Optimization of Soft Handover Parameters for UMTS Network in Indoor Environment

Transcription

1 Optimization of Soft Handover Parameters for UMTS Network in Indoor Environment J. Łcki, J. Niemelä, J. Lempiäinen Institute of Communications Engineering Tampere University of Technology P.O. Box 533 FI Tampere, Finland Abstract The aim of this paper is to present the impact of different parameters on UMTS network performance in indoor environment. The measurement results of the impact of the most important parameters, i.e., the thresholds for add and drop events and their time to trigger values are presented on UMTS indoor network performance in terms of average downlink SIR target, downlink transmit power, probability, and observed BER values. Due to indoor propagation channel, which causes smaller delay spread and hence less multipath diversity, a UMTS signal becomes inherently narrowband. Therefore, the gain of against fast and slow fading would be crucial. Moreover, the characteristics of indoor radio channel enforce to apply different parameter settings than for outdoor network. In the frame of this paper, the analysis is based on the downlink as it is seen more crucial from point of view due to additional link required for s. The measurement results indicate that relatively large thresholds for add and drop events simultaneously with a long time to trigger for drop event are the most suitable for UMTS indoor environment due to the lack of multipath diversity, and to combat fast and deep fluctuations of the indoor radio channel. Key words: indoor, optimization, soft handover, UMTS. 1. Introduction WCDMA (wideband code division multiple access) technology introduces other kind of handovers, namely (soft handover) and SfHO (softer handover). A soft handover occurs, when two or more BSs serve a MS simultaneously. During a soft handover, MS is in the coverage area of two or more sectors belonging to different BSs. On the other hand, in softer handover, the MS is in the coverage of two or more sectors, which belong to the same base station. and SfHO are supported in UTRA FDD (UMTS Terrestrial Radio Access Frequency Division Duplex) mode only. The performance and functionality of and SfHO is controlled through dedicated functionalities and their parameters (see Section II). So far, most of the research concerning (herein is understood as and SfHO as function does not separate these two) parameter optimization has been concentrated on outdoor UMTS networks (e.g., [1] and [2]). Moreover, other already accomplished simulations of in [3] shows that optimum static parameters, i.e., the thresholds for add and drop events and dynamic parameters, i.e., time to trigger values of add and drop events can improve the downlink performance, and therefore also the overall network performance, quality of service (QoS) and capacity. However, during the current evolution of the 3G radio networks, indoor UMTS networks are becoming more and more important and also more popular. Moreover, the indoor environment introduces different radio propagation characteristics than outdoor environment. Due to small delay spread in a typical indoor propagation channel, the resulting amount of multipath diversity is inherently small. Moreover, due to frequency non-selective channel, the fading of the signal is mostly characterized by large amplitude variations. In terms of function and parameters, small delay spread values and lack of multipath diversity should be taken into account when setting the different parameters for function. Due to the fact that diversity reception is typically not used in passive indoor antenna systems due to cost constrains, the possibility of using the diversity component of against fast and slow fading indoor radio channel should be studied. In this paper, the measurement results of thresholds for add and drop events together with their time to trigger values are shown for the indoor UMTS network in the downlink direction. 2. UTRA FDD soft handover 2.1. Soft handover procedure and algorithm The soft handover procedure is divided into three phases: measurements, decision, and execution. In the measurement phase, energy per chip over interference spectral density ratio (E c /N 0 ) is evaluated based on the measurements on the downlink primary common pilot channel (P-CPICH) and received signal strength indicator (RSSI) as shown in (1): E /N = RSCP (1) RSSI c o RSCP (received signal code power) is the power of decoded pilot channel. The performed measurements results are sent by the MS to the BS. All measurements parameters are contained in measurement report, and then passed to RNC (radio network controller), where the decision phase takes place. In this phase, the performed

2 Figure 1: The meaning of parameters. measurements are compared with predefined soft handover criteria. This process is carried out by RNC. After the decision phase, the execution of soft handover is accomplished, if the soft handover criteria are fulfilled. The effect of the execution phase is that the MS either enters or leaves area. The algorithm currently used in WCDMA systems and in measurements presented in this paper is adopted from [1]. In explanation of soft handover algorithm, the following terms are crucial, and have to be defined: Active set contains the list of cells, having the connection with MS, in other words, the cells, which are in soft handover connection. Monitored set contains the list of cells, which pilot channels power is not high enough to be added to the active set, or active set is already full. The RNC decides, which event is triggered based on continuous measurements performed by MS on P-CPICH channel. These events are radio link addition (event 1A), radio link removal (event 1B), and radio link replacement (event 1C). These events are executed depending on relative strengths the pilot signal from different cells as well as on the time to trigger value ( T). A cell is added or dropped from active set depending on the difference between its P-CPICH E c /N 0 level and the power level of the best pilot signal in active set. algorithm is illustrated in Figure 1 and more details can be found from [7] Soft handover performances Soft handovers provide many advantages to the WCDMA network. One of them is smaller amount of ping-pong event, which occurs, when a mobile moves closer and further from a cell boundary causing frequent handovers. Non-optimal parameters may cause unnecessary high power rise peaks, which result in high interference and reduce network capacity. This negative effect can be mitigated by providing a larger hysteresis margin for execution of. However, in turn, this solution may introduce longer handover delays and higher interference Figure 2: Antenna positions and orientations during the measurements. in neighbor cells. This ping-pong effect is strongly decreased by soft handovers. The other feature of soft handovers is smoother data transmission. During a soft handover, there is no break in data transmission like during hard handovers. Moreover, s eliminate possible data loss and decrease drop call rate. This feature is very essential, because high transmission data rates are utilized in UMTS networks, where even a short brake in the transmission can cause a loss of high amount of data. The s have also negative effects. There is more complexity in implementation of s than with, e.g., hard handovers (HHO), as well as additional consumption of downlink resources (transmit power and spreading codes) is caused by the mobile being in. advantages and disadvantages give the reason to consider its parameters, algorithms, etc. The soft handover function should be planned carefully and have to be compromise between power gain and possible losses. 3. Measurement environment and setup 3.1. General description UMTS indoor test nework The measurements were conducted in indoor UMTS test network implemented in a university building. In the measurements, two cells belonging to different BSs were used. Base stations were connected through Iub interface to RNC/Iub simulator. RNC/Iub was supporting soft handovers between BSs [8]-[9]. Coaxial cable (i.e., a passive distributed antenna system, DAS) was employed to connect transmitting antennas and radiating cables to BSs. Antennas used in measurements were two standard discrete UMTS antennas [10]. Used antennas are highlighted in Figure 2. Antennas gain was 7 dbi and the horizontal beam width was 90. The effective isotropic radiated power (EIRP) values for antennas were close to dbm Measurement equipment The equipment used to provide the result was two base stations, two discrete antennas, MS, RNC/Iub simulator [8] and WCDMA radio interface analyzer [11] RNC/Iub

3 simulator was used in this measurement campaign for setting parameters, and capturing the transmit powers of two BSs. The radio interface analyzer was used to measure and store the radio interface parameters during the measurements. The mobile equipment connected to a laptop was placed on small trolley. The height of the trolley was approximately 1 m. During the measurements the trolley was moved along a pre-defined route (see the black solid line in Figure 2), on which the measurements were performed. The speed of the trolley was approximately 2-3 km/h Measurement campaign Two antennas belonging to different BSs were symmetrically placed and their beams were partly overlapping in this area on each other. Measurements were partly conducted in corridors, where a clear area was obtained (Corridor C in Figure 2). Measurements were mostly conducted during weekends in order to minimize the impact of external interferences on the measurement results. During the measurements, the trolley with laptop and mobile was moved from corridor G to the corridor F, through the C corridor of the first floor (Figure 2). While the trolley was moved, the transmit powers from both base stations were recorded. Radio interface analyzer recorded the time, when the MS was in area, and when MS was connected to only one cell. Moreover, the radio interface analyzer stored also other parameters like BER (bit error rate) and downlink SIR (signal to interference) target values. The measurements were conducted for the combination of various static and dynamic parameters. During each measurement, the transmit powers of both base stations were recorded. Power levels were later averaged in such manner that first the power outside area was averaged and added to the sum of average power transmitted during from both base stations. Finally, the sum of these two values was averaged again. Hence, it impact of additional radio link and additional transmit power was taken into account in final transmit power values. In this paper, probability is the ratio of the time, when the mobile is in area to the total time of the connection. Only one mobile was used for making a call (12.2 kbps speech bearer), and this call was reflected from the RNC/Iub simulator. Reflection of the call means that the call was initiated and terminated by the same MS. 4. Measurement results Tables 1-4 presents the measurement results with all measured sets of considered parameters. The rows contain the list of thresholds for add and drop events (in db) in such a manner that add thresholds are in the left side of the brackets and the drop thresholds in the right side. The columns contain the time-to-trigger values for add and drop events expressed (in ms), respectively. Table 1 shows the required SIR target values for used radio bearer (12.2 kbps speech). SIR target is set per bearer basis in RNC, and corrected in outer loop power control. In practice, the lower the average SIR target is, the smaller is also the amount of required resources, i.e., downlink transmit power. If power control is able to compensate the changes in the radio channel, the average SIR (adjusted by closed-loop power control) should correspond to average SIR target. As indicated by the results, increasing the threshold for add event decreases slightly the required SIR target. However, increasing the time-to-trigger for add event increases the required SIR target. Hence, the results indicate that in indoor environment in order to achieve lower SIR target values, the threshold for add event should be high but the time-totrigger value short. In other words, a cell is easily and quickly included in the active set in order to provide as much diversity gain as possible. On the contrary, if the threshold for drop event is increased, the required SIR target decreases as well. However, this phenomenon is the most efficient with higher time-to-trigger values for drop event. Hence, the results indicate that also high threshold for drop event should be used but also with longer timeto-trigger values. In other words, a cell is not easily dropped from the active set, and if it is about to drop, the level of the signal should maintain for a longer time below the threshold for drop event. Nevertheless, the results clearly point out that in indoor environment, the used thresholds should be high with short time-to-trigger for add event and long for drop event. Table 2 presents the average downlink transmit power for different sets of parameters. The average downlink transmit powers are mostly inline with the required SIR target values. However, in some cases, the downlink transmit power is lower although the required SIR target has been higher. The reason for this might have been the different distribution of area and certain level of measurement inaccuracy. Nevertheless, the measured results show that the used thresholds should be high with short time-to-trigger for add event and long for drop event. Figure 3a shows the comparison of transmission powers between different time-to-trigger values for all measured combinations of the thresholds for add and drop events. With low thresholds, the difference between [160ms 1280ms] scenario and [100ms 640ms] is minimal. However, with higher thresholds the gain of longer timeto-trigger values is obvious. In addition, the trend of all time-to-trigger curves indicates that even higher thresholds than [6dB 9dB] should be used for indoor networks. For example, with [0dB 1dB] thresholds for add and drop events and with [100ms 240ms] time-to-trigger values, the resulting downlink transmit power was around 31 dbm where as with the highest measured add and drop thresholds [6dB 9dB] and with the longest time-to-trigger values [160ms 1280ms], the resulting average downlink transmit power was only 22.7 dbm. Hence, over the measurement route, the gain of more correct parameters is 8.3 db in the downlink transmit power. However, as a consequence of this, the resulting probability increases over the measurement route from 16% up to 51% (see Table 3). Meaning that instead of having 1/6 of the users in, there are half of the users

4 Table 1. Required SIR target values for kbps speech link [db]. threshold [0 1] [1 2] [1 4] [2 4] [1 5] [2 5] [3 4] [3 5] [3 6] [3 8] [4 7] [6 9] Table 2. Required DL transmit powers in [dbm]. [0 1] [1 2] [1 4] [2 4] [1 5] [2 5] [3 4] [3 5] [3 6] [3 8] [4 7] [6 9] Table 3. Resulting probabilities [%]. [0 1] [1 2] [1 4] [2 4] [1 5] [2 5] [3 4] [3 5] [3 6] [3 8] [4 7] [6 9] Table 4. BER values of P-CPICH [%]. [0 1] [1 2] [1 4] [2 4] [1 5] [2 5] [3 4] [3 5] [3 6] [3 8] [4 7] [6 9] continuously in. Note that the impact of additional radio links required for is already taken into account when calculating the average transmit powers of the base stations for a single user. However, in multi-user scenario, the gain in the downlink transmit power is not as huge as indicated by the results, but since the downlink power requirement for users in is considerably smaller than for mobiles not in, the overall gain of having large thresholds for add and drop events is expected to be significant. Hence, the only implication is the increased probability that causes a higher requirement of Iub capacity from the indoor base station. Table 3 gathers the BER values calculated at the mobile station on the P- CPICH. These results further verify the previous discussion about parameter settings for indoor environment. Figure 3a shows the comparison of transmission powers between different time-to-trigger values for all measured combinations of the thresholds for add and drop events. With low thresholds, the difference between [160ms 1280ms] scenario and [100ms 640ms] is minimal. However, with higher thresholds the gain of longer timeto-trigger values is obvious. In addition, the trend of all time-to-trigger curves indicates that even higher thresholds than [6dB 9dB] should be used for indoor networks. Figure 3b shows the cumulative distribution function (CDF) of the downlink transmit power with [160ms 1280ms] time-to-trigger values for [6dB 9dB] (solid) and for [0dB 1dB] (dashed) parameter setups. As clearly indicated by the figure, the fast fading dips with small thresholds for add and drop events requires larger dynamics for the downlink transmit power as the required power varies between 16 dbm and 36 dbm, whereas with higher thresholds, the required range is between 16 dbm and 22 dbm. The fundamental difference in the downlink transmit power comes directly from the possible area (i.e., from Corridor C in Figure 2) since in the non- areas (i.e., Corridors F and G) the required transmit power is the same (see Figure 3). Figure 4 shows the drop call rates for all measured timeto-trigger values with [3dB 4dB] thresholds for add and drop events. As indicated by the curve, the resulting drop call rate decreases tremendously from % down to 4% when time-to-trigger values are changed from [100ms 240ms] to [160ms 1280ms]. However, since the acceptable drop call rate is typically 1-2%, the results clearly indicate that in addition to long drop timer, higher thresholds for add and drop event are needed for indoor UMTS networks. The impact of different parameter values on the required downlink SIR target, downlink transmit power, probability and P-CPICH BER was analyzed in this paper. The analyzed parameters

5 DL transmission power [dbm] Best case [ ] Typical case [ ] Worst case [ ] Measurement point (a) CDF[%] Best case [6dB 9dB] Worst case [0dB 1dB] Transmit power [dbm] Figure 3: (a) An example of downlink transmit powers for three different sets of time-to-trigger values. Measurement points correspond to all measuremed combinations of the thresholds for add and drop event in order of appearance in the tables. (b) Cumulative distribution function of the transmit power with high [6 db 9 db] and low [0 db 1dB] add and drop window sizes. (b) were the threshold values for add and drop events together with their time-to-trigger values. Due to smaller delay spread values in indoor environment, UMTS signal is behaving inherently as a narrowband signal. Moreover, due to resulting lack of multipath diversity, the required SIR target for a radio bearer increases if is not utilized. Hence, the diversity gain provided by clearly reduces the required SIR target, and hence results in also smaller required TX power. This fundamentally increases the capacity of an indoor network. Drop call rate [%] Conclusions The smallest SIR target values and correspondingly the smallest downlink TX powers were observed with [6dB 9dB] thresholds for add and drop events and with [160ms 1280ms] time-to-trigger values, correspondingly. However, the measurement results and analysis indicate that even higher thresholds or longer time-to-trigger for drop event should be utilized. The required dynamic range for downlink TX power is considerably smaller with higher threshold for add and drop events. The future studies concentrate on providing the impact of diversity gain on the capacity of indoor network. Moreover, the impact of larger thresholds for add and drop events with longer time-to-trigger values is analyzed in uplink direction. Acknowledgement Authors would like to thank European Communications Engineering (ECE) Ltd for helpful comments, Elisa Mobile Finland for allocating the frequency for UMTS test network, Nemo Technologies for providing radio interface analyzer, and the National Technology Agency of Finland for funding the work. REFERENCES [1] I. Forkel1, M. Schinnenburg1, B. Wouters, Performance evaluation of soft handover in a realistic UMTS network, in Measurement point Figure 4: Drop call rates as a function of add and drop window sizes. Proc. IEEE 57 th Semiannual Vehicular Technology Conference, vol. 3, pp , April 03. [2] M. Schinnenburg1, I. Forkel1, B. Haverkamp, Realization and Optimization of Soft and Softer Handover in UMTS Networks, in Proc. IEEE 5 th Personal Mobile Communications Conference, pp , April 03. [3] D. Wong, T.J. Lim, Soft Handoffs in CDMA mobile systems, IEEE Transition on Personal Communications, vol. 4, issue 6, pp. 6-17, December [4] N. Binucci, K. Hiltunen, M. Caselli, Soft Handover gain in WCDMA, in Proc. IEEE 52 th Semiannual Vehicular Technology Conference, vol. 3, pp , September 00. [5] X. Yang, S. Ghaheri-Niri, R. Tafazolli, UTRA Soft Handover optimization, in Proc. IEE 3 rd International Conference on 3G Mobile Communication Technologies, pp , May 02. [6] 3GPP, TS , Radio resource control, Version 5.2.0, Release 5. [7] 3GPP, TR , Radio resource management strategies, Version 3.8.0, Release 6. [8] Web site, NetHawk Oyj, NetHawk RNC/Iub Simulator. [9] 3GPP, TS , UTRAN Iub interface NBAP signaling, Version 4.5.0, Release 4. [10] Web site, Indoor Directional Antenna MHz. [11] Web site, Nemo Technologies, Nemo outdoor, Radio Interface Analyzer.