Tài liệu Điện thoại di động băng thông rộng không dây P10 pptx

5.6.1 Transmission Power Control for Connection-less ServicesTransmission power control TPC is able to increase the maximum value of throughput in the uplink of CDMA cellular packet comm

The basic principle of DCC is shown in Figure 5.35(b) When BS2reduces its pilot-signal transmission power (PP2), the cell boundary moves toward BS2 The area of BS2decreases, and some MSs that were in the BS2cell are reallocated to the BS1cell At the same time, MSs reconnected to BS1increase their transmission power (TM) to meet the target power level of BS1 (SL1) This action, however, increases the interference with BS2 BS2thus raises its target power level (SL2) to improve the local anti-interference characteristics, and BS1decreases its target power level (SL1) to balance the volume of interference between cells These power adjustments take place independently at each BS so that the communication-quality level equals the target quality level

Figure 5.36 shows an example DCC structure at a BS Only three functions need to be added to a conventional BS: calculating the difference from the SIR target, calculating updated power level values, and determining pilot signal transmission powers and target powers The DCC control unit is located a layer above the TPC ones The DCC controls the power levels so the measured SIR level, SIR(t), meets to the SIR target level, SIRO The difference between the measured SIR and the target SIR is sent to a signal converter and used

as the update value for the pilot-signal transmission power The target power level, SL(t), is updated in inverse relation to the update of pilot-signal power, assuming that the propagation loss is the same in both the uplink and the downlink A TPC controller sends the power-up/ down information to each MS, therefore the uplink signal from each one meets SL(t) The interval of DCC control can be determined according to the variation in teletraffic and interference

5.5.6 Evaluation of DCC

Figure 5.37 shows the effect of DCC on traffic bias In this simulation, the traffic load at the centre cell was increased to five times the average load while the overall total load stayed unchanged The abscissa indicates the traffic bias (traffic load at centre cell/average traffic load) We evaluated the SIR of three schemes at the centre and at its adjacent cells:

† in fixed-level power control (FPC), the BS target power level was fixed;

† in adaptive-level power control (APC), only the target power level was controlled accord-ing to the SIR (this method is equivalent to SIR-based TPC);

† in DCC, the pilot-signal power level and the target power level were adjusted

Figure 5.36 An example of DCC structure

The SIR in the centre cell fell greatly with FPC as the traffic load of the centre cell increased The SIR in the adjacent cells decreased slightly because of increased interference from the centre cell With APC, the SIR in the centre cell remained above 215 dB up to a traffic bias level of 3.0 For levels above 3.0, the SIR in the centre cell decreased significantly because APC only controls the transmission power The SIR degradation in the adjacent cells was the largest among the three schemes With DCC, the centre cell SIR stayed close to the target of 214.5 dB at all traffic-bias levels The SIR in the adjacent cells gradually decreased because some traffic in the centre cell was reallocated to these adjacent cells When the traffic load increases (or the transmission rate increases) at certain cells, degradation in quality can

be prevented by configuring cell areas adaptively and setting power levels appropriately to improve the efficiency of the system

5.5.7 Characteristics in Up and Downlinks

The characteristics in the uplink and downlink of cellular radio systems are not coincident because these system structures are different When traffic is uneven, it influences on commu-nication quality differently for the two types of links [44,75,134] In multimedia communica-tions, the volume of teletraffic is greatly different in the up and downlink Therefore, controlling a cell area as DCC affects the links differently We calculated the optimum cell boundary for each uplink and downlink under uneven teletraffic conditions The traffic load in the centre cell was larger than that in the other cells The transmission power of the pilot signal was changed to control the cell areas The power levels in the uplink and downlinks – the target power level in the uplinks and the total transmission power in the downlinks – can

be controlled The evaluation results for the uplink are shown in Figures 5.38 and 5.39 The traffic load in the centre cell was 25, 35, or 50 erl, while that in the other cells was constant at 10 erl The target power level at each BS was adjusted to eliminate any SIR differences between cells for each pilot-signal power level at the centre cell, which is shown

on the abscissa The target-power level in the centre cell is plotted, as well as the SIR obtained

Figure 5.37 Effect of DCC on traffic bias

when the SIRs in the centre and adjacent cells were equal When the traffic load was 20 erl, the highest SIR was obtained at a pilot power level of 26 dB This means the cell boundary moved towards the centre cell BS, and the effective area of the centre cell was reduced to balance the traffic load: the cell radius was reduced by 80% and its area by 65% When the traffic load was 50 erl, the highest SIR was obtained at a pilot signal power level of 213 dB The SIR level improved by about 2.7 dB from when the pilot-signal power level was 0 dB,

Figure 5.38 Effect of area control in uplink: SIR

Figure 5.39 Effect of area control in uplink: target-power increase

where the cell boundary was positioned halfway between the BSs The target power level at the centre BS increased by 11.1 dB for a traffic load of 50 erl The centre BS required a higher target power level than the adjacent ones to resist the interference from adjacent cells that had enlarged their cell areas

Figures 5.40 and 5.41 show the downlink performance obtained when the total transmis-sion power was adjusted to equalise the SIRs of the centre and adjacent cells The traffic load in the centre cell was 50 erl The orthogonality factor, FO, in the downlink was assumed to be 1.0, 0.8, 0.5, or 0.2 When FO is 1.0, the signals are fully orthogonal, and when FO is 0.2, 80% of the transmission power from the connected BS becomes interfer-ence When FOis 1.0, the power level of the pilot signal had to be 7 dB to obtain the highest SIR The total transmission power at the centre cell increased by about 8.1 dB The effective area of the centre cell widened in spite of the large traffic load when perfect orthogonality was assumed The highest SIRs were obtained at pilot-signal power levels of 2 dB when FO

¼ 0.8, and at 213 dB when FO¼ 0.2 When FOwas 0.2, the centre cell’s total transmission power increased by 28.7 dB

The appropriate location of a cell boundary differed according to the expected downlink orthogonality If the area control is used in both the up and downlinks, the cell boundaries are not always coincident, and different BSs might be selected for the two links to manage uneven traffic [127]

5.5.8 Future Works

This section has described the issues of teletraffic and its preventive measures based on a dynamic cell configuration in CDMA systems Potential access methods are being considered

in 4G systems System configuration depending on the access method might be developed The method introduced in this section concentrates on CDMA systems, and it will need to be

Figure 5.40 Effect of area control in downlink: SIR

adapted for use in other systems In multimedia and personal communications, however, cell areas will be reduced further, and traffic variation and unevenness will expand Adapting cell configurations and power settings to teletraffic conditions are crucial to enhancing the system efficiency for all types of cellular systems

5.6 CDMA Cellular Packet Communications

Packet communication services are already being provided in current (2nd-generation) mobile communication systems These services, however, are achieved by utilising some channel resources of circuit-switching systems that mainly provide voice communications This system configuration has few capabilities of providing highly efficient packet commu-nication services Moreover, 3rd-generation systems provide packet commucommu-nication services through a similar system configuration Current trends of mobile communications suggest that data traffic will occupy a major portion of system traffic in the future mobile nication systems, and 4th-generation systems have to be especially suitable for data commu-nications From this point of view, the 4th-generation systems are expected to adopt a system configuration in which all information is transmitted on the basis of packet communications Research of radio packet communications for mobile communication systems has been quite active in recent years However, most of these researches focus on transmission schemes and medium access control (MAC) protocols [14,24,41,64,82,92,117,146] In this regard, research of system configuration technologies in a cellular environment is indis-pensable for constructing a mobile communication system in which all information is to be transmitted by packets In this section, we focus on CDMA packet communications and introduce some examples of research activities, which aim to achieve efficient CDMA packet communications in a cellular environment

Figure 5.41 Effect of area control in downlink: total-power increase

5.6.1 Transmission Power Control for Connection-less Services

Transmission power control (TPC) is able to increase the maximum value of throughput in the uplink of CDMA cellular packet communication systems [64] Under heavy channel load conditions, however, TPC also degrade throughput performance dramatically and throughput decreases below a level corresponding to no use of TPC [117] The cause of this intrinsic problem can be explained as follows In the situation that many packets arrive simultaneously

at a base station, the quality drops for all arriving packets due to multiple-access interference, and in the worst case, no packets at all can be received at a base station The degradation of throughput performance under heavy channel load has not been a serious problem in the conventional circuit-switching systems because traffic control such as call admission control and similar techniques is easy to perform in these systems

Fourth-generation mobile communication systems are assumed to provide new types of communication services such as connection-less services in which no call admission control

is performed In connection-less services, however, it is not easy to achieve traffic control through a simple method and traffic generated at each mobile station tends to be burst-like in nature Accordingly, traffic fluctuation is large compared to conventional circuit-switching systems If such a system employs conventional open-loop TPC in which received power at base stations is made constant for all packets, transmission efficiency would drop rapidly and system quality would be degraded when the system should temporarily fall into a heavy load condition This is a problem affecting system stability, and improvement of throughput performance is therefore desirable to maintain system stability under heavy channel load

In this regard, the capture effect caused by differences in received power is considered to be effective for improvement of system stability [42] In the following, we describe a transmis-sion power control method that adapts to channel load by making use of this capture effect in heavy-channel-load conditions [86]

5.6.1.1 Load-based transmission power control

In load-based transmission power control, the target received power at a receiving base station, that is, the target value of transmission power control at a mobile station, varies according to the location of the mobile station (Figure 5.42) Target received power at each location, moreover, is adaptively controlled according to channel load

In this method, the mobile station first estimates its distance from the connecting base station (the base station targeted for transmission power control) by measuring received power of a pilot signal Then, it determines its target received power Ptgtcorresponding to the estimated distance by referencing a target received power function which is controlled at the base station and broadcast to the mobile station After that, the mobile station computes transmission power based on target received power Ptgtand finally performs packet transmis-sion

The base station, on the other hand, adaptively controls the target received power function according to channel load This function relates target received power Ptgtto distance d from the connecting base station The example of this function is a linear function as illustrated in Figure 5.43 Using this function, the base station controls DPtgt, the difference in target received power according to distance DPtgt is set to a large value when channel load is large and a small value when channel load is small

Although it is difficult to correctly estimate channel load in the uplink at a base station in the case of connection-less services, it is possible, for example, to use Nrx, the number of correctly received packets in a certain period, to estimate channel load In DPtgt can be controlled by observing the number of packets Nrxreceived in a certain period at the base station and then comparing the Nrxobtained in this period with that of the previous observa-tion period DPtgt can be updated in the same direction as the previous control if Nrxhas increased, and updated in the opposite direction if Nrxhas decreased, with the result that DPtgt

is always updated in the direction of increasing Nrx

As described above, the load-based transmission power control method adjusts a target received power function so that the difference in the target received power of packets trans-mitted from different distances from the base station is made large in the case of heavy channel loads This makes it possible to obtain a capture effect due to differences in received power and to suppress degradation in transmission performance Furthermore, by setting target received power near the cell boundary smaller than that near the base station, the transmission power of packets transmitted near the cell boundary can be kept lower and the amount of interference on adjacent cells minimised

Figure 5.42 Transmission power control based on mobile-station position

Figure 5.43 Setting function for target received power

5.6.1.2 Transmission characteristics

Figure 5.44 shows the throughput performance on the uplink of CDMA cellular packet communication systems with load-based transmission power control Here, the cellular system is configured as a 5 £ 5 arrangement of regular hexagonal cells and the radio channel suffers from propagation loss and shadowing fluctuation Transmission power control, however, can compensate for propagation loss and shadowing fluctuation, and it is assumed

to be perfect Spread factor is 16 and the access protocol is slotted-ALOHA

In this figure, throughput performances are shown for the cases of no transmission power control (Non-TPC), conventional transmission power control in which target received power

is constant for all packets (Conventional TPC), and the proposed transmission power control (Proposed TPC) Conventional TPC achieves greatly improved maximum throughput compared to Non-TPC and this result is the same as described in Ref [3] After reaching maximum throughput, however, throughput performance of Conventional TPC drops off rapidly as channel load increases and eventually become worse than the performance obtained by Non-TPC On the other hand, performance degradation in the same region is extremely small in the case of Proposed TPC and maximum throughput is nearly maintained

In addition, Proposed TPC obtain throughput equivalent to those of Conventional TPC in the light-load region Therefore Proposed TPC achieves good performance over all load regions From these results, it can be seen that use of the proposed method can expand the practical channel load region

Figure 5.45 shows slot error rate as a function of channel load Here, slot error rate is defined as the possibility that a base station fails to receive all packets in a slot where mobile stations transmit one or more packets As shown in Figure 5.45, Proposed TPC exhibits greatly improved slot error rate under heavy channel load indicating that the capture effect

is well working in this region This improvement in slot error rate through the capture effect contributes to improved throughput performance in the proposed method

5.6.2 Service Fairness in a System with Site Diversity Reception

Soft handoff is widely used in circuit-switching CDMA cellular systems Mobile stations

Figure 5.44 Throughput performance

communicate with several base stations simultaneously in soft handoff state Site diversity reception in the uplink of radio packet communications is a similar technique to soft handoff from the viewpoint of simultaneous reception Channel utilisation (for example throughput performance) improves if site diversity reception is performed for the uplink in a narrow-band radio packet communications [109] This is because the possibility exists that a packet will be received by another base station with little interference even if the connecting base station does not receive that packet

On the other hand, transmission power control (TPC) is generally used in CDMA cellular systems, and when applying TPC, fair service can be provided to mobile stations in all areas within the cell Fair service is also provided by TPC for the uplink in CDMA cellular packet communications Applying site diversity in the uplink of CDMA cellular packet commu-nications, however, brings geographical fluctuation in successful reception across the entire cell because site diversity rescues the packets originating mainly from near the cell bound-aries As a consequence, service quality at a mobile station depends on the position of the mobile station, and as a result, the system has geographical unfairness in service provision within the cell

In packet communications, a packet that fails to receive at its destination will be retrans-mitted Packet retransmissions increase the traffic from areas with inferior quality and result

in geographical non-uniformity in amount of traffic This non-uniformity can have a negative influence on total system performance in CDMA cellular packet systems [132]

In the following, fairness in service provision is first examined from the viewpoint of successful packet reception rate when applying site diversity to the uplink in a CDMA cellular slotted-ALOHA system, and it is pointed out that geographical unfairness in service provision exists Next, transmission power control based on difference in pilot-signal received power is described as a technique for relieving this unfairness [87]

5.6.2.1 Examination of service fairness

As shown in Figure 5.46, site diversity reception is defined as simultaneous reception by MSD base stations from among all the base stations in a service area For given packets, these MSD base stations are the ones that have the highest power levels of received packets Site

Figure 5.45 Packet error rate characteristics

diversity is always applied for each packet and all error-free packets are correctly received without relation to their spread codes

Figure 5.47 shows an example of transmission performance when applying conventional transmission power control in which target received power is constant for all packets In Figure 5.47, a successful reception rate is shown for each area when cells are divided into ten concentric regions of equal area We assign each region with a number where smaller numbers correspond to regions closer to the base station As can be seen, site diversity significantly improves successful reception rate near the cell boundary – the closer is the region to the boundary the greater is the improvement

This is because that the possibility that another base station can receive the packet is higher for packets originating near the cell boundary On the other hand, when not applying site diversity, successful reception rate is essentially the same for all regions and fair service can

be provided Accordingly, the application of site diversity reception generates geographical fluctuation in successful reception rate and brings unfairness in service provision

Figure 5.46 Site diversity reception

Figure 5.47 Successful reception rate characteristics