Wireless digital communications systems are poised to offer a cost-effective alternative to cable and DSL data services. For example, the so-called “WiMAX” technology, based on the IEEE 802.16e air interface standard, is a promising framework for broadband wireless applications. It has the potential to enable full internet and digital voice services for both fixed and mobile users. The physical layer architecture for IEEE 802.16e Orthogonal Frequency Division Multiple Access (OFDMA) systems is based on orthogonal frequency-division multiplexing (OFDM) modulation.
OFDM, or one of its forms, provides a promising and cost effective technology to combat multipath effects of the wireless channel, including advantages at the link and system levels. The available frequency band is divided into a group of subcarriers where each subcarrier experiences frequency flat fading. Subcarriers can be used for transmitting pilots and data symbols. Subcarriers that do not have any data/pilot transmission and are called null subcarriers.
In OFDM systems, the channel between the transmitted symbols and the received symbols must be estimated in order to decipher the transmitted data. An important characteristic of the wireless channel is its delay spread, which is the range of delays of the multipath that the signal experiences. Some channels have small delay spread, and some have large delay spread. A channel may also be relatively flat in the frequency domain or equivalently have a low delay spread in the time domain. Pilot tones embedded in the transmission and known to the receiver are used to perform estimates of the channel.
In the context of the OFDMA in WiMAX, current channel estimation techniques utilize a uniform pilot spacing of 12 sub-carriers in a data zone, referred to as the full usage of sub-channels (FUSC) zone. However, this pilot spacing limits the delay spread of the channel that can be estimated. For most channels, this provides reasonable performance in the regular mode of transmission where a subscriber station (SS) tries to decode the signal transmitted by a single base station (BS). However, in a special mode of transmission called the MBS (Multicast and Broadcast Service) mode, multiple BSs transmit the same signal.
In the MBS mode, an SS attempts to decode the sum of the signals from multiple BSs. The effective channel seen by the SS in this mode, has a large delay spread relative to transmission from a single BS (i.e. relative to the Unicast case). Even if the inherent channel between any BS and the SS is frequency flat, the composite channel consisting of the channels from each BS to the SS will no longer be frequency flat. For wideband channels (e.g. 5, 10 and 20 MHz channel bandwidths), the individual channel itself is not flat, but frequency selective. This is greatly enhanced when multiple BS transmit the same signal. The delay spread that is experienced is larger than in the Unicast scenario. Thus, it is desirable to improve channel estimation quality and hence also the link level quality in such situations.