Wednesday, March 28, 2012

Cyclic Delay Diversity

Cyclic 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).

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.

Cyclic Delay Diversity (CDD) is a diversity scheme used in OFDM-based telecommunication systems, transforming spatial diversity into frequency diversity avoiding intersymbol interference.

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 OFDMA transmission

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.

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.

Tuesday, March 27, 2012

Spatial Multiplexing

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.

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.

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.

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.

Thursday, December 1, 2011

Orthogonal Frequency Division Multiple Access (OFDMA)

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.

OFDMA is a multi-user OFDM that allows multiple access on the same channel (a channel being a group of evenly spaced subcarriers).

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.

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.

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.

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).

OFDMA Advantages 
  • Averaging interference's from neighboring cells, by using different basic carrier permutations between users in different cells. 
  • Interference’s within the cell are averaged by using allocation with cyclic permutations. 
  • Enables orthogonality in the uplink by synchronizing users in time and frequency. 
  • Enables Multipath mitigation without using Equalizers and training sequences. 
  • Enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage. 
  • Enables spatial diversity by using antenna diversity at the Base Station and possible at the Subscriber Unit. 
  • 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). 
  • Offers Frequency diversity by spreading the carriers all over the used spectrum. Offers Time diversity by optional interleaving of carrier groups in time.