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OFDMA: The PHY transmission technology choice for 4G

Posted: 03 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:OFDMA? 4G wireless interface? wireless ecosystem?

Even as 3G wireless equipment continues deployment, the wireless ecosystem is identifying and designing 4G systems. Although there is no clear definition of what separates 3G from 4G systems, there seems to be a consensus forming within the standards bodies with respect to maximum supported data rates.

3G systems such as high speed packet access provide around 15-20Mbit/s downlink and about 5-10Mbit/s uplink. 4G systems are being designed to support five to 10 times those rates with greater than 100Mbit/s or more in the downlink and over 50Mbit/s in the uplink.

Regarding 4G standards, both major 3G standards bodies, the 3G Partnership Project (3GPP) and 3GPP2, have indicated that orthogonal frequency division multiple access (OFDMA) is their choice for the PHY transmission technology. However, the first standard to be deployed using the new multiplexing technique is IEEE 802.16e (WiMAX).

In OFDM, usable bandwidth is divided into a large number of smaller bandwidths that are mathematically orthogonal using FFTs. Reconstruction of the band is performed by the inverse FFT (IFFT). FFTs and IFFTs are well-defined algorithms that can be implemented very efficiently when sized as powers of 2. Typical FFT sizes for OFDM systems are 512, 1024 and 2048, with the smaller 128 and 256 sizes also possibilities. Among the bandwidths that will be supported are 5-, 10- and 20MHz. One benefit of this technique is the ease of adaptation to different bandwidths. The smaller bandwidth unit can remain fixed, even as the total bandwidth utilization is changed.

A challenge in today's wireless systems is an effect called "multipath." Multipath results from reflections between a transmitter and receiver whereby the reflections arrive at the receiver at different times. The time span separating the reflection is referred to as delay spread. This type of interference tends to be problematic when the delay spread is on the order of the transmitted symbol time. Typical delay spreads are microseconds in length, which are close to CDMA symbol times. OFDMA symbol times tend to be on the order of 100?s, making multipath less of a problem. To mitigate the effect of multipath, a guardband of about 10?scalled the cyclic prefixis inserted after each symbol.

Achieving higher data rates requires OFDM systems to make more efficient use of the bandwidth than CDMA systems. The number of bits per unit hertz is referred to as the spectral efficiency. One method of achieving this higher efficiency is through the use of higher order modulation. Modulation refers to the number of bits that each subcarrier transmits. Another benefit of OFDM is the use of advanced multi-antenna signal processing techniques. The two most common techniques are called multiple input, multiple output (MIMO) processing and beamforming.

Signal processing
In MIMO, the system exploits the fact that the received signal from one transmit antenna can be quite different from the received signal from a second antenna. This is most common in indoor or dense metropolitan areas where there are many reflections and multipaths between transmitter and receiver. In this case, a different signal can be transmitted from each antenna at the same frequency and still be recovered at the receiver by signal processing. A simple way to view this is in a standard N equations and N unknowns problem, which is solved using a well known matrix inversion technique. Reusing frequency in this way is known as Re-use 1, where the same frequency is used for different signals at the same time.

Beamforming, on the other hand, is mostly a transmit technology and attempts to form a coherent construction of the multiple transmitters at the receiver. This can yield a higher SNR at the receiver and can provide higher bandwidth or longer reach for the same transmitted power. Rather than exploiting the different air interface responses between antennas, beamforming modifies the signal to unify the signal. Thus, beamforming does not reuse frequency in the same way as MIMO. Dividing the frequency into separate bands for separate cells is called Re-use 3. This comes from the common practice of dividing wireless cell sites into three distinct sectors.

It is also possible to combine both MIMO and beamforming in some cases, particularly in four-antenna systems. An ideal system would switch between modes depending on the characteristics of the deployment.

Shown is frame allocation within OFDM, where each burst allocation can be changed from frame to frame and within the modulation order.

OFDMA was developed to move OFDM technology from a fixed-access wireless system to a true cellular system with mobility. The underlying technology is the same, but more flexibility was defined in the operation of the system. In OFDMA, subcarriers are grouped into larger units referred to as subchannels. These subchannels are further grouped into bursts, which can be allocated to wireless users. Each burst allocation can be changed from frame to frame and within the modulation order. This allows the base station to dynamically adjust the bandwidth usage according to the current system requirements.

In addition, since each user consumes only a portion of the total bandwidth, the power of each user can also be modulated according to the current system requirements. QoS is another feature that can be adapted for different users depending on their specific application such as voice, streaming video or Internet access.

Bandwidth flexibility
OFDM and OFDMA allow systems to easily adapt to the available spectrum. The stated goal of both LTE and WiMAX is to support bandwidth allocations from 1.25MHz to 20MHz. In addition, the systems can support either time division or frequency division multiplexing. This flexibility will allow service providers to roll out 4G systems in different ways for different areas depending on the needs of the markets.

As the early stages of 4G wireless networking unfold, system developers are beginning to consider the solutions that might be best suited for WiMAX and other OFDMA-based equipment. In many respects, the same generic considerations hold for OFDMA that have held for earlier wireless applications: high computational performance, low power consumption, programming flexibility, integration of high-speed peripherals, complete software platforms and comprehensive development tools. DSP suppliers who are able to bring together all these elements in their products will provide the solutions that best enable 4G networks. One DSP currently being offered as a solution is the TMS320TCI6487 from TI. It combines three 1GHz C64x+ DSP cores with a full 3Mbytes of on-chip memory and high-speed interfaces.

- Arnon Friedmann
Software Manager, Communications Infrastructure Group
Texas Instruments Inc.

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