ile WiMAX system profile [15] and system evaluation methodology [16] to study VoIP. The VoIP model included 1.2 kbps AMR (Adaptive Multi-rate audio codec) with 50% voice activity factor and with two receive diversity antennas for each terminal. A VoIP call was considered failed if more than 2% of its packets were not delivered within a given delay bound to the receiving party. It was shown that a sector can support 105 active VoIP users with an over-the-air delay of 50 ms and system outage of less than 5%. 4.5.2.3 Performance with Mobile Receive Diversity Using the same methodology and simulator stated in the case of VoIP performance authors also look into the benefits that can be achieved by having more than one receiving antenna on subscriber stations. It was shown that uplink system performance was marginally improved with subscriber stations that have only two receive antennas and using SIMO However, with SDMA, MMSE and having eight antennas per subscriber unit the system performance is improved by more than 80% [14]. Hence, SDMA along with eight receive antennas and MMSE provides higher spectral efficiency (approximately 0.88 with SDMA VS. 0.72 with SIMO) than SIMO with eight receive antennas and MRC. It is also noticeable all these MIMO techniques provide higher spectral efficiency with 1x3x1 than with 1x3x3 frequency reuse pattern. However, the practicality of having more than two antennas per subscriber unit is highly unlikely. 4.6 WiMAX Certification The WiMAX Forum has developed a certification program that tests subscriber devices and base stations to ensure that these products conform to the standards, perform as expected, and interoperate with equipment from other vendors. This was made possible through extensive collaboration with IEEE, ETSI, testing labs and WiMAX Forum members. Next Generation Mobile Communications Ecosystem WiMAX Forum Service Provider Technical Working Network Working Group (TWG) Working Group Group (NWO) (SPWG) Certification Working Group (CWG) Certification labs Figure 4.19 Wi