Orthogonal Frequency Division Multiplexing (OFDM)

Some of the key technologies used in WiMAX systems include orthogonal frequency division multiplexing, frequency reuse, adaptive modulation, diversity transmission and adaptive antennas.

OFDM is a process of transmitting several high speed communication channels through a single communication channel using separate sub-carriers ( frequencies) for each radio channel. The use of OFDM reduces the effects of multi-path and delay spread, which is especially important for lower frequencies and near line of sight (NLOS) transmission.

Multi-path propagation is the transmission of a radio signal which travels over two or more paths from a transmitter to a receiver. Multi-path transmission can cause changes in the received signal level as delayed signals can either add or subtract from the received signal level. Multi-path is not usually a challenge on systems that use higher frequencies as these systems tend to use highly directional (high-gain) antennas for direct line of sight transmission.

Multi-path propagation is frequency dependent meaning that the multiple paths radio signals travel will vary depending on its’ frequency.

Figure 1illustrates how a transmitted signal may travel through multiple paths before reaching its destination. In this example, the same signal is reflected off an office building where it is received by the subscriber device. The reflected signal is delayed (travels a longer path) and subtracts from the direct signal resulting in a dead spot (fade) at the receiver. Furthermore, mutli-path propagation is sensitive to frequency and that distortion occurs at different points when other frequencies are used. When a different frequency is used, the reflected signal is redirected and it does not subtract from the direct signal.

Figure 1: Multi-path Propagation

For a wide radio channel that is divided into several sub-carriers, each subcarrier channel operates at a different frequency and can have different transmission characteristics than other sub-carriers. Because multiple sub-carriers are typically combined for a single subscriber, this can reduce the effects of multi-path fading.

The use of multiple sub-carriers also has the effect of reducing the symbol rate, which can reduce the effects of delay spread. Delay spread is a product of multi-path propagation where symbols become distorted and eventually overlap due to the same signal being received at a different time. It becomes a significant problem in mountainous areas where signals are reflected at great distances. Delay spread can be minimized by either using an equalizer to adjust for the multi-path distortions or to divide a communication channel into sub-carriers (e.g. OFDM) where each sub-carrier transfers data at a much slower data transmission rate thereby reducing the effects of delay spread.

Figure 2 demonstrates how OFDM divides a single radio channel into multiple coded sub-channels. A high-speed digital signal is divided into multiple lower-speed sub channels that are independently from each other and can be individually controlled. The OFDM process allows bits to be sent on multiple sub channels. The channels selected can be varied based on the quality of the sub channel. In this figure, a portion of a sub channel is lost due to a frequency fade. As a result of the OFDM encoding process, the missing bits from one channel can be transmitted on other channels.

Figure 2: Orthogonal Frequency Division Multiplexing (OFDM)

OFDM Variants 2–11 GHz

The need for NLOS operation drives the design of the 2—11 GHz PHY. Because residential applications are expected, rooftops may be too low (possibly due to obstruction by trees or other buildings) for a clear sight line to a BS antenna. Therefore, significant multipath propagation must be expected. Furthermore, outdoor-mounted antennas are expensive, due to both hardware and installation costs. The four 2—11 GHz air interface specifications are described in the following paragraphs.

WirelessMAN-OFDM This air interface uses OFDM with a 256-point transform (see OFDM description later in this chapter). Access is by TDMA. This air interface is mandatory for license-exempt bands.

The WirelessMAN-OFDM PHY is based on OFDM modulation. It is intended mainly for fixed access deployments where SSs are residential gateways deployed within homes and businesses. The OFDM PHY supports subchannelization in the UL. There are 16 subchannels in the UL. The OFDM PHY supports TDD and FDD operations, with support for both FDD and H-FDD SSs. The standard supports multiple modulation levels including Binary Phase Shift Keying (BPSK), QPSK, 16-QAM, and 64-QAM. Finally, the PHY supports (as options) transmit diversity in the DL using Space Time Coding (STC) and AAS with Spatial Division Multiple Access (SDMA).

The transmit diversity scheme uses two antennas at the BS to transmit an STC-encoded signal to provide the gains that result from second-order diversity. Each of two antennas transmits a different symbol (two different symbols) in the first symbol time. The two antennas then transmit the complex conjugate of the same two symbols in the second symbol time. The resulting data rate is the same as without transmit diversity.

Figure 1 illustrates the frame structure for a TDD system. The frame is divided into DL and UL subframes. The DL subframe is made up of a preamble, Frame Control Header (FCH), and a number of data bursts. The FCH specifies the burst profile and the length of one or more DL bursts that immediately follow the FCH. The downlink map (DL-MAP), uplink map (UL-MAP), DL Channel Descriptor (DCD), UL Channel Descriptor (UCD), and other broadcast mes-sages that describe the content of the frame are sent at the beginning of these first bursts. The remainder of the DL subframe is made up of data bursts to individual SSs.

Figure 1: Frame structure for a TDD system (Source: IEEE)

Each data burst consists of an integer number of OFDM symbols and is assigned a burst profile that specifies the code algorithm, code rate, and modulation level that are used for those data transmitted within the burst. The UL subframe contains a contention interval for initial ranging and bandwidth allocation purposes and UL PHY protocol data units (PDUs) from different SSs. The DL-MAP and UL-MAP completely describe the contents of the DL and UL subframes. They specify the SSs that are receiving and/or transmitting in each burst, the subchannels on which each SS is transmitting (in the UL), and the coding and modulation used in each burst and in each sub-channel.

If transmit diversity is used, a portion of the DL frame (called a zone) can be designated to be a transmit diversity zone. All data bursts within the transmit diversity zone are transmitted using STC coding. Finally, if AAS is used, a portion of the DL subframe can be designated as the AAS zone. Within this part of the subframe, AAS is used to communicate to AAS-capable SSs. AAS is also supported in the UL.

WirelessMAN-OFDMA This variant uses orthogonal frequency division multiple access (OFDMA) with a 2048-point transform. In this system, addressing a subset of the multiple carriers to individual receivers provides multiple access. Because of the propagation requirements, the use of AASs is supported.

The WirelessMAN-OFDMA PHY is based on OFDM modulation. It supports subchannelization in both the UL and DL. The standard supports five different subchannelization schemes. The OFDMA PHY supports both TDD and FDD operations. The same modulation levels are also supported. STC and AAS with SDMA are supported, as is multiple input, multiple output (MIMO). MIMO encompasses a number of techniques for utilizing multiple antennas at the BS and SS in order to increase the capacity and range of the channel.

The frame structure in the OFDMA PHY is similar to the structure of the OFDM PHY. The notable exceptions are that subchannelization is defined in the DL as well as in the UL, so broadcast messages are sometimes transmitted at the same time (on different subchannels) as data. Also, because a number of different subchan-nelization schemes are defined, the frame is divided into a number of zones that each use a different subchannelization scheme. The MAC layer is responsible for dividing the frame into zones and communicating this structure to the SSs in the DL-MAP and UL-MAP. As in the OFDM PHY, there are optional transmit diversity and AAS zones, as well as a MIMO zone.

Wireless High Speed Unlicensed Metro Area Network (WirelessHUMAN) WirelessHUMAN is similar to the aforementioned OFDM-based schemes and is focused on Unlicensed National Information Infrastructure (UNII) devices and other unlicensed bands.

What OFDM Means to WiMAX

To the telecommunications industry, an WiMAX OFDM-based system can squeeze a 72 Mbps uncoded data rate (~100 Mbps coded) out of 20 MHz of channel spectrum. This translates into a spectrum efficiency of 3.6 bps per Hz. If five of these 20 MHz channels are contained within the 5.725 to 5.825 GHz band, giving a total band capacity of 360 Mbps (all channels added together with 1 ×frequency reuse). With channel reuse and through sectorization, the total capacity from one BS site could potentially exceed 1 Gbps.

OFDM has manifold advantages in WiMAX, but among the more notable advantages is greater spectral efficiency. This is especially important in licensed spectrum use, where bandwidth and spectrum can be expensive. Here, OFDM delivers more data per spectrum dollar. In unlicensed spectrum applications, OFDM mitigates interference from other broadcasters due to its tighter beam width (less than 28 Mhz) and guardbands, as well as its dispersal of the data across different frequencies so that if one flow is "stepped on" by an interfering signal, the rest of the data is delivered on other frequencies.

QoS: Error Correction and Interleaving

Error correcting coding builds redundancy into the transmitted data stream. This redundancy allows bits that are in error or even missing to be corrected. The simplest example would be to simply repeat the information bits. This is known as a repetition code. Although the repetition code is simple in structure, more sophisticated forms of redundancy are typically used because they can achieve a higher level of error correction. For OFDM, error correction coding means that a portion of each information bit is carried on a number of subcarriers; thus, if any of these subcarriers has been weakened, the information bit can still arrive intact.

Interleaving is the other mechanism used in OFDM systems to combat the increased error rate on the weakened subcarriers. Interleaving is a deterministic process that changes the order of transmitted bits. For OFDM systems, this means that bits that were adjacent in time are transmitted on subcarriers that are spaced out in frequency. Thus errors generated on weakened subcarriers are spread out in time; that is, a few long bursts of errors are converted into many short bursts. Error correcting codes then correct the resulting short bursts of errors.

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