS_OAakJzJMVKgTU_5u1g1RXxg/s1600/Walsh+Code+-+Base+Station.PNG" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiau_9OdPUSnKTn8Spt3X9l4BC7Na9dVn7twMfCiyn4-bkjBwLRnFBjGrNTzAZudXEJc3O448SP5nLAb0Sznv_wfXONvgYr7YR1C_V5HxSjlfR3Z01OszS_OAakJzJMVKgTU_5u1g1RXxg/s1600/Walsh+Code+-+Base+Station.PNG" /></a></div><br />
The above is simplified look at the use of these codes. Assume there are three users in<br />
one cell. Each is trying to talk to someone else. User 1 wants to talk to someone who is<br />
outside its cell and is in cell 2. User 3 wants to talk to someone in cell 3.<br />
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Let’s take User 1. Its data is first covered by a channel Wash code, which is any Walsh code from 8 to 63. It is assigned to the user by the base station 1 in whose cell the mobile is located. The Base Station has also assigned different Walsh codes to users 2 and 3. All three of these are different are assigned by base station 1 and are orthogonal to each other. This keeps the data apart at the base station. Now based on the random number assigned by the BS, the mobile generates a long code mask (which is just the starting point of the long code sequence and is a scalar number). It now multiplies the signal by this long code starting at the mask ID. Now it multiplies it by the short code of the base station to whom it is directing the signal.<br />
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When the base station receives this signal, it can read the long code and see that the message needs to be routed to base station 2. So it strips off 1st short code and adds on the short code of base station 2 which is then broadcast by the BS 1 to BS 2 or sent by landlines. BS2 then broadcasts this signal along to all mobiles in its cell. The users who is located in this cell, now does the reverse. It multiplies the signal by the BS 2 short code (it knows nothing about BS 1 where the message generated) then it multiplies the signal by the same long code as the generating mobile. How? During the call paging, the mobile was given the same random number from which it creates the same long code mask. After that it multiplies it by the Walsh code sequence (also relayed during call setup).Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com1tag:blogger.com,1999:blog-7810716034849241025.post-12866452259390896032010-10-11T14:54:00.000+05:302010-10-11T14:54:39.192+05:30Spread Spectrum Signals - CDMA<div style="text-align: justify;">Spread Spectrum uses wide band, noise-like signals. Because Spread Spectrum signals are noise-like, they are hard to detect. Spread Spectrum signals are also hard to Intercept or demodulate. Further, Spread Spectrum signals are harder to jam (interfere with) than narrowband signals. These Low Probability of Intercept (LPI) and anti-jam (AJ) features are why the military has used Spread Spectrum for so many years. Spread signals are intentionally made to be much wider band than the information they are carrying to make them more noise-like.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">Spread Spectrum signals use fast codes that run many times the information bandwidth or data rate. These special "Spreading" codes are called "Pseudo Random" or "Pseudo Noise" codes. They are called "Pseudo" because they are not real gaussian noise.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">Spread Spectrum transmitters uses similar transmit power levels to narrow band transmitters. Because Spread Spectrum signals are so wide, they transmit at a much lower spectral power density, measured in Watts per Hertz, than narrowband transmitters. This lower transmitted power density characteristic gives spread signals a big plus. Spread and narrow band signals can occupy the same band, with little or no interference. This capability is the main reason for all the interest in Spread Spectrum today.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">Since the development of CDMA technology there has been many new releases and platforms. The original CDMA is now referred to as CDMAone. Several different variants of CDMA technology been developed continuously improving quality and data transfer speeds. Third generation CDMA technology, commonly referred to as CDMA2000 encompasses a wide variety of different standards, each continually improving upon the first including; 1X EV, 1XEV-DO, and MC 3X. CDMA2000 is the current standard used by most US carriers today. The first release of CDMA2000 was refereed to as either 3G1X, 1XRTT, or X.Designed to provide data transmissions of ten times faster then the previous technology and double the voice capacity of CDMAone.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">Depending on the phone you have and its capabilities you will notice symbols in the default screen of your phone reading either 1X, 1XEV-DO or some variation of the two. This symbol defines the CDMA2000 standards your phone is operating on. Newer phones will display EV or EV-DO using the newer faster, more reliable CDMA technology.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">Qualcomm the original developer of CDMA owns patents of this technology. They have granted royalty-bearing licenses to over 100 network operators.</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-71463470333893081432010-10-08T17:25:00.000+05:302010-10-08T17:25:14.119+05:30PN Offset in CDMA<div style="text-align: justify;">Offset is one of the 512 short code sequences used to differentiate sectors on base stations for communication with mobile units. PN stands for pseudo random noise that appears in a repetitive manner. The PN sequence forms a “short” code that is 32,768 chips in length and repeats every 26.666 milliseconds. This short code is combined with the data and transmitted in each of the forward channels. 512 points within the sequence have been selected as the PN offsets (from 0-511). Each base station uses a different point in the sequence to create a unique PN offset or identifier in its pilot signal which can be used to identify the base station sector.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">For CDMA networks, the most common form of interference is pilot pollution. Each base-station sector is assigned an identifier called a PN offset, which is a timing offset based on the GPS even-second clock. Since each base station assigned to a particular frequency carrier operates at the same center frequency, the PN offset is used to distinguish base stations from one another.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">When a CDMA phone searches for the strongest base-station signal, it identifies the PN offset of each signal it receives. It only looks for PNs for which the network tells it to search. This list of PNs, the neighbor list, constantly is changing since it depends on the phone's current location. Pilot pollution occurs when the CDMA mobile phone's rake receiver receives more than three (four for newer phones) pilot signals having approximately the same Ec/Io relative power levels.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">Each base station sector in a cdmaOne network may transmit on the same frequency, using the same group of 64 Walsh codes for pilot, paging, sync and forward traffic channels. Therefore, another layer of coding is required so that a mobile phone can differentiate one sector from another.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">The PN offset plays a key role in this code layer. The abbreviation “PN” stands for pseudo-random noise – a long bit sequence that appears to be random when viewed over a given period of time, but in fact is repetitive. In cdmaOne transmissions, the entire PN sequence is defined to form a short code that is 32,768 chips in length and repeats once every 0.027 seconds. The short code is exclusive OR’d with the data and transmitted in each of the forward channels (pilot, paging, sync, and traffic). Within the 32,768 chip sequence, 512 points have been chosen to provide PN offsets. Each base station transceiver uses a different point in the sequence to create a unique PN offset to the short code in its forward link data. As a result, a mobile phone can identify each base station sector by the PN offset in the received signal.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">Each base station transmits a version of the long code that is shifted in time by a different multiple of the chip time. The PN offset represents the number of chip times by which a particular base station delays transmission of the long code. The cellular telephone receives the long code offset when the cellular telephone enters a cell or powers on, and stores the PN offset in a nonvolatile portion of memory.</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com1tag:blogger.com,1999:blog-7810716034849241025.post-91467248871424573802010-10-08T17:24:00.000+05:302010-10-08T17:27:40.495+05:30Distance-based Location Update & RouteUpdateRadius in CDMA 1X EV-DO<ul><li style="text-align: justify;"><b>Introduction</b></li>
</ul><div style="text-align: justify;">Idle-mode mobility management in cellular systems involve location updates and paging. Idle-mode mobility is not tracked at the granularity of individual cells. Instead, it is tracked at a coarser granularity of a group of contiguous cells termed as a "location area". A location update mechanism involves the reporting of this location area information by an idle mode mobile to the network, whenever it moves from one location area to another. Because the network knows the location of the mobile only at the location area-level, when there is an incoming call for the mobile, the network needs to page the mobile in all the cells in the location area. Both the location update mechanism and paging will generate signaling load on the network, and reducing the load due to one of these would involve an increased load due to the other. </div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">In CDMA 1X EV-DO, specified in 3GPP2 C.S0024-A v1.0 (2004), a dynamic distance-based location update mechanism is used. In this scheme, a mobile makes a location update if the distance between the BTS in which it is currently camped, and the BTS where it made its last location update is greater than a parameter called RouteUpdateRadius. This scheme provides a significant performance benefit over the static location update mechanism used in GSM/GPRS/UMTS networks. However, the distance-based mechanism does not utilize the knowledge of the direction of mobiles’ movement.<br />
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</div><ul><li style="text-align: justify;"><b>1X EV-DO Route Update Protocol</b></li></ul><br />
<div style="text-align: justify;">In CDMA 1X EV-DO, the idle-mode mobility management procedures are handled by the route update protocol, and it uses the distance-based location update approach. In 1X EV-DO, each cell broadcasts its latitude, longitude, and a parameter called <i>RouteUpdateRadius</i>. An idle mode mobile as it moves from cell to cell, monitors these three parameters. after each cell change, the mobile computes the distance between the site locations of the current cell and the cell in which it last sent a location update message. If this distance is greater than the <i>RouteUpdateRadius</i> parameter broadcast in the cell in which it last sent a location update message, the mobile sends another update to the network.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">Otherwise mobile does not send a location update message. To perform this operation, the mobile would have to store the latitude, longitude, and <i>RouteUpdateRadius</i> parameters of the last cell in which it did a location update operation.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">The distance computed is the distance between the site locations, and it does not depend on the location of mobile within the serving site. The latitude and longitude information is broadcasted by each cell is used for computing the distance.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">Because the mobile sends a location update message only after it moves to a cell that is sufficiently far apart from the cell from which the mobile last sent a location update, the problem of ping-ponging is eliminated. Essentiall, as soon as a mobile sends a location update, it draws a circle around the serving cell of radius <i>RouteUpdateRadius</i> and sends the next location update only if it goes outside that circle. Clearly, this approach eliminates the ping-ponging problem of the static location area approach.</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-12931427535755107192010-10-08T17:06:00.000+05:302010-10-08T17:11:54.542+05:30CDMA Introduction<div style="text-align: justify;">Code Division Multiple Access (CDMA) has gained widespread international acceptance by cellular radio system operators as an upgrade that will dramatically increase both their system capacity and the service quality. CDMA is a "spread spectrum" technology, allowing many users to occupy the same time and frequency allocations in a given band/space. As its name implies, CDMA (Code Division Multiple Access) assigns unique codes to each communication to differentiate it from others in the same spectrum.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">In a world of finite spectrum resources, CDMA enables many more people to share the airwaves at the same time than do alternative technologies. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to point communication at the same data rate.</div><div style="text-align: justify;"><br />
</div><b>1. INTRODUCTION</b><br />
<div style="text-align: justify;"><br />
</div><div style="text-align: justify;">CDMA stands for Code Division Multiple Access, but was originally known as IS-95. Qualcomm was the first to created this technology and by 1993 it was adopted by the Telecommunication Industry Association. Later this technology was enhanced and refined by Ericsson.The world is demanding more from wireless communication technologies than ever before as more people around the world are subscribing to wireless. Add in exciting Third-Generation (3G) wireless data services and applications - such as wireless email, web, digital picture taking/sending, assisted-GPS position location applications, video and audio streaming and TV broadcasting - and wireless networks are doing much more than just a few years ago. This is where CDMA technology fits in. CDMA consistently provides better capacity for voice and data communications than other commercial mobile technologies, allowing more subscribers to connect at any given time, and it is the common platform on which 3G technologies are built. </div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">The CDMA air interface is used in both 2G and 3G networks. 2G CDMA standards are branded cdmaOne and include IS-95A and IS-95B. CDMA is the foundation for 3G services: the two dominant IMT-2000 standards, CDMA2000 and WCDMA, are based on CDMA.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;"><b>1.1 CDMAONE: The Family of IS-95 CDMA</b></div><div style="text-align: justify;"><b><br />
</b></div><div style="text-align: justify;">Technologies cdmaOne describes a complete wireless system based on the TIA/EIA IS-95 CDMA standard, including IS-95A and IS-95B revisions. It represents the end-to-end wireless system and all the necessary specifications that govern its operation. cdmaOne provides a family of related services including cellular, PCS and fixed wireless (wireless local loop).</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;"><b>1.2 CDMA2000: Leading the 3G revolution</b></div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">CDMA2000 represents a family of ITU-approved, IMT-2000 (3G) standards and includes CDMA2000 1X and CDMA2000 1xEV technologies. They deliver increased network capacity to meet growing demand for wireless services and high-speed data services. CDMA2000 1X was the world's first 3G technology commercially deployed (October 2000).</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;"><b>2. SPREAD SPECTRUM COMMUNICATIONS</b></div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread Spectrum communications is distinguished by three key elements:</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">1. The signal occupies a bandwidth much greater than that which is necessary to send the</div><div style="text-align: justify;">information. This results in many benefits, such as immunity to interference and jamming and multiuser access, which we'll discuss later on.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">2. The bandwidth is spread by means of a code which is independent of the data. The independence of the code distinguishes this from standard modulation schemes in which the data modulation will always spread the spectrum somewhat. </div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">3. The receiver synchronizes to the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time.</div><div style="text-align: justify;">In order to protect the signal, the code used is pseudo-random. It appears random, but is actually deterministic, so that the receiver can reconstruct the code for synchronous detection. This pseudo random code is also called pseudo-noise (PN).</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">There are three ways to spread the bandwidth of the signal:</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">• Frequency hopping. The signal is rapidly switched between different frequencies within the hopping bandwidth pseudo randomly, and the receiver knows before hand where to find the signal at any given time.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">• Time hopping. The signal is transmitted in short bursts pseudo-randomly, and the receiver knows beforehand when to expect the burst.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">• Direct sequence. The digital data is directly coded at a much higher frequency. The code is generated pseudo-randomly, the receiver knows how to generate the same code, and correlates the received signal with that code to extract the data.</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-56642853464465298782010-10-08T16:50:00.000+05:302010-10-08T17:17:20.057+05:30What is Ec/I0<div style="text-align: justify;">In CDMA refers to the portion of the RF signal which is usable. It's the difference between the signal strength and the noise floor.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">Ec/Io (pronounced "ee-see over eye-not") is basically a measure of how well your phone can hear the tower over all the other traffic on the channel.</div><div style="text-align: justify;"><br />
</div><div style="text-align: justify;">A reading near 0.0 is very good. You can find low readings late at night on weekdays when traffic is low. When the reading is high (-12.0 to -15.0), quality will drop and you may even lose the call.</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-18501377027533769652010-10-01T15:18:00.000+05:302010-11-15T18:30:29.239+05:30BTS – Base Transceiver Station<div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiNWDV3yMsDUYt6ko91929zyt4rlPN-enfejPPiRNg90xUCxDwnouxzk8zboUPcAnbXpRhFqZUrTHEINScWW6yCX0_PKe9fCwxjam_Pv98PM2iK0YjxZ_ACOgHSjJusBRBzDYW1mHWuOVg/s1600/BTS-1.jpeg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiNWDV3yMsDUYt6ko91929zyt4rlPN-enfejPPiRNg90xUCxDwnouxzk8zboUPcAnbXpRhFqZUrTHEINScWW6yCX0_PKe9fCwxjam_Pv98PM2iK0YjxZ_ACOgHSjJusBRBzDYW1mHWuOVg/s320/BTS-1.jpeg" width="240" /></a></div><br />The base transceiver station, or BTS, contains the equipment for transmitting and receiving radio signals (transceivers), antennas, and equipment for encrypting and decrypting communications with the base station controller (BSC). Typically a BTS have several transceivers (TRXs) which allow it to serve several different frequencies and different sectors of the cell (in the case of sectorised base stations).<br /><br />A BTS is controlled by a parent BSC The BTSs are equipped with radios that are able to modulate layer 1 of interface Um; for GSM 2G+ the modulation type is GMSK, while for EDGE-enabled networks it is GMSK and 8-PSK.<br /><br />Frequency hopping is often used to increase overall BTS performance; this involves the rapid switching of voice traffic between TRXs in a sector. A hopping sequence is followed by the TRXs and handsets using the sector. Several hopping sequences are available, and the sequence in use for a particular cell is continually broadcast by that cell so that it is known to the handsets. A TRX transmits and receives according to the GSM/CDMA standards.<br /><br />In short BTS<br /><br />1. Encodes,encrypts,multiplexes,modulates and feeds the RF signals to the antenna.<br />2. Frequency hopping<br />3. Communicates with Mobile station and BSC<br />4. Consists of Transceivers (TRX) unitsVikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-29636511368333181082010-06-30T11:36:00.000+05:302010-11-15T18:30:29.256+05:30Fast handoffFast handoff is a particular type of hard handoff that applies only to packet data sessions. Fast handoff allows the target PDSN to connect to an anchor PDSN where the packet data session was first established, eliminating the need to re-establish a PPP session while the packet data session is active. Fast handoff allows for early establishment of the A10 connections on the target side.Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-4169264669941441532010-06-30T11:34:00.000+05:302010-11-15T18:30:29.269+05:30Dormant Handoff<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">A handoff that occurs when an MS with a dormant packet session determines that it has crossed a packet zone boundary. Dormant handoff results in A10 connection(s) being established between the target PCF and the target PDSN. A dormant handoff may require exchange of higher layer protocol messages between the MS and the PDSN, and thus, reactivation of the packet data session. Note that no air interface channels are handed off or re-configured as the result of a dormant handoff.</span>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-31263307469306745342010-05-29T13:07:00.000+05:302010-11-15T18:30:29.296+05:30Handover BasicsAlthough the concept of cellular handover or cellular handoff is relatively straightforward, it is not an easy process to implement in reality. The cellular network needs to decide when handover or handoff is necessary, and to which cell. Also when the handover occurs it is necessary to re-route the call to the relevant base station along with changing the communication between the mobile and the base station to a new channel. All of this needs to be undertaken without any noticeable interruption to the call. The process is quite complicated, and in early systems calls were often lost if the process did not work correctly.<br /><br />Different cellular standards handle hand over / handoff in slightly different ways. Therefore for the sake of an explanation the example of the way that GSM handles handover is given.<br /><br />There are a number of parameters that need to be known to determine whether a handover is required. The signal strength of the base station with which communication is being made, along with the signal strengths of the surrounding stations. Additionally the availability of channels also needs to be known. The mobile is obviously best suited to monitor the strength of the base stations, but only the cellular network knows the status of channel availability and the network makes the decision about when the handover is to take place and to which channel of which cell.<br /><br />Accordingly the mobile continually monitors the signal strengths of the base stations it can hear, including the one it is currently using, and it feeds this information back. When the strength of the signal from the base station that the mobile is using starts to fall to a level where action needs to be taken the cellular network looks at the reported strength of the signals from other cells reported by the mobile. It then checks for channel availability, and if one is available it informs this new cell to reserve a channel for the incoming mobile. When ready, the current base station passes the information for the new channel to the mobile, which then makes the change. Once there the mobile sends a message on the new channel to inform the network it has arrived. If this message is successfully sent and received then the network shuts down communication with the mobile on the old channel, freeing it up for other users, and all communication takes place on the new channel.<br /><br />Under some circumstances such as when one base transceiver station is nearing its capacity, the network may decide to hand some mobiles over to another base transceiver station they are receiving that has more capacity, and in this way reduce the load on the base transceiver station that is nearly running to capacity. In this way access can be opened to the maximum number of users. In fact channel usage and capacity are very important factors in the design of a cellular network.Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-87739100639305619102010-05-28T12:50:00.000+05:302010-11-15T18:30:29.309+05:30UMTS HandoverThere are following categories of handover (also referred to as handoff):<br /><ul><li>Hard Handover<br />Hard handover means that all the old radio links in the UE are removed before the new radio links are established. Hard handover can be seamless or non-seamless. Seamless hard handover means that the handover is not perceptible to the user. In practice a handover that requires a change of the carrier frequency (inter-frequency handover) is always performed as hard handover.<br /></li><li>Soft Handover<br />Soft handover means that the radio links are added and removed in a way that the UE always keeps at least one radio link to the UTRAN. Soft handover is performed by means of macro diversity, which refers to the condition that several radio links are active at the same time. Normally soft handover can be used when cells operated on the same frequency are changed.<br /></li><li>Softer handover<br />Softer handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B (i.e. the site of co-located base stations from which several sector-cells are served. In softer handover, macro diversity with maximum ratio combining can be performed in the Node B, whereas generally in soft handover on the downlink, macro diversity with selection combining is applied.</li></ul>Generally we can distinguish between intra-cell handover and inter-cell handover. For UMTS the following types of handover are specified:<br /><br />The most obvious cause for performing a handover is that due to its movement a user can be served in another cell more efficiently (like less power emission, less interference). It may however also be performed for other reasons such as system load control.<br /><br />Active Set is defined as the set of Node-Bs the UE is simultaneously connected to (i.e., the UTRA cells currently assigning a downlink DPCH to the UE constitute the active set).<br /><br />Cells, which are not included in the active set, but are included in the CELL_INFO_LIST belong to the Monitored Set.<br /><br />Cells detected by the UE, which are neither in the CELL_INFO_LIST nor in the active set belong to the Detected Set. Reporting of measurements of the detected set is only applicable to intra-frequency measurements made by UEs in CELL_DCH state. <br /><br />The different types of air interface measurements are:<br /><br /><b>Intra-frequency measurements:</b> measurements on downlink physical channels at the same frequency as the active set. A measurement object corresponds to one cell.<br /><br /><b>Inter-frequency measurements:</b> measurements on downlink physical channels at frequencies that differ from the frequency of the active set. A measurement object corresponds to one cell.<br /><br /><b>Inter-RAT measurements:</b> measurements on downlink physical channels belonging to another radio access technology than UTRAN, e.g. GSM. A measurement object corresponds to one cell.<br /><br /><b>Traffic volume measurements:</b> measurements on uplink traffic volume. A measurement object corresponds to one cell.<br /><br /><b>Quality measurements:</b> Measurements of downlink quality parameters, e.g. downlink transport block error rate. A measurement object corresponds to one transport channel in case of BLER. A measurement object corresponds to one timeslot in case of SIR (TDD only). <br /><br /><b>UE-internal measurements:</b> Measurements of UE transmission power and UE received signal level.<br /><br /><b>UE positioning measurements:</b> Measurements of UE position.The UE supports a number of measurements running in parallel. The UE also supports that each measurement is controlled and reported independently of every other measurement.Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-62533933497588113372010-02-05T11:52:00.000+05:302010-11-15T18:30:29.331+05:30DSCH - Downlink Shared ChannelThe Downlink Shared Channel is a downlink transport channel that may be shared by several UE (User Equipment). It is used to carry dedicated control or traffic data from the SRNC/BTS (Serving Radio Network Controller). The DSCH will be associated with one or several downlink DCH (Dedicated Channel).Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com1tag:blogger.com,1999:blog-7810716034849241025.post-69645566322657934372010-02-03T15:44:00.000+05:302010-11-15T18:30:29.353+05:30GSM Technology<div style="text-align: justify;">What is GSM?</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">GSM (Global System for Mobile communications) is an open, digital cellular technology used for transmitting mobile voice and data services</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">The origins of GSM can be traced back to 1982 when the Groupe Spécial Mobile (GSM) was created by the European Conference of Postal and Telecommunications Administrations (CEPT) for the purpose of designing a pan-European mobile technology. </div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">It is approximated that 80 percent of the world uses GSM technology when placing wireless calls, according to the GSM Association (GSMA), which represents the interests of the worldwide mobile communications industry. This amounts to nearly 3 billion global people.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">GSM supports voice calls and data transfer speeds of up to 9.6 kbit/s, together with the transmission of SMS (Short Message Service).</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">GSM operates in the 900MHz and 1.8GHz bands in Europe and the 1.9GHz and 850MHz bands in the US. The 850MHz band is also used for GSM and 3G in Australia, Canada and many South American countries. By having harmonised spectrum across most of the globe, GSM’s international roaming capability allows users to access the same services when travelling abroad as at home. This gives consumers seamless and same number connectivity in more than 218 countries.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">Terrestrial GSM networks now cover more than 80% of the world’s population. GSM satellite roaming has also extended service access to areas where terrestrial coverage is not available.</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-77928529405680918412010-02-03T12:30:00.000+05:302010-11-15T18:30:29.367+05:30What is CDMA2000 1xEV-DO<div style="text-align: justify;">EV-DO is a high-speed network protocol used for wireless data communications, primarily Internet access. EV-DO is considered a broadband technology like DSL or cable modem Internet services.</div><div style="text-align: justify;">Certain classes of cellular phones support EV-DO. These phones may be available from various phone carriers around the world including Sprint and Verizon in the U.S. Additionally, various PCMCIA adapters and external modem hardware exists to enable laptops and handheld devices for EV-DO.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">The EV-DO protocol uses asymmetric communications, allocating more bandwidth for downloads than for uploads. The original EVDO Revision 0 standard supports up to 2.4 Mbps data rates down but only 0.15 Mbps (about 150 Kbps) up.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">An improved version of EV-DO called Revision A increases download speeds up to 3.1 Mbps and uploads to 0.8 Mbps (800 Kbps). EV-DO providers have gradually been upgrading their equipment from Rev 0 to support Rev A.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">A future version of EV-DO called Revision B (not yet widely deployed) promised to offer much higher data rates as this protocol is capable of aggregating bandwidth from multiple wireless channels. Early trials have achieved EV-DO Rev B downloads of greater than 9 Mbps.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">As with many other network protocols, the theoretical maximum data rates of EV-DO are not achieved in practice. Real-world networks may run at 50% or less of the rated speeds.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">CDMA2000 1x EV-DO cell phone system is a standard that has evolved from the CDMA2000 mobile phone system and it is now firmly established in many areas of the world. The letters EV-DO stand for Evolution Data Only or Data Optimised. From the title it can be seen that it is a data only mobile telecommunications standard that can be run on CDMA2000 networks.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">The EV-DO cell phone system is capable of providing the full 3G data rates up to 3.1 Mbps now that release A of the standard has been issued. The first commercial CDMA2000 1xEV-DO network was deployed by SK Telecom (Korea) in January 2002. </div><div style="text-align: justify;"><br /></div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-6688048419708664412010-02-03T12:22:00.000+05:302010-11-15T18:30:29.380+05:30EV-DO Basics<ul><li>EV-DO - Evolution Data Optimized</li><li>Personal broadband wireless service for a wide range of customers, from business people to students</li><li>Always on - - similar to DSL (wherever 3G capability is available)</li><li>Rides on CDMA signal- 1x data capability available everywhere CDMA voice service available</li><li>Up to 10 times the peak data rate of the next best public wireless solution - 800 - 1,000 Kbps (kilobits per second) average download speeds, comparable to DSL speeds</li><li>Allows the user to be connected herever they are are not only for email, but for downloads, large files, photos, spreadsheets, etc.</li><li>Advantages over WiFi:</li><ul><li>Always on with seamless roaming!</li><li>Signal can travel on same cell sites as cell phones</li><li>No 300-ft range from the cell tower or "hotspot"</li><li>Customers can access their corporate VPN (virtual private network) anywhere they can get a cellular signal via a secure, encrypted signal</li><li>Can download and run video clips in real time</li><li>Can provide service to customers outside of cable-modem or DSL areas</li></ul><li>Relatively low cost with high capacity - allows rich web browsing and application usage</li><li><b>1xRTT</b>: 50Kbps - 100Kbps Upload and Download (bursts to 144Kbps)</li><li><b>EVDO Rev 0</b>: 400kbps-1000kbps Download (bursts up to 2.0Mbps), 50kbps-100kbps Upload (bursts to 144Kbps)</li><li><b>EVDO Rev A</b>: 600Kbps-1,400Kbps Download (bursts to 3.1Mbps), 500Kbps-800Kbps Upload (bursts to 1.8Mbps)</li></ul>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com0tag:blogger.com,1999:blog-7810716034849241025.post-33319172452758455402010-02-03T11:53:00.000+05:302010-11-15T18:30:29.392+05:30What are Femtocells?<div style="text-align: justify;">Femtocells are low-power access points that can combine mobile and Internet technologies within the home. The femtocell unit generates a personal mobile phone signal in the home and connects this to the operator’s network through the Internet. This will allow improved coverage and capacity for each user within their home. </div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">Femtocells have an output power less than 0.1 Watt, similar to other wireless home network equipment, and will typically allow up to about 4 simultaneous calls/data sessions at any time. Mobile phones connected to a femtocell will typically operate at levels similar to other wireless phones used in the home.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">Femto cells or femtocells are small cellular telecommunications base stations that can be installed in residential or business environments either as single stand-alone items or in clusters to provide improved cellular coverage within a building. It is widely known that cellular coverage, especially for data transmission where good signal strengths are needed is not as good within buildings. By using a small internal base station - femtocell (femto cell), the cellular performance can be improved along with the possible provision of additional services.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">In order to link the femtocells with the main core network, the mobile backhaul scheme uses the user's DSL or other Internet link. This provides a cost effective and widely available data link for the femtocells that can be used as a standard for all applications.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">There are many advantages for the deployment of femtocells to both the user and the mobile network operator. For the user, the use of a femto cell within the home enables far better coverage to be enjoyed along with the possible provision of additional services, possible cost benefits, and the use of a single number for both home and mobile applications. For the network operator, the use of femtocells provides a very cost effective means of improving coverage, along with linking users to their network, and providing additional revenue from the provision of additional services.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">Although there are advantages and disadvantages to the use of femtocells, their use has many advantages for both user and network provider.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">Typically, a single femtocell will deliver voice services simultaneously to at least four users within the home, while allowing many more to be connected or ‘attached’ to the cell, accessing services such as SMS. Additionally, femtocells will deliver data services to multiple users, typically at the full peak rate supported by the relevant air interface technology, currently several megabits per second and rising to tens and hundreds of megabits per second in the future. But by removing the capacity hungry indoor mobile users from the outdoor network, femtocells also in effect improve performance for consumers outside. Indeed, for each additional indoor femtocell user, system resources are freed to serve about ten outdoor users. The femtocell behaves like a normal base station in that as users enter or leave the home their voice or data services are seamlessly handed over from or to the outdoor network as required.</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">Subscribers benefit from perfect cellular coverage and faster mobile broadband in the home as well as a more competitive voice and data tariff. Operators get optimum cellular coverage and more mobile usage in the home and dramatically reduced operating costs especially through backhaul - their single largest OPEX - and power savings. Equally importantly the cellular operators’ capital expenditure will significantly drop because accelerating data usage means they will inevitably have to heavily invest in their outdoor network in terms of new cell sites and backhaul to meet expected demand - something femtocells do at a fraction of the cost. In fact, Paul Jacobs, Qualcomm’s CEO, recently said that the gains in throughput available to femtocell users are “equivalent to that brought by the cell phone’s shift from analogue to digital.”</div><div style="text-align: justify;"><br /></div><div style="text-align: justify;">Finally, as mobile operators look beyond 3G to LTE or WiMAX, femtocells offer a new, dramatically lower-cost model for network rollout. For example, LTE femtocells could be employed using higher frequencies to deliver targeted intense high bandwidth requirements inside buildings - exactly where subscribers most demand it. Operators can then use their existing networks outdoors as demand slowly builds up and then use the scarce lower frequency spectrum to provide good quality LTE coverage across entire markets with the minimum number of outdoor network cells. As we have seen, the simple proposition of lower costs, for both operators and consumers, combined with improved coverage and services is compelling. Yet there are also challenges which must be overcome before widespread commercial deployments can become a reality.</div>Vikramhttp://www.blogger.com/profile/04778775507951515622noreply@blogger.com2