Channel Assignment

Channel Assignment Channel assignment schemes attempt to achieve a high degree of spectrum utilization for a given grade of service with the lease number of database lookups and the simplest algorithm employed in both the MS and the network. It includes     Service quality Implementation complexity of the channel assignment algorithm Number of database lookups Spectrum utilization Handoff requests and initial access requests compete for radio resources. At a busy BS, call attempts that fail because there are no available channels are called blocked calls. Handoff requests for existing calls that must be turned down because there are no available channels are called forced terminations. Note that the successful handoff access is intimately tied to the radio technology of the channel assignment process, which may be dynamic channel assignment (DCA), fixed channel assignment (FCA), quassi-static autonomous frequency assignment (QSAFA), or some other fixed, flexible or dynamic process. Several channel assignment strategies have been developed to reduce forced termination at the cost of increasing the number of lost or blocked calls. Several handoff initial access channel assignment schemes such as the non-prioritized scheme, the reserved channel scheme, the queuing priority scheme and the subtracting scheme have been proposed. Non-prioritized Scheme and the Reserved Channel Scheme(NPS), In the non-prioritized scheme (NPS), the BS handles a handoff call in exactly the same manner as a new call that is the handoff call is blocked immediately if no channel is available This scheme is employed by most PCS radio technologies. Fig: Flowchart for non-prioritized scheme. The reserved channel scheme (RCS) is similar to NPS except that a number of channels or transceivers in each BS are reserved for handoffs. In words the channels are divided into two groups – the which serve both new calls and handoff calls and the reserved channels, which for RCS is show only serve handoff calls. Fig: Flowchart for reserved channel scheme Queuing Priority Scheme(QPS) The queuing priority scheme (QPS) is based on the fact that adjacent cells in a PCS network overlap. Thus, there is a considerable area where a call can be handled by either BS of the adjacent cells, called the handoff area. The time that an MS spends in the overlapped area is referred to as the degradation interval. The channel assignment for a QPS new call is the same as that for NPS. If a channel is available after the MS moves out of the handoff area –the degradation interval expires the call is forced to terminate. In this scheme, when a channel is released, the BS first checks if the waiting queue is empty. If not, the released channel is assigned to a handoff call in the queue. The next handoff to be served is selected based on the queuing policy. Two scheduling policies for the QPS waiting queue have been considered. In the FIFO scheme the next handoff call is selected on a first- infirst-out basis. The measured-based priority scheme (MBPS) uses a non-preemptive dynamic priority policy. The priorities are defined by the power level that the MS receives from the BS of the new cell. The network dynamically monitors the power levels of the handoff calls in the waiting queue. We may view a handoff call as having a higher priority if its degradation interval is closer to expiration. Subrating Scheme The subrating scheme (SRS) creates a new channel on a blocked on blocked BS for a handoff access attempt by subrating an existing call. Subrating is the process of temporarily dividing an occupied full rate channel into two channels at half the original rate, one to serve the existing call and the other to serve the handoff request. For a PCS radio system to take advantage of a priority based scheme for handoff and initial-access channel assignment, there must be a way for the MS to signal to the blocked BS of the access request for the handoff. In general, when a subscriber makes or receives a call, the MS has to acquire an available traffic channel for the connection. For some PCS radio systems using dynamic channel assignment, the MS launches an access request on a common signaling channel (CSC), and is then directed to a traffic channel. In these cases, there are limited number of serves or transceivers in a BS. When a BS is blocked, there is often no transceiver for the CSC since they have all been used for existing calls. In other PCS radio systems, the access attempt can be made directly on an available control channel. The PACS protocol provides for a dedicated time slot at each BS for a system broadcast channel (SBC), which is available when all the traffic channels at a BS are in use. The PACS protocol also allows for MSs to indicate to the network the need to make a priority call. With SRS the cost for temporarily switching the call to a lower bit rate can be absorbed in two ways. The simplest way is to use one adaptive differential pulse code modulation (ADPCM) codec, which can operate in both modes (i.e., 16 kbps and 32 kbps) and can be switched between the two. In this case there is no impact on the channel delay, the current drain, or the cost of the MS. Alternatively a high-quality 16Kbps codec could be implemented along with the 32 Kbps ADPCM codec and the conversation can be switched between two.