6 - Overview

Summary

Orthogonal frequency division multiplexing (OFDM) divides an entire channel into many orthogonal narrow-band subchannels (subcarriers) to deal with frequency-selective fading and to support a high data rate. Furthermore, in an OFDM-based wireless network, different subcarriers can be allocated to different users to provide a flexible multi-user access scheme [52, 122] and exploit multi-user diversity.

There is plenty of room to exploit the high degree of flexibility of radio resource management in the context of OFDM. Since channel frequency responses are different at different frequencies and for different users, data rate adaptation over each subcarrier, dynamic subcarrier assignment (DSA), and adaptive power allocation (APA) can significantly improve the performance of OFDM networks. Using data rate adaptation [144, 83], the transmitter can send higher transmission rates over the subcarriers with better conditions so as to improve throughput and simultaneously to ensure an acceptable bit error rate (BER) at each subcarrier. Despite the use of data rate adaptation, deep fading on some subcarriers still leads to low channel capacity.

Channel characteristics for different users are almost mutually independent in multiuser environments; the subcarriers experiencing deep fading for one user may not be in a deep fade for other users; therefore, each subcarrier could be in a good condition for some users in a multi-user OFDM wireless network. By dynamically assigning subcarriers, the network can benefit from multi-user diversity.

Resource allocation issues and the achievable regions for multiple access and broadcast channels have been investigated in [195] and [123], respectively, which have proved that the largest data rate region is achieved when the same frequency range is shared with overlap by multiple users in broadcast channels. However, when optimal power allocation is used, from [84], there is only a small range of frequency with overlapping power sharing. Thus, optimal power allocation with dynamic subcarrier (non-overlap) assignment can achieve a data transmission rate close to the channel capacity boundary.