Tuesday, May 8, 2007

[WiMAX] Fundamentals of WiMAX - (2)

In this part, we are going to give an overview of the wireless channel that may be experienced by WiMAX.

Generally speaking, the basic and important issues of wireless channel contain three aspects: pathloss, large-scale Shadowing and small-scale fading. You can find detailed descriptions of them in most of the books regarding wireless communications (chapter 4 and 5 of [1]). In summary, pathloss is inversely propotional to the distance between the transmitter and the receiver, Shadowing is mainly caused by obstructions within the propagation environment and is often log-normal distributed, small-scale fading is caused by multipath reflections (modeled by delay spread or coherence bandwidth) or motion between the transmitter and the receiver (modeled by Doppler frequency shift or coherence time).

So we will skip those fundamentals issues and assume that you have already known them very well. However, channel also varies over space if multiple antennae are used. Multiple antenna techniques emerged as a breakthrough in the last several years and some traditional books may lack of description on this part, thereby next we will give an outline of the spatial aspect of wireless channels.

Angular Spread

Like delay spread or Doppler spread, a parameter used to model the channel in space is the angular spread. It refers to the statistical distribution of the arriving signal's angle. A small angular spread means the received signal's energy is more focused, which implies less statistical diversity; contrarily, large angular spread means more statistical diversity available.

Coherence Distance

The dual of angular spread is coherence distance. A coherence distance Dc means that any physical positions separated by Dc have an uncorrelated received signal amplitude and phase. Coherence distance is often related to the wave length. Thus, higher frequency system has shorter coherence distance.

Coherence distance is often used to specify how far apart antennae should be separated to make received signals statistically independent. Small coherence distance means antenna array can be easily applied to provide rich diversity. This is particularly useful for mobile terminals which has the size constraint. Large coherence distance implies that large separation between two antennas is needed. In this case, it would be preferable to use beamforming with antenna array.

Channel model for OFDM

Since WiMAX is using OFDM, the channel model with multidimension correlation should be considered. When channel is highly frequency selective, it requires a large number of closely spaced subcarriers to combat the ISI (Inter Symbol Interference). On the other hand, when the channel is with large Doppler frequency shift (high mobility), it will degrade the subcarrier orthogonality. In the frequency domain, it will cause significant ICI (Inter Carrier Interference) as subcarriers become more closely spaced.

Emperical model

An emperical channel model for MIMO was proposed and developed by 3GPP (Third Generation Partnership Project). It is described in the technical report TR 25.996 [2]. This model can be used for systems such as WiMAX, 802.11n and 802.20 which use MIMO techniques.

[1] Theodore S. Rappaport, "Wireless Communications: Principles and Practice (2nd Edition)," Prentice Hall PTR, December, 2001
[2] Spatial channel model for Multiple Input Multiple Output (MIMO) simulations http://www.3gpp.org/ftp/Specs/archive/25_series/25.996/

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