MIMO Techniques for Mobile WiMAX Systems
Multiple input multiple output (MIMO) techniques are an essential part of of the research 802.16e – 2005 specifications, which form the basis of mobile WiMAX systems. In this chapter, we first discuss the basic tradeoffs between diversity, interference cancellation and spatial multiplexing in MIMO systems, and we compare optimum combining (OC), maximum-ratio combining (MRC) and interference cancellation for different numbers of receive antennas. Then, we focus on the two mandatory MIMO profiles in the research specifications (Alamouti’s STC and the 2×2 spatial multiplexing scheme) and compare them when the first is combined with MRC at the receiver. The simulations made using the ITU pedestrian B channel indicates that Alamouti’s STC outperforms spatial multiplexing when the two schemes are operated at the same spectral efficiency. We next give signal-to-noise ratio (SNR) thresholds for the operating regions of different modulation, coding and MIMO schemes included in WiMAX system specifications.
Mobile WiMAX systems are based on the research 802.16e-2005 specifications which define a physical (PHY) layer and a medium access control (MAC) layer for mobile and portable broadband wireless access systems operating at microwave frequencies below 6 GHz. The research 802.16e-2005 specifications actually define three different PHY layers: Single-carrier transmission, orthogonal frequency-division multiplexing (OFDM), and orthogonal frequency-division multiple access (OFDMA). The multiple access technique used in the first two of these PHY specifications is pure TDMA, but the third mode uses both the time and frequency dimensions for resource allocation. From these 3 PHY technologies, OFDMA has been selected by the WiMAX Forum as the basic technology for portable and mobile services. Compared to TDMA-based systems, it is known that OFDMA leads to a significant cell range extension on the uplink (from mobile stations to base station). This is due to the fact that the transmit power of the mobile station is concentrated in a small portion of the channel bandwidth and the signal-to-noise ratio (SNR) at the receiver input is increased. Cell range extension is also achievable on the downlink (from base station to mobile stations) by allocating more power to carrier groups assigned to distant users. Another interesting feature of OFDMA is that it eases the deployment of networks with a frequency reuse factor of 1, thus eliminating the need for frequency planning.