Adaptive modulation and scheduling for fading channels

In future packet based wireless communication systems, transmission in the downlink will often dominate the traffic load. High bit-rate applications like WWW-browsing, file transfer, and full motion video will impose strong requirements on the system capacity. An obstacle in this context is the time-variability of the channel: for mobile users, frequently occurring fading dips will cause unnecessary and capacity degrading retransmissions. To achieve a high throughput also over fading channels, adaptive methods for adjustment of, for example, the modulation alphabet, and the coding complexity, can be used. The idea is to make efficient use of the bits: whenever the channel conditions are adequate, transmission of redundant bits should be avoided. In this paper we investigate the effect of adaptive modulation, along with time-slot scheduling, in a scenario involving several mobiles and one base station. We found that the bit error rate can be kept at an adjustable low level, well suited for forward error correction (FEC) codes. Moreover, the more mobiles that share a frequency channel, the more efficiently the scheduler uses the channel bandwidth.