**2.1. Statistical channel models**

in hostile wireless channel. The OFDM based physical layer of the IEEE 802.16 standard has been standardized in close cooperation with the European Telecommunications Standards Institute (ETSI) High PERformance Metropolitan Area Network (HiperMAN) [4]. Thus, the HiperMAN standard and the OFDM based physical layer of IEEE 802.16 are nearly identical. Both OFDM based physical layers shall comply with each other and a global OFDM system should emerge [5]. The WiMAX forum certified products for BWA comply with the both

Some researchers investigate the effect of nonlinear amplifier in WiMAX [6] and apply Clipping as a simple method [7], CORDIC Algorithm [8], and Tone Reservation [9] to reduce

In this chapter, proposed WiMAX system will be studied. This system will be compared with the conventional system. Where the companding technique is used to reduce PAPR based on the properties of the µ-law that uses for decreasing dynamics range of the signal. Moreover, the performance of the proposed system will be compared with the system that uses clipping as a reduction of PAPR. These systems will be investigated under SUI channels and AWGN. In Section 1.2, the broadband channel will be explained. In section 1.3 SUI multipath Channel Models will be investigated. The performance of MMSE equalizer over SUI model will be study in section 1.4. In section 1.5, a system model of WiMAX for PAPR will be explained. PAPR Reduction Technique Using µ-Law Compander is studied in Section 1.6. Finally, summary

One of the more intriguing aspects of wireless channels is fading. Unlike path loss or shad‐ owing, which are large-scale attenuation effects owing to distance or obstacles, fading is caused by the reception of multiple versions of the same signal. The multiple received versions are caused by reflections that are referred to as *multipath*. The reflections may arrive nearly simultaneously— for example, if there is local scattering around the receiver—or at relatively longer intervals— for example, owing to multiple paths between the transmitter and the

When some of the reflections arrive at nearly the same time, their combined effect is as in Figure 2. Depending on the phase difference between the arriving signals, the interference can be either constructive or destructive, which causes a very large observed difference in the amplitude of the received signal even over very short distances. In other words, moving the transmitter or the receiver even a very short distance can have a dramatic effect on the received amplitude, even though the path loss and shadowing effects may not have changed at all. One of the key parameters in the design of a transmission system is the maximum delay spread

In order to design and benchmark wireless communication systems, it is important to develop channel models that incorporate their variations in time, frequency, and space. Models are

standards.

36 Selected Topics in WiMAX

will be made.

receiver (Figure 1).

value that it has to tolerate.

**2. Broadband wireless channel models**

PAPR in WiMAX system.

As we have noted, the received signal in a wireless system is the superposition of numerous reflections, or multipath components. The reflections may arrive very closely spaced in time — for example, if there is local scattering around the receiver—or at relatively longer intervals.

Figure 2 shows that when the reflections arrive at nearly the same time, constructive and destructive interference between the reflections causes the envelope of the aggregate received signal *r*(t) to vary substantially.

In this section, we summarize statistical methods for characterizing the amplitude and power of *r*(t) when all the reflections arrive approximately at the same time.
