The UGS-AD algorithm is designed to support real-time service flows that generate fixed-size data packets on a semi-periodic basis (e.g., VoIP using on–off voice codec). It incorporates activity detection, which makes it suitable for use with on/off voice codecs. The algorithm uses combined features of UGS and rtPS. UGS-AD has two scheduling modes: UGS and rtPS, and can switch between these modes depending on the status of the voice users (on or off). On initialization of VoIP services, this algorithm starts with the rtPS mode. While in rtPS mode, if the voice user requests bandwidth size of zero bytes, the BS maintains this (rtPS) mode. However if the user requests bandwidth size greater than zero, the BS switches its mode to UGS. While in UGS mode, if the voice user requests bandwidth size = 0, the BS switches to rtPS, and if the user requestsbandwidth greater than zero, the BS stays in UGS. By switching between rtPS and UGS modes, the UGS-AD algorithm significantly addresses the problem of UL resources wastage in the UGS algorithm, and the MAC over head and access delay in the rtPS. This is however only for the case where the voice user uses voice codecs with only two data rates (on–off). Where voice codecs with variable data rates like EVRC is used, resources wastage still occur in the UGS-AD algorithm. In this case, the wastage occurs during the on duration, when full resources is assigned eventhough the variable data rate of voice codecs means that it will not operate at full rate for all of the time the resources is allocated. The operation of the UGS-AD algorithm is illustrated in Figure 1.
