Sunday, May 6, 2012


In wired networks the channel impairments tend to be constant or at least very slowly varying. Wireless networks in contrast are well known for rapidly fluctuating channel conditions even when the transmitter and receiver are stationary. Broadly speaking, the lower the modulation and coding rate, and the higher the transmitted power, the more channel fading a system can tolerate and still maintain a link at a constant error level. It is desirable therefore to be able to dynamically change the transmitted power, modulation, and data rate to best match the channel conditions at the moment to continually support the highest capacity channel possible. WiMAX systems support adaptive modulation and coding on both the downlink (DL) and uplink (UL) and adaptive power control on the UL. Adaptive modulation allows the WiMAX MAC layer to adjust the signal modulation rate depending on the channel or radio link quality. When the channel quality is good, the MAC layer chooses the highest modulation rate, e.g., 64QAM, giving the system the highest throughput. When the channel quality degrades, the MAC layer reduces the modulation rate, e.g., 16QAM, reducing the throughput. In practice, adaptive modulation and coding rate control are used in conjunction with power control. In the PMP network deployments with multiple users in a cell serviced by a BS, when a link degradation arises for a user, the BS first increases the transmitted power of the user to provide extra link budget gain, until it reaches the maximum permitted. If the received signal quality does not improve, then the coding rate is reduced. Extra redundancy is added to provide more coding gain for better error correction performance. If the received signal quality still does not improve, then the modulation rate is reduced as a last resort (as this significantly affects the throughput than others). Similar (reverse) process is also followed when link quality appreciates. For WiMAX mesh networks using the amplify-and-forward relaying option, mesh relaying cannot exploit adaptive modulation technology because relaying nodes are not able to decode the contents of the received OFDM symbols to retrieve the modulated data and remodulate them at higher or lower rate, to increase or reduce the transmission rate (or throughput) of the mesh streams in response to link quality condition. However for mesh networks using the decode-and-forward relay option, adaptive modulation and coding rate control can benefit the mesh relaying operation as mesh nodes can decode the mesh data streams and adjust the coding and modulation rate, depending on the forwarding link quality. For example in Figure 1 a relay node decodes a data stream originally transmitted from the source node using 16QAM modulation, and remodulates the data stream using 64QAM as it has good channel quality to the destination node that can support this modulation rate. This results in fast and efficient use of the mesh links. Power control is applicable in WiMAX networks (PMP and mesh) in two ways: One, when nodes are transmitting data, they are regulated to transmit only the minimum power required to achieve successful reception at the receiver. Two, when mobile nodes do not have data (mesh relay or access service data) to transmit or receive, they go on sleep modes to save battery life.

Figure 1: Adaptive modulation at WiMAX mesh nodes.
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