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INHALTE

CARMA - Comparative Analysis of Radio resource Management strAtegies

Summary

This project investigates the UMTS system capacity based on the load of the wireless access network in association with the algorithms used for radio resource control (RRC). The UMTS simulator CARMA was developed by the our department with the support of Lucent Technologies. It allows the simulation of a wide variety of traffic scenarios, cost structures and RRC algorithms. The results leads to a new understanding of the importance of these algorithms and their relationship to the capacity of UMTS systems. By using advanced algorithms it is possible to enhance the capacity by 20% and reduce number of blocked and dropped telephone and data calls by 30% as shown in various publications.

Description

Three kinds of resources are crucial at the radio interface of CDMA systems. These are:

  • the radio resources, i.e. the maximum allowed transmit power of the cell,
  • the code resources, and
  • the processing resources to handle signalling.

    UMTS systems are limited by each of these resources. For packet-switched (PS) calls, the time during which the mobile is inactive is crucial. Different from circuit-switched (CS) calls, there is no explicit call release for many PS services. Thus, the issue of when to release resources for inactive mobiles arises.

    In UMTS, the resource usage for packet data depends on the state of the Radio Resource Control (RRC) protocol. Depending on the state, different amounts of cost for the different resources are consumed. However, also state changes cause cost, especially for processing resources. Thus, there is a trade-off between (1) remaining in the state or (2) changing to a cheaper state. Staying in a state typically pays off if the inactivity is of short duration only. Otherwise changing to a cheaper state may be recommended. An additional drawback of the transition to a cheaper RRC state is the larger delay (call setup time) encountered when new data arrive.
    In order to reduce the resource consumption when mobiles are inactive, timeouts are typically used to move inactive PS sessions to cheaper RRC states. The timeout values are up to the operator. Unfortunately, the optimal timeout values depend on many parameters, which are not under full control of the operator (like the traffic model for PS traffic or the mix of PS and CS traffic).
    In summary, it can be stated that the static selection of the timeout values is highly error prone and the optimal values may change during the day, e.g. due to changes of the traffic characteristics of the users. In addition, timeout values are typically dimensioned to ensure proper operation at a load close to the limit of the cell. Different timeout values may be optimal for smaller cell loads, e.g. to minimize the call setup delay.

    Based on our analytical investigations and simulations with CARMA we are able to design and verify new and innovative algorithms aside from the standard timeout algorithm. As mentioned in the summary it is possible to enhance the capacity of an UMTS system by 20%. With constant capacity it is possible to reduce the number of blocked and dropped calls by more than 30% without negative impacted on other parameters.

    Screenshot
    CARMA screenshot