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.

Advantages & Disadvantages of OFDM

Advantages of OFDM

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.

Spectral Efficiency

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.

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.

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.

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.

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.

Reduced Inter Symbol Interference (ISI)

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.

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.

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.

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.

Other Advantages

• Flexibility of deployment across various frequency bands with little needed modification to the air interface. 
• Averaging interferences from neighboring cells, by using different basic carrier permutations between users in different cells. 
• Interferences within the cell are averaged by using allocation with cyclic permutations. 
• Enables orthogonality in the uplink by synchronizing users in time and frequency. 
• Enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage. 
• 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.
• 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.

Disadvantages of OFDM

• Peak to average power ratio (PAPR) is high 
•  High power transmitter ampli ers need linearlization 
•  Low noise receiver ampli ers need large dynamic range
• Capacity and power loss due to guard interval 
• Bandwidth and power loss due to the guard interval can be signi cant 
• The guard interval consumes 20% of the bandidth and transmit power in IEEE802.11a
• Frequency off sets and phase noise sensitivity 
•  Phase noise is especially acute at high carrier frequencies
• 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. 
• It is more sensitive to carrier frequency offset and drift than single carrier systems are due to leakage of the DFT.

OFDM Basics

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.

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.

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.

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.

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.

OFDM is present in

 – LTE

 – Mobile WiMax IEEE 802.16e

 – xDSL

 – Wireless LAN IEEE 802.11a,g,

What is OFDM?

• Orthogonal FDM – it’s multiplexing

• It’s more: – Multi Carrier – Digital modulation (PSK, QAM) – Digital processing

• Demultiplexing

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.

In OFDM

       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.

A parallel data transmission system offers possibilities for alleviating this problem encountered with serial systems.

    - Resistance to frequency selective fading.

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. 

Modulation types over OFDM systems

 - Phase shift keying (PSK)

 - Quadrature amplitude modulation (QAM)

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.

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.

Public Land Mobile Network (PLMN)

PLMN 

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. 

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). 

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. 

PLMN Area

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. 

Terminating network users can also set up calls to the PLMN. 

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.

Gateway GPRS Support Node [GGSN]

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.

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.

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.

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.

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).

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.

Other functions include subscriber screening, IP Pool management and address mapping, QoS and PDP context enforcement.