Praise for Fundamentals of WiMAX

410 Chapter 12 • System-Level Performance of WiMAX11 OFDM symbols are used for the various control messages, such as DL frame preamble,FCH, DL-MAP, UL-MAP, DCD, and UCD, and the MSs are not scheduled over the first 11DLs. Thus, the calculated average throughput and data rate do not include any MAP overheadsand represent the effective layer 1 capacity available for data plane traffic. However,overheads due to the MAC header, convergence sublayer, mobility management, QoS management,and so on, are not explicitly modeled. As a result, the net layer 3 capacity availablefor IP traffic is expected to be less that what is indicated here.12.2.2 System ConfigurationsThe IEEE 802.16e-2005 standard offers a wide choice of optional PHY and MAC features, andit is not within the scope of this book to provide system-level performance results for all the possiblecombinations of such features. As shown in Table 12.3, this chapter considers only fourconfigurations, based on the number of transmit and receive antennas. A SISO antenna configurationhas not been considered in this chapter, since all WiMAX BSs and MSs are expected tohave at least two antennas as per the WiMAX Forum’s Mobile WiMAX System Profile. In orderto preserve a competitive edge of 3G cellular networks, such as HSDPA and 1xEv-DO, theWiMAX Forum has decided to make the support for MIMO mandatory in all devices.The basic configuration consists of a 2 × 2 open-loop MIMO in the DL and a 1 × 2 open-loopMIMO in the UL. In this basic configuration, collaborative MIMO between two MSs is alsoallowed in the UL, where two MS are allocated the same slots for their UL transmission, and theBS is able to separate the two MSs using the two receive antennas. The three enhanced configurationsadd various flavors of higher-order open-loop and closed-loop MIMO on top of the basicconfiguration. Enhanced configuration 1 increases the number of receive antennas in the DLfrom two to four thus providing higher order receive diversity in the DL. The UL in enhancedconfiguration 1 is essentially the same as that of the basic configuration. Enhanced configuration2 increases the number of transmit antennas in DL from two to four over the basic configuration,thus providing higher-order transmit diversity. The UL in enhanced profile 2 also increases thenumber of receive antennas from two to four. Finally, enhanced configuration 3 uses 4 × 2closed-loop MIMO in the DL, increases the number of transmit antennas in the UL from one totwo, and increases the number of receive antennas in the UL from two to four.Enhanced configuration 3 uses the antenna selection and the quantized channel-feedbackbasedclosed loop MIMO in the DL, (see Section 8.9). Channel state information (CSI) is providedby the MS, using the fast-feedback channel. The MS provides a single feedback for theCSI over two bands (18 subcarriers) once every 10 msec. The BS uses the quantized feedback tocalculate an optimum precoding or beamforming matrix that minimizes the mean square error(MSE) at the output of the receiver. (See Chapter 5 for the MMSE calculation of the precodingmatrix.) In the case of the basic, enhanced 1, and enhanced 2 configurations, two MSs are simultaneouslyscheduled over the same subchannels. In this 2 × N r collaborative MIMO, N r is thenumber of receive antennas at the BS. However, in the case of the enhanced 3 configuration,