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2.4 IEEE 802.16-2004
As aforementioned, the first standard of 802.16 addressed the LOS commu- nication in the 10–66 GHz band. 802.16a extended its operation to include
NLOS communication in the lower-frequency band of 2–11 GHz. Thus, IEEE 802.16-2004 (LAN/MAN committee, 2004) supports communication in the 2–66 GHz band. LOS and NLOS propagation are quite different. Thus, to design a standard that supports both bands, the physical and the medium access control (MAC) layer should support these differences. For example, signal propagation in high-band frequency is highly affected by obstacles, consequently LOS propagation is utilized, which in turn results in alleviat- ing the effect of multipath interference. Multipath results from receiving the signal at the receiver through more than one path owing to reflection and refraction of obstacles. However, operation in the lower band that includes licensed and unlicensed spectrum requires its own regulations. For example, operation in the unlicensed spectrum requires management of transmitter output power, techniques to avoid frequency interference, etc. These issues and others not only affect the physical layer design but also influenced the MAC layer. Thus, the scope of 802.16-2004 standard covers the specifications of these two lower layers in the OSI model.
Physical Layer
A 10–66 GHz frequency wave is a focused beam, which theoretically can reach multiple miles through LOS propagation. Designers deemed that single-carrier modulation was a sufficient choice and the physical layer standard version of this band is called WirelessMAN-SC (single carrier). WirelessMAN-SC can support frequency division duplex (FDD) and time division duplex (TDD) modes. However, operation in the 2–11 GHz band required changes in the physical layer specification to support NLOS propagation. Mainly, three new PHYsical layer (PHY) specifications were introduced to meet this requirement—a single-carrier PHY, a 256-point FFT OFDM PHY, and a 2048-point FFT OFDMA PHY. The single-carrier PHY, designated as WirelessMAN-SCa, is based on the WirelessMAN-SC. How- ever, there are some differences such as framing elements that enable improved equalization and channel estimation performance over NLOS prop- agation, extended delay spread channels, parameter settings, and MAC/PHY messages that facilitate optional adaptive antenna systems (AAS) imple- mentations. The second and third PHY specifications employ orthogonal frequency division multiplexing (OFDM), which is a multicarrier transmis- sion technique suitable for high-speed NLOS. OFDM uses 256 RF subcarriers to transmit different signals simultaneously. The neighboring subcarriers are allowed to overlap; however, they are orthogonal to each other to prevent inter-carrier interference (ICI). The key difference between WirelessMAN SCx and OFDM is that OFDM is more resilient to the multipath effect. OFDM has higher bandwidth efficiency since it allows neighboring subcarriers to overlap. Thus, OFDM modulates data at a rate of 72 Mbps over a channel bandwidth of 20 MHz, which provides a spectral efficiency of 3.6 bps/Hz (WiMAX Forum, 2004).
Orthogonal frequency division multiple access (OFDMA) is a 2048 sub- carrier OFDM scheme. The difference between OFDM and OFDMA is that OFDMA organizes the time (i.e., the symbols) and the frequency (i.e., sub- carriers) resources into subchannels for allocation to individual receivers, which allows for multiple access. Thus, OFDMA operates over two dimen- sions, time and frequency. There are two types of subcarrier permutations for subchannelization—diversity and contiguous (WiMAX Forum, 2006). The diversity permutation draws subcarriers pseudorandomly to form a subchannel. The contiguous permutation groups a block of contiguous sub- carriers to form a subchannel. OFDM PHY is common between 802.16 and ETSI HiperMAN because, for example, OFDM requires weaker frequency synchronization and faster Fast Fourier Transform (FFT) calculation. Conse- quently, WiMAX Forum focuses on 256-carrier OFDM PHY in all its profiles. One may ask, why not use code division multiple access (CDMA) as a sig- naling format? CDMA requires a bandwidth that is much larger than the data throughput to maintain a processing gain capable of overcoming interfer- ence. Furthermore, OFDM and OFDMA support NLOS performance making
maximum use of the available spectrum.
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