Radio network planning and optimisation for umts 2nd edition phần 5 pot

inner-loop PC to recover. In the case of CM by HLS, larger TGLs require the use of lower transport format combinations and result in lower L2 throughput. In the case of CM by SF/2, larger TGLs require the use of the double-frame approach meaning that two radio frames rather than a single radio frame have their spreading factor reduced. Table 4.5 presents the relationship between TGL and the minimum requirement for the UE’s ability to sample GSM RSSI. These figures have been extracted from [9]. The third column shows the efficiency with which measurements are made. Also included in the table is the equivalent time required to complete eight GSM RSSI measurements based upon three samples per measurement and a TGPL of four radio frames. GSM RSSI measurements are made without acquiring GS M synchronisation and do not require the CM transmission gap to coincide with a particular section of the GSM radio frame. The measurement efficiency becomes relatively poor for TGLs of less than seven slots. A TGL of seven slots balances the efficiency but with an impact on the inner-loop PC. In the case of BSIC verific ation, the frame structure and timing of the GSM system has a more significant impact on the required TGL. The GSM system is based on an eight-slot radio frame structure with a duration of 4.615 ms. The first slot of each frame is dedicated to the BCCH. The BSIC is broadcast periodically within the SCH of the BCCH. The UE has no knowledge of the timing of the GSM system and must capture 9 slots’ worth of GSM data to be sure of capturing the BCCH. A CM TGL of 7 slots is equivalent to 4.667 ms and provides a high probability of capturing the BCCH. The fact that the BSIC is broadcast 5 times per 51 frames means that multiple transmission gaps are likely to be required. Table 4.6 presents the relationship between the TGL and the BSIC identification time that guarantees the UE at least two attempts at decoding the BSIC. These figures have been extracted from [9]. In practice BSIC identification times may be less than those presented in Table 4.6. It is possible that the UE manages to identify the BSIC within the first transmission gap. Longer TGLs and shorter TGPLs result in more rapid BSIC identification times. The TGPL provides a tradeoff between the time spent in CM and the potential impact on L1 and L2 performance. Long TGPLs increase the time spent in CM. This means that CM must be triggered relatively early to prevent radio-link failure occurring prior to completing a successful IS-HO. Triggering CM relatively early means Radio Resource Utilisation 231 Table 4.5 The impact of transmission gap length on GSM received signal strength indicator measurements. TGL [slots] No. of GSM RSSI No. of GSM RSSI Time to complete eight GSM RSSI samples samples per slot measurements (three samples per measurement) 3 1 0.33 960 ms 4 2 0.50 480 ms 5 3 0.60 320 ms 7 6 0.86 160 ms 10 10 1.00 120 ms 14 15 1.07 80 ms that it will also be triggered more frequently. TGPL should be defined such that CM can be triggered relatively late and less frequently. The benefit of using a long TGPL is that the inner-loop PC has more time to recover between transmission gaps. Throughput reductions caused by higher layer scheduling and L2 retransmissi ons will also be less frequent and thus will have lower average impact. CM may be configured such that the UE has a fixed number of radio frames within which to complete its GSM RSSI measurements and a fixed number of radio frames to complete BSIC verification. The drawback of this approach is that the UE may complete its RSSI measurements very rapidly and subsequently have to wait until it can start BSIC verification. Alternatively the UE may not manage to complete its RSSI measurements within the fixed time and would then be forced to start BSIC verification without successful RSSI measuremen ts. In this case, BSIC verification would ha ve to be completed using the entire GSM neighbour list and the UE would have to report the GSM RSSI at the same time as report ing the BSIC. A different approach is to allow the UE to remain in CM for GSM RSSI measurements until instructed otherwise by the RNC. The RNC would be able to reconfigure the CM measurements for BSIC verification once the UE has provided sufficient RSSI measurements. In this case, BSIC verification could be completed using only the best GSM neighbour. 4.3.7.5 Common Issues The definition of good inter-system neighbour cell lists is essential for reliable IS-HO performance. If neighbour lists are too short then missing neighbours may lead to failed IS-HOs. If the neighbour lists are too long then the UE measurement time increases and important neighbours may be removed from the list when the UE is in SHO. The initial definition of inter-system neighbour lists is part of the radio network planning process. The initial definition should be refined during pre-launch optimisation when, for example, RF scanner measurements or network performance statistics can be used to detect missing neighbours (see also Section 9.3.4.1). If the RNC has reduced the GSM neighbour list to a single neighbour for BSIC verification then it is possible that the single neighbour is no longer available – i.e., the UE has moved out of its coverage area. This is more likely if the RNC has based its decision of which is the best GSM neighbour upon a single measurement report. Otherwise the UE may have difficulties synchronising and extracting the BSIC within the CM transmission gap. When GSM RSSI measurements or BSIC verification fail 232 Radio Network Planning and Optimisation for UMTS Table 4.6 The impact of the transmission gap length on GSM BSIC verification. TGL [slots] Transmission gap pattern No. of transmission gap Equivalent time length patterns 7 3 frames 51 1.5 s 7 8 frames 65 5.2 s 10 12 frames 23 2.7 s 14 8 frames 22 1.8 s 14 24 frames 21 5.0 s then the UE is unable to complete an IS-HO. It is then likely that the UE will trigger a further CM cycle and reattempt the HO procedure. Otherwise the UE may have moved back into good coverage or moved completely out of coverage and dropped the connection. Once the HO command or cell change order has been issued by the RNC then the UE has a limited period of time to successfully connect to the GSM system. If connection is not achieved within this limited period of time then the UE returns to the UMTS system and issues a failure message. In the case of packet switched data services, GSM cell reselection after receiving the cell change order can slow down the IS-HO procedure. This may occur if the UE has moved onto a non-ideal GSM neighbour. 4.4 Congestion Control In WCDMA it is of the utmost importance to keep the air interface load unde r pre- defined thresholds. The reasoning behind this is that excessive loading prevents the network from guaranteeing the needed requirements. The planned coverage area is not provided, capacity is lower than required and the QoS is degraded. Moreover, an excessive air interface load can drive the network into an unstable condition. Three different functions are used in this context, all summarised here under congestion control: . Admission Control (AC), handling all new incoming traffic. It checks whether a new packet or circuit switched RAB can be admitted to the system and produces the parameters for the newly admitted RABs. . Load Control (LC), managing the situation when system load has exceeded the threshold(s) and some countermeasures have to be taken to get the system back to a feasible load. . Packet Scheduling (PS), which handles all the NRT traffic – i.e., packet data users. Basically, it decides when a packet transmission is initiated and the bit rate to be used. 4.4.1 Definition of Air Interface Load Since WCDMA systems have the possibility of uplink and downlink being asym- metrically loaded, the tasks of congestion control have to be done separately for both links. Two different approaches can be used for measuring the load of the air interface. The first defines the load via the received and transmitted wideband power; the second is based on the sum of the bit rates allocated to all currently active bearers. The quantities have already been introduced in Chapter 3 and are thus only summarised here. Wideband Power-based Uplink Loading In this approach the Node B measures the total received power, PrxTotal, which can be split into three parts: PrxTotal ¼ Iown þIoth þ P N ð4:15Þ Radio Resource Utilisation 233 where Iown is the received power from users in the own cell; Ioth comes from users in the surrounding cells; and P N represents the total noise power, including background and receiver noise as well as interference coming from other sources (see Section 5.4). Two quantities representing the uplink loading can be derived from Equation (4.15). The first is called the uplink load factor,  UL , and is defined as:  UL ¼ Iown þIoth PrxTotal ð4:16Þ The second quantity is called the uplink noise rise, NR, and can be derived as follows : NR ¼ PrxTotal P N ¼ 1 1 À UL ð4:17Þ Throughput-based Uplink Loading The definition of uplink loading follows the derivation in Section 3.1.1.1 and is based on the sum of the individual load fact ors of each user k:  UL ¼ X k 1 1 þ W  k Á R k Á  k Áð1 þiÞð4:18Þ where W is the chip rate; and  k , R k and  k are the E b =N 0 requirement, the bit rate and the service activity of user k, respect ively. Wideband Power-based Downlink Loading One method of defining the air interface loading in the downlink direction is simply by dividing the total currently allocated transmit power at the Node B, PtxTotal, by the maximum transmit power capability of the cell, PtxMax:  DL ¼ PtxTotal PtxMax ð4:19Þ Throughput-based Downlink Loading The first way to define the downlink loading based on throughput is similar to that used in the wideband power-based approach: The loading is the sum of the bit rates of all currently active connections divided by the specified maximum throughput for the cell:  DL ¼ X N k¼1 R k Rmax ð4:20Þ where R k is the bit rate of connection k;andN is the total number of connections. Note that in the summation the bit rates from the common channels also have to be included. Alternatively, downlink loading can be defined as derived in Section 3.1.1.2 and simplifying Equation (3.9) by introducing an average orthogonality   and an average downlink other-to-own-cell-interference ratio i DL :  DL ¼½ð1 À  Þþi DL Á X N k¼1   k Á R k Á  k W  ð4:21Þ 234 Radio Network Planning and Optimisation for UMTS where W is the chip rate; and  k , R k and  k are the E b =N 0 requirement, the bit rate and the service activity of connection k, respectively. 4.4.2 Admission Control This section describes the tasks performed in AC and the parameters involved. AC is the main location that has to decide whether a new RAB is admitted or a current RAB can be modified. Because of the different nature of the traffic, AC consists of basically two parts. For RT traffic (the delay-sensitive conversational and streaming class es) it must be decided whether a UE is allowed to enter the network. If the new radio bearer would cause excessive interference to the system, access is denied. For NRT traffic (less delay-sensitive interactive and background classes) the optimum scheduling of the packets (time and bit rate) must be determined after the RAB has been admitted. This is done in close cooperation with the packet scheduler (Section 4.4.3). The AC algorithm estimates the load increase that the establishment or modification of the bearer would cause in the RAN. Separate estimates are made for uplink and downlink. Only if both uplink and downlink admission criteria are fulfilled is the bearer setup or modification request accepted, the RAB established or modified, or the packets sent. Load change estimation is done not only in the access cell, but also in the adjacent cells to take the inter-cell interference effect into account, at least in the cells of the active set. The bearer is not admitted if the predicted load exceeds particular thresholds either in the uplink or downlink. In the decision procedure, AC will use thresholds produced during radio network planning and the uplink interference and downlink transmission power information received from the wideband channel. To be able to decide whether AC accepts the request, the current load situation of the surrounding cells in the network has to be known and the additional load due to the requested service has to be estimated. Therefore, AC functionality is located in the RNC where all this information is available. 4.4.2.1 Wideband Power-based Admission Control The uplink admission decision is based on cell-specific load thresholds given during radio network planning. An RT bearer will be admitted if the non-controllable uplink load, PrxNC, fulfils Equation (4.22) and the total received wideband interference power, PrxTotal, fulfils Equation (4.23): PrxNC þ DI PrxTarget ð4:22Þ PrxTotal PrxTarget þ PrxOffset ð4:23Þ where PrxTarget is a threshold and PrxOffset is an offset thereof, defined during radio network planning. For NRT bearers only the latter condition is ap plied. The non- controllable received power, PrxNC, consists of the powers of RT users, other-cell users, and noise. DI is the increase of wideband interference power that the admission of the new bearer would cause. For its estimation in [2] two methods are Radio Resource Utilisation 235 proposed. The first is called the derivative method and defines the power increase as: DI % PrxTotal 1 À Á DL ð4:24Þ where  is calculated with Equation (4.16). The second approach is called the integration method. Here the power increase is estimated to be: DI % PrxTotal 1 À À DL Á DL ð4:25Þ In both Equations (4.24) and (4.25) the fractional load DL of the new user can be calculated as derived in Section 3.1.1: DL ¼ 1 1 þ W  ÁR Á ð4:26Þ where W is the chip rate;  the required E b =N 0 ; and  the service activity of the new bearer. For the downlink direction a similar admission algorithm as in the uplink is defined. An RT bearer will be admitted if the non-controllable downlink load, PtxNC, fulfils Equation (4.27) and the total transmitted wideband power, PtxTotal, fulfils Equation (4.28). PtxNC þ DP PtxTarget ð4:27Þ PtxTotal PtxTarget þPtxOffset ð4:28Þ where PtxTarget is a threshold; and PtxOffset is an offset thereof defined during radio network planning. For NRT bearers only the latter condition is ap plied. The non- controllable transmitted power, PtxNC, consists of the powers of RT users, other- cell users and noise. DP can be based on the initial transmit power estimated by the open-loop PC as specified in Section 4.2.1. 4.4.2.2 Throughput-based Admission Control The throughput-based AC is pretty simple by nature. The strategy is simply that a new bearer is admitted only if the total load after admittance stays below the thresholds defined during radio network planning. In the uplink this means that:  oldUL þ DL  thresholdUL ð4:29Þ must be fulfilled, and in the downlink:  oldDL þ DL  thresholdDL ð4:30Þ where  oldUL and  oldDL are the network load before the bearer request, estimated with Equations (4.20) and (4.21); and DL is the load increase calculated with Equation (4.26). 236 Radio Network Planning and Optimisation for UMTS 4.4.3 Packet Scheduling 4.4.3.1 Packet Data Characteristics The RAN provides a capability to allocate RAB services for communication between the CN and the UE. RAB services realise the RAN part of end-to-end QoS. They have different characteristics according to the demands of different services and applications. In the UMTS QoS concept, RAB services are divided into four traffic classes, according to the delay sensitivity of the traffic. These traffic classes are: . conversational class; . streaming class; . interactive class; . background class. Conversational class is meant for traffic that is very delay-sensitive, while background class is the most delay-insensitive traffic class. Conversational and streaming classes are intended to carry RT services between the UE and either a circuit or packet switched CN. Typical examples of packet switched RT services are Voice over IP (VoIP) and multimedia streaming of audio, video or data. Interactive and background classes are intended to carry NRT services between the UE and a packet switched CN. The characteristics of interactive and background class bearers are that they do not have transfer delay or guaranteed bit rates defined. Due to looser delay requirements, compared with conversational and streaming classes, both NRT classes provide better error rate by means of channel coding and retransmission. Retransmissions over the radio interface allow the use of a much higher BLER for NRT packet data on the radio link, while still fulfilling the residual BER target that is part of the QoS definition. Typical characteristics of NRT packet data are the bursty nature of traffic. A packet service session contains one or several packet calls depending on the application. The packet service session can be considered as an NRT RAB duration and the packet call as an active period of packet data transmission. During a packet call several packets may be generated, meaning that the packet call constitutes a bursty sequence of packets. UMTS QoS classes and traffic modelling are described in more detail in Chapter 8. PS can be considered as the scheduling of data of the NRT RABs – i.e., interactive and background class bearers over the radio interface in both the uplink and downlink. Conversational and streaming classes are delay-sensitive and require dedicated resources for the whole duration of the connection. Radio resource allocation for RT packet switched bearers is an AC function and thus not considered in this section. 4.4.3.2 WCDMA Packet Access WCDMA packet access is controlled by the packet scheduler, which is part of the RRM functionality in the RNC. The functions of the packet scheduler are: . to determine the available radio interface resources for NRT radio bearers; . to share the available radio interface resources between the NRT radio bearers; . to monitor the allocations for the NRT radio bearers; Radio Resource Utilisation 237 . to initiate Tr CH-type switching between common, shared and dedicated channels when necessary; . to monitor the system loading; . to perform LC actions for the NRT radio bearers when necessary. As shown in Figu re 4.13, AC and the packet scheduler both participat e in the handling of NRT radio bearers. AC takes care of admission and release of the RAB. Radio resources are not reserved for the whole duration of a connection but only when there is actual data to transmit. The packet scheduler allocates appropriate radio resources for the duration of a packet call – i.e., active data transmission. As shown in Figure 4.13, short inactive periods during a packet call may occur, due to bursty traffic. PS is done on a cell basis. Since asymmetric traffic is supported and the load may vary a lot between the uplink and downlink, capacity is allocated separately for both directions. However, when a channel is allocated to one direction, a channel has to be allocated in the other direction as well, even if the capacity need was triggered only for one direction. The packet scheduler allocates a channel with a low data rate for the other direction, which carries higher layer (TCP) acknowledgements, data link layer (RLC) acknowledgements, data link layer control and PC information. This low bit rate channel is typically referred as the ‘return channel’. Packet scheduler functionality consists of UE- and cell-specific parts. The main functions of the UE-specific part are traffic volume measurement management for each UE TrCH, taking care of UE radio access capabilities and monitoring allocations for NRT radio bearers. SHO is also possible for the DCHs allocated to NRT radio bearers. During SHO, PS is done in every cell in the active set, and the UE-specific part of the PS function is the controlling entity between the cell-specific functions. The cell’s radio resources are shared between RT and NRT radio bearers. The proportions of RT and NRT traffic fluctuate rapidly. It is characteristic of RT traffic that the load caused by it cannot be controlled efficiently. The load caused by RT traffic, interference from other-cell users and noise together is called 238 Radio Network Planning and Optimisation for UMTS Packet scheduler handles bit rate Packet call RACH/FACH, CPCH, DSCH or DCH allocation NRT RAB allocated, packet service session Admission control handles time Figure 4.13 Admission control and packet scheduler handle non-real time radio bearers together. the non-controllable load. The available capacity that is not used for non-controllable load can be used for NRT radio bearers on a best effort basis, as shown in Figure 4.14. The load caused by best effort NRT traffic is called controllable load. PS as well as RRM in general can be based on, for exampl e, powers, throughputs and spectrum efficiency. Figure 4.15 shows the input measurements for a packet scheduler. The Node B performs received uplink total wideband power (RSSI) and downlink transmitted carrier and radio link power measurements, and rep orts them to the RNC over the Iub interface using the NBAP signalling protocol. Throug hput measurements can be performed in the RNC. If spectrum efficiency is taken into account, the P- CPICH E c =I 0 measurement can be used to estimate transmission power. Traffic volume measurements can trigger radio resource allocation for NRT radio bearers. Traffic volume measurements are controlled by the RNC. The UE measures uplink TrCH traffic volumes and sends measurement reports to the RNC. Measurement Radio Resource Utilisation 239 load time planned target load free capacity, which can be allocated for controllable load on best effort basis non-controllable load Figure 4.14 Capacity division between non-controllable and controllable traffic. UE UE UE Uu RNC Iub Node B Node B CN Iu u p l i n k i n t e r f e r e n c e a n d d o w n l i n k t r a n s m i s s i o n p o w e r m e a s u r e m e n t s uplink traffic volume measurements uplink and downlink throughput measurements downlink traffic volume measurements Figure 4.15 Measurements for WCDMA packet scheduler. reporting can be periodical or event-triggered. In the latter case the measurement report is sent when the uplink TrCH traffic volume exceeds the threshold given by the RNC. Downlink traffic volume measurements are performed by the RNC. According to the UE state and current channel allocations, system load, the radio performance of different TrCHs, the load of common channels and TrCH traffic volumes the packet scheduler selects an appropriate TrCH for the NRT radio bearer of the UE . The following TrCHs are applicable for packet data transfer: . Dedicated transport Channel (DCH); . Random Access Channel (RACH); . Forward Access Channel (FACH); . Common Packet Channel (CPCH); . Downlink Shared Channel (DSCH). Table 4.7 shows the key properties of these TrCHs. Applicable TrCH configurations for packet data in the uplink/downlink are DCH/DCH, RACH/FACH, CPCH/FACH, DCH/DSCH. A comparision of DSCH and HS-DSCH can be found in Table 4.8. 240 Radio Network Planning and Optimisation for UMTS Table 4.7 Properties of WCDMA transport channels applicable for packet data transfer (HS-DSCH see Table 4.8). TrCH DCH RACH FACH CPCH DSCH TrCH type Dedicated Common Common Common Shared Applicable UE state Cell_DCH Cell_FACH Cell_FACH Cell_DCH Cell_DCH Direction Both Uplink Downlink Uplink Downlink Code usage According to Fixed code Fixed code Fixed code Codes shared maximum allocations allocations allocations between bit rate in a cell in a cell in a cell several users Power control Fast closed- Open-loop Open-loop Fast closed- Fast closed- loop loop loop SHO support Yes No No No No Targetted data Medium or Small Small Small or Medium or traffic volume high medium high Suitability for Poor Good Good Good Good bursty data Setup time High Low Low Low High Relative radio High Low Low Medium Medium performance [...]... That is, for P multicodes starting at offset O the following codes are allocated: Cch;16;O Á Á Á Cch;16;OþPÀ1 The number of multi-codes and the corresponding offset for HS-PDSCHs is signalled in the HS-SCCHs The controlling RNC is responsible for the allocation of the spreading codes Radio Network Planning and Optimisation for UMTS 256 4.7 Impact of Radio Resource Utilisation on Network Performance... codes can be used to scramble the DPCCH and DPDCH In the uplink both the channelisation and the scrambling codes are allocated by the system and require little action during radio network planning Uplink scrambling codes are call-specific and are allocated in connection establishment by the RNC The uplink scrambling code 248 Radio Network Planning and Optimisation for UMTS space is divided between RNCs Each... maximum Doppler frequencies 5, 20, 40, 100 and 250 Hz, respectively, for the Pedestrian A channel These numbers are, however, only for a single isolated cell 262 Radio Network Planning and Optimisation for UMTS because SHO is not taken into account The effect of SHO is studied further in the next section 4.7.1.2 Impact of Soft Handover on Transmit Power Control Headroom and Transmit Power Increase The... Figure 4.24 the theoretical average transmit power rise from Equations (4.38) and (4.40) is plotted as a function of the average power difference of the two propagation paths Radio Network Planning and Optimisation for UMTS 258 9 Aver Tx power raise [dB] 8 7 without diversity 6 5 4 with diversity 3 2 1 -30 - 25 -20 - 15 -10 -5 0 Power difference [dB] Figure 4.24 Theoretical average transmit power rise... were compared with the single-link case and gains were estimated The gains were measured for the average transmitted and received power and for the required PC headroom as a function of the average level difference between the SHO links and the UE speed The gains estimated here are different from the ‘traditional’ SHO 266 Radio Network Planning and Optimisation for UMTS gains which are against shadow fading... short codes from a code tree must be made 4 .5. 2 Code Management The WCDMA system divides spreading and scrambling (randomisation) into two steps The user signal is first spread by the channelisation code and then scrambled by the scrambling code This is similar to IS- 95, but as 3G’s WCDMA system is asynchronous, Radio Network Planning and Optimisation for UMTS 246 +1 Channelisation code (OVSF) -1 +1... values for DACK , DNACK and DCQI are parameters set by higher layers, which can be quantised into nine steps (0; ; 8) Mapping onto amplitude ratios can be found in [17, table 1A]; for other details see also [1] Radio Network Planning and Optimisation for UMTS 252 HS-DPCCH CQI Report ACK/NACK ∆ACK; ∆NACK ∆CQI ∆CQI DPCCH Figure 4.22 HS-DSCH–DPCCH power offsets 4.6.2 4.6.2.1 Congestion Control for High-speed... Power Control and Soft Handover on Network Performance The results presented in this section are based on [20] and [21] Simulations have been performed with parameters that are not fully compatible with the current 3GPP specifications, but the trends visible in the results do also apply to the current standard 4.7.1.1 Impact of Fast Power Control In WCDMA radio network dimensioning and planning the link... specific to phone manufacturers Scrambling code planning in the network This task is carried out during radio network planning and described together with scrambling code optimisation in detail in Section 4 .5. 2.4 4 .5. 2.1 Cell Search Procedure The purpose of the cell search procedure is to find a suitable cell and to determine the downlink scrambling code and frame synchronisation of that cell The cell... Compared with the DCHs in Release ’99, the fundamental difference between the HC and mobility management involving cells where HSDPA is enabled comes from the issue that downlink channels involved in the HSDPA transmission (HS-PDSCH and 254 Radio Network Planning and Optimisation for UMTS HS-SCCH) can neither be in soft nor in softer handover – i.e., they can only belong to one link in the active set of a UE . R k Á  k W  ð4:21Þ 234 Radio Network Planning and Optimisation for UMTS where W is the chip rate; and  k , R k and  k are the E b =N 0 requirement, the bit rate and the service activity of. the network load before the bearer request, estimated with Equations (4.20) and (4.21); and DL is the load increase calculated with Equation (4.26). 236 Radio Network Planning and Optimisation for. secondary 246 Radio Network Planning and Optimisation for UMTS -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 Scrambling code Combined code Channelisation code (OVSF) Figure 4.20 Spreading (SF ¼8) and scrambling for