VoIP-Specific Control Channel Resource Allocation for LTE Downlink

Abstract

Long Term Evolution (LTE) is a novel digital mobile radio technology developed by 3rd Generation Partnership Project (3GPP). The most important improvements in comparison to Wideband Code Division Multiple Access (WCDMA) technology are entirely packet switched architecture, new multiple access scheme and higher datarates. Target peak datarates of LTE are 100 Mbit/s in downlink and 50 Mbit/s in uplink. The improved performance is stemming in many respects from dynamic radio resource management which enables frequency reuse factor one. Furthermore, LTE is the first digital mobile radio technology which uses packet based architecture for the transmission of speech traffic, that is Voice-over-IP (VoIP), also in radio interface. Packet based voice transmission sets certain demands on the signaling of the radio system. The dynamic multi-user channel sharing yields substantial amount of signaling traffic, for the VoIP-packets are small and they are being transmitted relatively often in order to meet the strict delay requirements. If dynamic packet scheduling is used, the VoIP capacity of LTE, that is the maximum number of simultaneous calls, is limited by the control signaling resources. The capacity is bounded to such a degree that the data channel utilization rate in downlink is only 60%, as users are being blocked due to the insufficient control channel capacity. This thesis presents two approches to rationalize the control channel resource usage with a view to improving the maximum VoIP capacity of the system. First of all, the problem of control signaling resource allocation seems to be that the control channel bandwidth is running out while there is still plenty of transmission power left. The performance of the two different approaches is compared using a cellular network radio interface system simulatior. Furhermore, the modeling of control channel power allocation is presented, which is the basis of the performance simulations. First of the two control channel resource allocation methods is so-called initial offset algorithm. It balances the control signaling resources using a power offset value that is applied to every control channel in order to reduce the bandwidth usage by allocating a higher transmission power. The second approach iteratively boosts the control channel power of the most favorable users thereby making room for new schedulable users. The voice capacity of initial offset algortihm is 358 users/cell on 5 MHz system bandwidth, whereas power boosting algorithm provides a slightly inferior capacity, 335 users/cell. The capacity without any control channel resource balancing is 295 users/cell, thus the capacity gain is 14% - 21% depending on the algorithm. This can be considered as a significant improvement from the point of view of the future LTE implementations. /Kir10