On the Capacity of Different Channel Selection Strategies for Multicast Transmissions in WCDMA

In wide code-division multiple access (WCDMA), data transfer to a group may either take place on multiple point-to-point (PTP) channels transmitted to individual group members separately or on a single point-to-multipoint (PTM) channel that is broadcast over the entire cell. We evaluate the capacity of three channel selection strategies for one-to-many multicast over the WCDMA air interface in terms of outage probability. With the first scheme, dedicated or PTP traffic channels carry multicast data to individual users, whereas the second scheme relies on a single common or PTM channel for broadcasting multicast data within the entire cell. The third scheme finally combines the PTP and PTM approaches by transmitting a single PTM channel over the cell center while transmitting PTP channels to multicast users close to the cell boundary, allowing these users to benefit from macrodiversity gain and, in doing so, improving handover performance. We study the capacity of each of the three schemes by first deriving suitable outage probability expressions and then calculating the overall capacity, i.e., the number of multicast users that can be supported by each of the three schemes. Our comparative study gives insight into the tradeoffs between the three schemes under varying system parameters.