Impact of Radio Resource Allocation Policies on the TD-CDMA System Performance JSAC, Vol. 19, No. 10, October 2001

Impact of Radio Resource Allocation Impact of Radio Resource Allocation Policies on the TD-CDMA System Policies on the TD-CDMA System PerformancePerformance

JSAC, Vol. 19, No. 10, October 2001

Main Research AxesMain Research Axes

Assessment of how important the impact of various parameters and policies could be on the overall TD-CDMA system performance.– User behavior: mobility

– Service constraints: source activity, delay constraint

– Radio algorithms: power control, handover, joint detection

– Network topology: inter-site distance

– Radio resource management

– Loading metric: number of codes vs. interference (per time slot)

– Neighboring list size

Simulation ScenarioSimulation Scenario


It is assumed that all the BSs are synchronized. The propagation model takes into account both

propagation along street segments and above rooftops. Hard handover is supported. Simulated services are speech service in both circuit and

packet switched mode, and 64kb/s circuit data service. Only downlink case is presented.


System performance measure– capacity (Erlangs) at a certain level of satisfied users (e.g. 98%).

– capacity at a target value for the frame error rate (FER).

Quality of Service Criteria– Blocking criterion

– Call quality criterion

– Dropping criterion

Impact of Environmental PropertiesImpact of Environmental Properties

A. Circuit vs. ON/OFF Sources


FER Behavior in light traffic load– the FER is even worse for packet switching than for circuit due

to the activity of ON/OFF sources.

FER Behavior in medium-heavy traffic load– The gap between the two operation modes increases dramatically

in favor of packet switching.


B. Impact of Manhattan Structure


The bloc size affects the capacity.– A slight improvement in terms of capacity is expected with

smaller bloc size. This is mainly due to shorter distances between BSs, which increases the number of accepted users connected to farther BSs.

– A higher FER is introduced with smaller bloc size, for users being connected to farther BSs have stronger impact on the global interference level and contribute more to the FER statistic. The impact becomes more important at heavy traffic load since users connected to farther BSs suffer from combined effect of loading (more new calls accepted) and interference.

Impact of User BehaviorImpact of User Behavior

C. Impact of Mobility

Impact of User BehaviorImpact of User Behavior

Mobility has serious impact on FER statistic and satisfied users statistic, both for circuit and ON/OFF sources.– There is less FER loss, with respect to the static scenario, in

packet switching mode than in circuit mode.

– For circuit connections there was more limitation due to satisfied user criteria than to FER, while for ON/OFF sources FER imposes a strong capacity limitation. It is why a more complex It is why a more complex satisfaction users’ criterion (considers also blocking statistic for satisfaction users’ criterion (considers also blocking statistic for new access requests) is more adapted for circuit switching while new access requests) is more adapted for circuit switching while an FER criterion is more suitable for packet switching.an FER criterion is more suitable for packet switching.

Impact of Service ConstraintsImpact of Service Constraints

D. Influence of User Dropping Threshold

Which action (dropping /keeping alive) should be taken for users falling in bad radio conditions.

Which finite value of the “dropping threshold” is desirable? Which value approaches the “no dropping” boundary?


E. Impact of Handover Margin

At a similar performance, it is better to operate at higher margin values to reduce the signaling.

Optimal handover margin has to be evaluated for each particular network and propagation models since it is highly dependent on the particular scenario.


F. Impact of Initial Power Value

Basic algorithms require a constant initial value, this they are not sensitive to the current loading and individual radio conditions.


1) FER statistic:– When starting at the highest allowed power level, it takes time

for the MS to converge.• Once the convergence is reached, other new calls are likely to be

generated, entering always at the maximum allowed power. Consequently C/I value might be no longer good, thus the power control loops will try to ask for more power, and all frames will be lost until the C/I condition is again satisfied.

– When starting at the lowest allowed power level, the applied initial power is very likely to be less than the required value for an acceptable quality.

• The power control loop will request more power to reach the satisfactory C/I value. During the convergence steps, the erroneous frames will be lost.


G. Joint Detection Imperfections

Joint detection receivers are multiuser detectors which combat both ISI and MAI by exploiting the knowledge about the spreading sequences and channel impulse responses.

Impact of RRM PoliciesImpact of RRM Policies

H. Evaluation of Some Resource Allocation Schemes



Channel allocation strategies:– Neighboring list codes loading (NLCL)

• The load is expressed as the number of codes in use per time slotthe number of codes in use per time slot.

• A call is admitted to any BSany BS in the current neighboring list, provided the C/I target is reached and there are enough resources available.

– Best available codes loading (BACL)• The load is expressed as the number of codes in use per time slotthe number of codes in use per time slot.

• A call can only be connected to the best BSbest BS among those in the list.

– Neighboring list interference loading (NLIL)• The load is expressed as the interference level measured in each the interference level measured in each

time slottime slot. Time slots are ordered from the lowest to the highest interference level.

• The TS of the lowest interference level is selected.


a) Circuit-switched mode– At light traffic load, a large neighboring list of potential servers

and a strategy based on the interference level measured by the user exhibits great advantage. This is because resources This is because resources available are better distributed among BSs.available are better distributed among BSs.

– At heavy traffic load, a strategy based on a smaller neighboring list is suitable. This is due to the fact that power budget is less This is due to the fact that power budget is less affected since connections to farther BSs are not allowed.affected since connections to farther BSs are not allowed.

– The neighboring list size can be tailored based on the desired tradeoff between carried traffic and individual radio link quality (the operators’ job)(the operators’ job).


Implementation trend: a large neighboring list at light traffic load while a small list at medium-heavy one (the switching point is operator-defined)– The access to farther BSs is excluded at medium-heavy traffic

load.– Advantages

• Better link quality.• Less measurements.• Reduce the time need to scan the neighboring list at heavy load.• Reduced signaling.• Better battery life.• Simplified power control and DCA operations.

– Main inconvenience is less carried traffic at heavy traffic loadless carried traffic at heavy traffic load.


b) ON/OFF sources– Due to the ON-OFF properties, the FER performance is worse,

and an explosion of the number of users having bad quality calls is resulted. And hence there’s no similar interest to have a reduced neighboring list at heavy traffic load (but the advantages in signaling, battery life, DCA and PC simplification hold true).

– An aggressive CAC is absolutely necessary in packet switching An aggressive CAC is absolutely necessary in packet switching mode to support a desirable FER at a reasonable traffic load and mode to support a desirable FER at a reasonable traffic load and to avoid severe call dropping.to avoid severe call dropping.

– In packet switching mode there’s no similar interest to have a strong reduction of neighboring list size at heavy traffic load: the link quality is not really much better.

Impact of Mobility on High-Rate Impact of Mobility on High-Rate SourcesSources I. 64kb/s LCD Service (circuit mode)

ConclusionConclusion

The system capacity depends highly on the selected satisfaction criteria.– More complex criteria mixing new call blocking and individual

radio link quality are more appropriate for circuit switching while a simple FER criterion is more suitable for packet switching.

Packet switching is much more sensitive to the “bad “bad quality call”quality call” criterion than to other criteria.

Packet switching generally outperforms circuit switching.– However, at light traffic load the FER is worse, for the activity of

ON-OFF sources induces more disturbance into the system. Shorter intersite distance introduces higher capacity.

– The number of available servers is increased.– The FER is worsened, however.

ConclusionConclusion

Mobility has strong impact on the system capacity. Fewer users are dropped with a higher dropping

threshold, but more calls will end up as “bad quality calls”.

An adaptive initial power control to C/I at the channel assignment phase gives the best capacity performance.

In circuit mode, there’s a considerable increase in capacity if the neighboring list size is adaptive. It is not the same case with packet mode, however.

The most exigent criterion is the FER and it is the real satisfaction indicator for packet switching operation.

Documents

Impact of Radio Resource Allocation Policies on the TD-CDMA System Performance JSAC, Vol. 19, No. 10, October 2001