Sunday, May 20, 2012

Unsolicited Grant Service

The UGS algorithm is designed to support real-time service flows, such as Voice over Internet Protocol (VoIP), that generate fixed size data packets periodically. BS periodically assigns fixed-size grants to voice users. These grants are sufficient to send voice data packets generated by the maximum data rate of enhanced variable rate “voice” codec (EVRC). The grant period are negotiated during the initialization process of the connection. Thus, MAC overhead and UL access delay caused by bandwidth request process are minimized. The drawback of the UGS algorithm is the following. Generally, voice users do not always have voice data packets to send throughout the duration of a connection, because voice users have frequent silence periods. A typical voice codec switches intermittently between “on” and “off” states as illustrated in Figure 1. While in “on” state, popular voice codecs like the EVRC also have variable data rates. For example, the EVRC operates at 1/8 of the full data rate during the off state, while the device has three different rates during the on state (rates 1, 1/2, and 1/4). Therefore for a UGS algorithm that reserves a flat amount of resources capable of sending data at the maximum rate of EVRC periodically, a significant amount of UL resources is wasted when the codec is in silence (or off) mode as well as when the codec is on but not operating at the full rate. This is illustrated in Figure 2. A number of other algorithms have thus been designed to adaptively determine the actual UL needs of each connection during frame periods, so as to minimize these resources wastage.

Figure 1: Voice codec status.

Figure 2: UL resource allocation using UGS algorithm.
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