Saturday, September 1, 2007

[WiMAX] Fundamentals of WiMAX - (6)

We have introduced the basic idea of OFDM before. In this post, we will discuss OFDMA (Orthogonal Frequency Division Multiple Access) in Q&A style.

Q: What's the relationship between OFDM and OFDMA?
A: OFDMA can be considered as a version of OFDM optimized for multiple users. The basic idea behind OFDMA is Sub-Channelization, i.e. assigning subsets of subcarriers to different users.

Q: What's the advantage of OFDMA over OFDM?
A: We know OFDM has immuity to frequency-selective fading channel because it breaks a single high data rate stream into multiple low data rate streams, thereby the subcarrier's bandwidth is smaller than the coherence bandwidth of the channel. For a wireless system with multi users, the total system resources are shared by all users. However, in one time slot, OFDM only assigns the whole resource to only one user, i.e. one user occupies all subcarriers in one time. This makes OFDM not efficient in resource allocation. A new multiple-access technique becomes necessary. This is the purpose of OFDMA. By allowing users share subcarriers and time slots, additional flexibility can be provided by OFDMA: multi-user diversity. In addition, adpative modulation can be used in OFDMA for different users according to channel condition.

Q: How is the 'Sub-Channelization' realized?
A: In OFDMA, the sub-carriers are divided into groups of sub-carriers. Each group is called a sub-channel. The sub-carriers that form a sub-channel need not be continuous. Based on their channel conditions and QoS requirements, different sub-channels can be assigned (scheduled) to different users. This multiple access is performed before the IFFT operation, thereby the scheduling algorithm used for OFDMA is realized in both frequency and time domain.

(to be continued)

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