Power Control in MBMS
Power control is one of the most critical aspects in MBMS due to the fact that downlink transmission power in UMTS networks is a limited resource and must be shared efficiently among all MBMS users in a cell. Power control aims at minimizing the transmitted power, eliminating in this way the intercell interference. However, when misused, the use of power control may lead to a high level of wasted power and worse performance results.
On the Point-to-Point (PTP) downlink transmissions, fast power control is used to maintain the quality of the link and thus to provide a reliable connection for the receiver to obtain the data with acceptable error rates. Transmitting with just enough power to maintain the required quality for the link also ensures that there is minimum interference affecting the neighboring cells. However, when a user consumes a high portion of power, more than actually is required, the remaining power, allocated for the rest of the users, is dramatically decreased, thus leading to a significant capacity loss in the system.
During Point-to-Multipoint (PTM) downlink transmissions, Node B transmits at a power level that is high enough to support the connection to the receiver with the highest power requirement among all receivers in the multicast group. This would still be efficient because the receiver with the highest power requirement would still need the same amount of power in a unicast link, and by satisfying that particular receiver's requirement, the transmission power will be enough for all the other receivers in the multicast group. Consequently, the transmitted power is kept at a relatively high level most of the time, which in turn, increases the signal quality at each receiver in the multicast group. On the other hand, a significant amount of power is wasted and moreover intercell interference is increased.
Currently, LDST works on the maximization of power efficiency during MBMS multicast transmissions. Due to the fact that power is the most limited resource in UMTS networks, an important aspect in MBMS is the selection of the most efficient transport channel for the transmission of MBMS multicast content. A wrong channel selection could result to a significant decrease in the total capacity of the system. Release '99 transport channels have already been standardized for the delivery of MBMS multicast sessions. More specifically, in PTP mode multiple Dedicated Channels (DCHs) can be configured, while in PTM mode a single Forward Access Channel (FACH) is transmitted throughout a cell. Another transport channel that could be used for the delivery of multicast content, and is not currently standardized in MBMS specifications, is the High Speed-Downlink Shared Channel (HS-DSCH). HS-DSCH is the transport channel added by the HSDPA technology and comprises several performance enhancements. MBMS over HS-DSCH is an open issue, still under investigation. However, all the key features characterizing HS-DSCH constitute it an ideal candidate for the delivery of multicast data, mainly in PTP mode.
The main contribution of LDST in this research field is the development of an efficient MBMS Counting Mechanism. MBMS Counting Mechanism constitutes a mechanism that decides whether it is more economic to use PTP or PTM MBMS transmission mode. LDST investigates the power characteristics of all three types of transport channels (including HS-DSCH). An optimum switching threshold between different bearers is investigated as well as various power saving techniques that further improve MBMS performance.