WiMAX and the Effects of Multipath

WiMAX signals suffer from the effects of multipath and exhibit broadband fading characteristics that are very apparent in the spectrum of the received WiMAX signal OFDM modulation does not eliminate fading and is not always received with a flat spectrum. Processing of the received OFDM signal can flatten the spectrum with a relatively simple channel estimator.

Multipath, delay spread inter-symbol interference and the cyclic prefix all determine the complexity of, and the performance of the algorithms needed at the receiver to flatten the spectrum. Let’s examine and differentiate these and how they apply to WiMAX.

Multipath is the propagation of an RF signal to a receiver via two or more paths. There are three distinct paths shown, and one path is delayed over 12μS relative to the first path. The multipath display was generated by correlating, over time, the preamble index code and the received signal. Multipath can result in constructive interference, destructive interference and phase shifting of the signal. The two most obvious cases are two sinusoids of the same frequency that are in phase, constructive, and two sinusoids of the same frequency that are a ½ cycle out of phase, hence destructive.

Considering the Effects of ISI
Combining any number sinusoids of the same frequency with any arbitrary phase still results in a sinusoid of the same frequency, but it may have a different amplitude and phase. These sinusoids, the RF carriers, are OFDM modulated in both amplitude and phase to carry data and this modulation requires that we also consider the effects of ISI, if present, on the received spectrum.

Delay spread, Td, intuitively, is the amount of time between the first (non-negligible) path and the last (non-negligible) path received. WiMAX adds a guard time between each OFDM symbol that is called the cyclic prefix, of length Tcp, if the Tcp <> Td and ISI is not present. If ISI exists, the receiver would need knowledge of both the multipath and the data modulated onto the RF carrier to flatten the received spectrum.

Source of Information : Wireless Design and Development May-June 2010

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