Understanding OFDMA, the interface for 4G wireless

Even as third generation (3G) wireless equipment continues deployment, the wireless ecosystem is identifying and designing fourth generation (4G) systems. Although there is no clear definition on 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 (HSPA), provide up to around 15-20 Mbits/s downlink and about 5-10 Mbits/s uplink. 4G systems are being designed to support 5 to 10 times those rates with greater than 100 Mbits/s or more in the downlink and over 50 Mbits/s in the uplink.

Current 3G wireless communications have successfully increased the bandwidth available for new applications through the use of code division multiple access (CDMA) for the physical-layer transmission. Unlike older schemes that multiplex data for individual channels through frequency or time divisions, CDMA spreads data throughout the telecom operator's spectrum, using the constructive interference properties of the codes associated with each channel to perform the multiplexing. CDMA has proven effective in the packet-switched voice wireless world and spread spectrum techniques have allowed a more efficient and flexible use of bandwidth than previous systems.

Regarding 4G standards, both major 3G standards bodies, the 3rd Generation Partnership Project (3GPP) and 3rd Generation Partnership Project 2 (3GPP2), have indicated that orthogonal frequency division multiple access (OFDMA) is their choice for the physical-layer transmission technology. However, the first standard to be deployed using the new multiplexing technique is IEEE 802.16e, or WiMAX (Worldwide Interoperability for Microwave Access). An earlier version of WiMAX, known as 802.16d, is already on-line in some areas for fixed-access, wide-area data networks. The main difference between the two standards is that 802.16e provides features for mobility. View a comparison table at wikipedia.org/wiki/Wimax.

What is OFDM?
OFDMA is based on orthogonal frequency division multiplexing (OFDM). This technology has been around for some time and has been used in ADSL, Wi-Fi (802.11a/g), DVB-H and other high-speed digital transmission systems. It is not surprising that the first foray of OFDM into the cellular wireless world was fixed-access WiMAX 802.16d. This wireless standard has been used to provide high-speed internet access either as a replacement for other access technologies like ADSL or cable, or to provide service in regions where the other access technologies were not deployed.

In OFDM, usable bandwidth is divided into a large number of smaller bandwidths that are mathematically orthogonal using fast Fourier transforms (FFTs). Reconstruction of the band is performed by the inverse fast Fourier transform (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 20 MHz. One beneficial feature 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. For example, a 10-MHz bandwidth allocation may be divided into 1,024 smaller bands, whereas a 5-MHz allocation would be divided into 512 smaller bands. These smaller bands are referred to as subcarriers and are typically on the order of 10 kHz.

Benefits of OFDM
One 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 microseconds, making multipath less of a problem. In order to mitigate the effect of multipath, a guardband of about 10 microseconds, called the cyclic prefix, is 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. For example, in a quaternary amplitude modulation (QAM) there are 2 bits transmitted per tone. In 16 QAM, there are 4 bits and 64 QAM yields 6 bits per subcarrier. It is expected that all 4G systems will be spectrally efficient with the use of modulation up to 64 QAM.

Another benefit of OFDM is the use of advanced multiantenna signal processing techniques. The two most common techniques are called multiple input multiple output (MIMO) processing and beamforming (commonly referred to as AAS).

In MIMO, the system exploits the fact that the received signal from one transmit antenna can be quite different than 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 uses a well known matrix inversion technique to solve. 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 signal-to-noise ratio (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. Therefore, 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 4-antenna systems. An ideal system would switch between modes depending on the characteristics of the deployment.

Page 2: So, what is OFDMA?
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