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John Wiley &. Sons, 2006. [5] R. Anderson, B. Arend, and K. Baker. Power Controlled Re- peaters for Indoor CDMA Networks. white paper, Qualcomm.
The 11th International Symposium on Wireless Personal Multimedia Communications (WPMC’08)

IMPROVING HSDPA INDOOR PERFORMANCE USING OUTDOOR REPEATER Panu L¨ ahdekorpi, Tero Isotalo, Ali Mazhar, Jukka Lempi¨ ainen Department of Communications Engineering, Tampere University of Technology P.O. Box 553, FIN–33101, Tampere, Finland [email protected], http://www.cs.tut.fi/tlt/RNG/ Abstract This paper presents how HSDPA performance can be improved indoors by using radio frequency repeaters. The radio signal is captured from outdoor macrocellular network and guided to indoor distributed antenna system through an amplifying repeater. Measurements were performed indoors in a typical office building in Finland. Two different locations were measured: open area and corridor. The results indicate that high average downlink throughput can be achieved by using repeater if the received signal strength is raised to an adequate level. However, the sufficient average signal level has been measured to be different between the two measured location. An average throughput of 3 Mbps was measured in both the open area and the corridor, when repeater was used. However in corridor environment, clearly lower signal strengths were measured to be enough providing the high throughput when comparing with the open area environment. Finally, the HSDPA performance increase was limited by the noise rise in the uplink direction. I.

Introduction

Due to introduction of many high speed wireless applications, cellular network operators have challenges in providing adequate coverage and capacity for the customers. High-speed Downlink Packet Access (HSDPA) technology was released by 3GPP to make it possible to share high bit rate downlink (DL) packet data with the customers. On the day of writing, HSDPA is already implemented to the most of the macrocellular third-generation base stations. Since customers requiring high data rates are mainly located indoors, new ways need to be introduced in order to intelligently avoid the losses caused by radio signals penetrating through the walls of buildings. In many cases, deployment of a new indoor base station may be far too expensive in the operator point of view. Thereby, repeaters could be used as a way to provide cost-efficient coverage for indoor mobile customers. A method of bringing the macrocellular radio signal to indoors using a repeater can be used (so called ’outdoor-to-indoor repeating’). Generally, repeaters have been studied by performing simulations and field measurements. However, these have mainly been done in outdoor environment. Indoor radio network planning aspects have been studied in [1] by performing corridor measurements with different antenna configurations without repeater. The indoor studies in [1] indicated that indoor radio network performance

can be significantly improved if decent coverage can be provided. Repeater performance in indoor environment has been previously studied by measurements in [2] for WCDMA Release ’99. The measurements indicated clear improvement in signal-to-interference ratio (SIR) at the mobile station, which can be directly mapped into increased HSDPA performance [2]. In this paper, functional HSDPA connection has been measured to study the capability of repeater in increasing HSDPA performance in indoor environment. Furthermore, the effect of environment type has been evaluated using two different indoor environment types (open area and corridor). Distributed Antenna System (DAS) was used to distribute the radio signals inside the building. This paper is divided into five chapters. First, the used radio technology is shortly described in Chapter II in the point of view of the measurements. The principles of repeaters and distributed antenna systems are given in Chapter II as well. The measurement setup and environment is described in Chapter III as detailed as possible. Measurement results are presented in Chapter IV in forms of figures, tables, and descriptive analysis. The results are concluded and discussed in the last chapter (Chapter V). II. A.

Radio Technology and Applied Methods

Distributed Antenna System

DAS consists of a trunk cable and a set of branch cables and antennas installed in a building. The purpose of DAS is to divide the signal into parts, which are then fed to the indoor antennas. The division of signal into parts (i.e., to several antennas) will provide lower effective isotropic radiated power (EIRP) levels and more smoothly distributed coverage compared to a single antenna. Radiating cables can be used to cover corridors or long spaces if needed. [3] B.

High Speed Downlink Packet Access

HSDPA technology is based on a fast packet switching principle in a shared downlink physical data channel. The shared radio channel can be re-allocated to different users in 2 ms time slots. Adaptive modulation and coding (AMC) takes care of the effective channel utilization. By these rapid link adaptations (by changing the modulation type, channel coding rate, and the number of utilized codes per user), HSDPA tries to provide the best throughput for the channel quality conditions measured and reported by the mobile station. The channel quality is reported every 2 ms time slot back to the transmitting

The 11th International Symposium on Wireless Personal Multimedia Communications (WPMC’08)

Table 1: Repeater details. parameter max. repeater DL power max. repeater UL power repeater noise figure repeater donor antenna gain donor antenna beamwidth

value 35 20 3 17.1 65

unit dBm dBm dB dBi ◦

Corridor route

1st floor

base station in form of channel quality indicator (CQI). In case of erroneous data packet reception at the mobile station, fast re-transmissions will be initiated by the serving base station to correct the misinterpreted packet (so called hybrid automatic repeat request). [4] C.

Outdoor-to-Indoor Repeating

Outdoor-to-indoor repeating can be used as an alternative method to improve coverage in indoor environment. Single antenna or DAS can be used to provide the extension in signal coverage offered by repeater. The building penetration loss can be avoided by feeding the signal through repeater amplifier using cables and antennas. The signal can be captured on the roof using outdoor antenna and forwarded inside the building using cables. Repeaters in this paper stand for simple linear amplifiers that can be installed to the cell area to provide amplified replica of the received UMTS bands (frequency division duplex UL and DL bands). As the whole band is amplified without signal regeneration, interference from other UMTS mobiles is included in the amplification and only out-of-band interference is filtered out using band pass filters. Repeater amplification ratio (repeater gain, GR ) can be adjusted to tune the EIRP level at the serving antennas. The repeater gain also affects the amount of received noise in uplink (noise originated from repeater circuits) [5]. III. A.

Measurement setup

Network

The measurements were done in a fully operating macrocellular UMTS network (equipped with HSDPA capability) during off-peak hours. Thus, no other traffic was present in the network. This was ensured by performing short EC /I0 test measurements at the time of starting each measurement round. The presence of other HSDPA traffic would directly affect the measured throughput due to the shared high speed DL data channel. The network and measurement equipment supported HSDPA data rates up to 3.5 Mbps (medium access control layer), which can be achieved with modulation type of 16-QAM using 5 parallel codes for a single connection. The maximum supported coding rate is 3/4. B.

Measurement equipment

The measurement equipment consisted of an HSDPA PC data card connected to a laptop computer and measure-

Open area route

Corridor service antennas

Open area service antennas

2nd floor

Donor antenna

5th floor

Figure 1: Measurement scenario, measurement routes, and antenna locations. Gain {55,65,75} +17.1 dB 35 m 95 m dBi -2.5 dB -12.6 dB

RNC

Node B

Mother cell antenna

Donor antenna

Repeater

-5 dB

25 m -4.2 dB

+2 dBi

HSDPA UE Splitter

Service antennas

Figure 2: Antenna line configuration. ment software. The measurement event was performed by walking through a designed route using a wheeled trolley for the laptop computer. A WCDMA repeater was used in the measurement campaign. The main properties of the repeater equipment and used antennas are listed in Table 1. The only repeater parameter adjusted during the measurements was the repeater gain for uplink and downlink. Same values were used for both directions (link symmetry requirement). C.

Repeater configuration

Repeater donor antenna was located on the rooftop of the measured building as shown in Fig. 1. The antenna was directed to the best hearable macro cell at the distance of 450 m. The mother cell antenna was visible from the donor antenna location, but some obstacles (e.g. rooftops and some trees) were present inside the effective link path. The strength of the radio link can be estimated using the received signal code power indicator (RSCP). It is measured by the mobile station on the primary common pilot

The 11th International Symposium on Wireless Personal Multimedia Communications (WPMC’08)

channel (P-CPICH). The received power is measured after de-spreading the wideband signal using a designated code. The difference in the P-CPICH RSCP between the best and the second best cell was 10 dB with the selected donor antenna location, which is considered as an adequate value to prevent interference between cells. With lower other cell level difference values, worse HSDPA performance was observed. The RSCP level from the mother cell at the donor antenna location was measured to be around −65 dBm (omnidirectional antenna used in measuring this value), which indicates a sufficient repeater-todonor connection (clearly above the repeater noise floor level). A typical outdoor macro cell antenna equipped with 65◦ horizontal half-power beamwidth was used as the donor antenna. Indoor environment and DAS configuration

The measurements were done in one of the buildings of Tampere University of Technology, Finland. Fig. 1 shows the floor plan of the building that was used in the measurements. Two types of locations were measured: open area and corridor. All the measurements were performed using three antennas. The antenna locations are marked in Fig. 1 with cross signs. Fig. 2 lists the antenna line components for the DAS and repeater. As presented in Fig. 2, the signal from repeater was divided between the three antennas by using a splitter. All of the indoor antennas used in the DAS were omnidirectional with 2 dBi gain. Measurement routes are presented in Fig. 1 with solid lines. Measurements were performed using a single route for each environment type. The route was walked through six times in each measurement event in order to get statistically reliable results and to minimize the measurement errors. In both, the open area and the corridor measurements, the antennas were located on the next floor above the measured route. The open area measurement was done with no critical obstacles between the antennas and measurement equipment. The serving antennas were placed on a footbridge in the second floor thereby giving an opportunity for the radio signals to reflect from walls towards the first floor. The corridor measurements were done one floor below the chosen antenna locations (Fig. 1) in a typical office corridor. IV.

Results

The measurement results are numerically presented in Table 2. The values represent an average of the samples gathered on the measured route. In Table 2, RSCP and EC /I0 are radio channel measurements reported by the data card. Throughput represents the achieved medium access control (MAC) layer throughput reported by the card. CQI is generated by the data card basing on the radio channel measurements and other vendor specific rules. A high CQI represents a good radio channel quality whereas a low value indicates problems in the signal reception. Uplink interference level gives indication of the uplink cell load

results in numbers. TP CQI UL int. [kbps] [dBm] 1892 2416 2895 3007

15.5 16.7 17.5 18.4

-105 -105 -104 -100

N/A 2212 2948 2974

N/A 15.9 18.1 18.5

-105 -105 -104 -100

3500

Average MAC throughput [kbps]

D.

Table 2: Measurement RSCP EC /I0 [dBm] [dB] Open area measurements Rep. off -101.4 -13.3 55 dB -82.9 -10.6 65 dB -73.6 -10.6 75 dB -63.5 -10.2 Corridor measurements Rep. off -119.1 -17.6 55 dB -97.5 -11.4 65 dB -88.5 -10.5 75 dB -77.9 -9.8

3000

2500

2000

1500 −110

Open Corridor Open − rep. off −100

−90 −80 Average RSCP [dBm]

−70

−60

Figure 3: Average MAC layer throughput as a function of average RSCP. experienced by the base station and is measured at the base station receiver. UL interference value is reported to the mobile station in DL using broadcast channel (BCH). The measurements were done first with repeater switched off. Then, repeater was turned on and the measurements were completed with different, pre-selected, repeater gain settings. At this point, the signal from the outdoor network was captured on the roof and fed to the indoor antennas through the repeater amplifier. Excluding the repeater gain adjustment, the configuration was remained untouched. The chosen repeater gain settings were 55 dB, 65 dB, and 75 dB. This repeater setup was chosen to reveal the behavior of the HSDPA performance in clearly different repeater operating areas (low gain, high gain). According to the ’repeater off’ measurements, continuous HSDPA coverage was not available in some parts of the measured routes, and the overall HSDPA indoor coverage was poor in general. RSCP levels were measured to be roughly around −100 dBm in the open area case (Table 2, ”rep. off”). In the corridor route, no HSDPA coverage was available and the measured RSCP levels remained as low as −120 dBm in average. However in the

The 11th International Symposium on Wireless Personal Multimedia Communications (WPMC’08)

1 0.9 0.8

rep. off GR 55 dB GR 65 dB GR 75 dB

0.7

CDF

0.6 0.5 0.4 0.3 0.2 0.1 0 −130 −120 −110 −100

−90 −80 −70 RSCP [dBm]

−60

−50

−40

Figure 4: RSCP CDF – open area route. 1 0.9 0.8 0.7 0.6 CDF

open area case, modest throughput was still offered by the HSDPA despite of the low received signal strengths. To summarize the observations, there was a clear need to improve the situation inside the building regarding to the HSDPA availability in order to fully utilize the higher total throughput offered by the HSDPA technology. Fig. 3 shows the performance improvement after switching the repeater on. In addition, the initial ’repeater off’ performance is presented in open area case. In Fig. 3, the measurement points connected with lines represent the different repeater gain settings for each scenario: 55 dB, 65 dB, and 75 dB. As already shown in Table 2, the RSCP values increase hand in hand with the repeater gain. Thus, the repeater equipment is not limiting the performance. The offset of 15 dB in the RSCP values between the open area and corridor case comes from the differences in the radio propagation environment. The propagation conditions are likely to be clearly better in terms of RSCP in the open area case compared to the corridor case, while the used repeater- and antenna line configurations were the same (visualized in Fig. 3 as the horizontal offset between the two graphs). It is also clearly visible in Fig. 3 how the average HSDPA throughput is significantly increased as a result of coverage improvement provided by the repeater in both measured indoor environments. After repeater gain of 65 dB, the throughput saturates to the maximum achievable average rate of 3 Mbps, which actually is quite close to the theoretical maximum of 3.5 Mbps. The difference in the achieved throughput is impressive in both environments, when repeater gain is increased from 55 dB to 75 dB: Table 2 reveals an average improvement of 600 kbps in the open area case and 800 kbps in the corridor case. The improvement from the original, ’repeater off’ case is in the range of 1.1 Mbps with the open area environment. The repeater HSDPA measurements give support to the earlier studies done without repeater and with slightly different antenna configuration ( [1]). When the indoor RSCP goes above a certain level, the achieved throughput is significantly increased. However based on the measurement results in open area (Fig. 3), the repeater gain has somewhat lower effects on the average throughput (the difference between the maximum and minimum measured average throughput is smaller) compared to the corridor case. This can be explained by the better initial radio conditions in open area location (HSDPA coverage originally available). By comparing the CQI values and average throughput values in Table 2, the achieved average throughput seems to closely follow the behavior of the reported average CQI values. This is a natural behavior, since the decision for offered throughput is made based on the reported CQI (when no other traffic present). However, the reported CQI values are not following the measured average RSCP values (although the CQI calculation is partly based on the channel measurements such as RSCP and EC /I0 ). Since there is no exact, vendor specific, information available concerning the CQI calcu-

0.5 0.4 0.3

rep. off

0.2

G 55 dB

0.1

GR 65 dB

R

GR 75 dB

0 −130 −120 −110 −100

−90 −80 −70 RSCP [dBm]

−60

−50

−40

Figure 5: RSCP CDF – corridor route. lation, only speculations could be made concerning the behavior of the average HSDPA throughput. At this point it is essential to notice that there is a limit for the maximum repeater gain setting. As stated in Table 2, uplink interference has increased from −105 dBm to −100 dBm (with no traffic), when the repeater gain has been set to 75 dB. The increase in the received base station noise level comes from the amplified noise of the repeater. The measured noise rise of 5 dB corresponds to a highly loaded cell, which directly indicates a really poor uplink performance. However with repeater gain of 65 dB, the noise rise is only 1 dB. Thus, significant performance improvement can be achieved even by using low repeater gains. Cumulative distribution functions (CDF) of the measured RSCP and throughput samples were plotted to give insight of the effects of repeater. Based on the open area RSCP CDF (Fig. 4), repeater is capable to increase the coverage up to a level, where nearly all of the samples are above −100 dBm. With repeater off, only 30% of the samples are above the −100 dBm level. Thus even with small repeater gains, the open area coverage can be significantly improved with repeater. Fig. 5 presents the RSCP CDF for the corridor measurement samples. It is interesting to notice that some deep fades (RSCP below −110 dBm) exist with repeater gain 55 dB, but the average through-

The 11th International Symposium on Wireless Personal Multimedia Communications (WPMC’08)

1

rep. off

0.9

GR 55 dB

0.8

GR 65 dB GR 75 dB

0.7

CDF

0.6 0.5 0.4 0.3 0.2 0.1 0 0

500

1000 1500 2000 2500 3000 HSDPA MAC layer throughput [kbps]

3500

Figure 6: Throughput CDF – open area route. 1

GR 55 dB

0.9

GR 65 dB

0.8

GR 75 dB

0.7

CDF

0.6 0.5 0.4 0.3 0.2 0.1 0 0

500

1000 1500 2000 2500 3000 HSDPA MAC layer throughput [kbps]

3500

Figure 7: Throughput CDF – corridor route. put is still at relatively high level. This is characteristic to HSDPA technology, since the short 2 ms transmission time interval (TTI) can effectively be utilized during the good radio channel periods. The CDF of the gathered throughput samples are presented for the two measurement locations. ’Repeater off’ results for the open area measurement show a clear peak for the samples in the range of 1.5 - 1.8 Mbps in Fig. 6. This phenomenon may be caused by the AMC function that controls the modulation and coding of the connection. The radio conditions are too weak to support better modulation and coding schemes during the measured route. However in some parts of the route, higher data rates can be utilized even with repeater switched off (slow linear progress of the solid plain curve in Fig. 6 in the range of 1.7 Mpbs - 3.5 Mbps). Figures 6 and 7 show, how the maximum MAC layer throughput of 3.5 Mbps is instantaneously measured during the routes. It is also shown, how the amount of high throughput samples is clearly increased as the repeater gain is increased. Even with the 55 dB repeater gain setting in open area case (Fig. 6), the amount of low data rate samples is clearly reduced and the amount of higher data rate samples is steadily increased when compared with the ’repeater off’ case. Furthermore, the effects of the repeater on the uplink interference are minimal with this repeater gain setting (Table 2, UL int.,

55 dB). Hence, the HSDPA performance can be easily improved without major drawbacks in the uplink direction. Similar behavior of achieved throughput can be observed in the corridor measurement (Fig. 7). However, the repeater gain of 55 dB is not high enough to prevent the losses coming from the more difficult propagation environment of the corridor area (measured through a ceiling). Thus, large amounts of low data rate samples can still be observed in Fig. 7 with repeater gain 55 dB. When the repeater gain is increased to 65 dB, the behavior is very similar compared with the open area 65 dB case. However in uplink, 1 dB is lost in form of increased uplink interference compared to the ’repeater off’ case (Table 2). V.

Discussion and Conclusions

Deployment of outdoor repeater was tested together with indoor DAS, which was implemented using three omnidirectional antennas. The measurements were made in two indoor locations: open area and corridor. Successful repeater configuration was detected by means of increased average RSCP (up to 41 dB improvement) and throughput (up to 1.1 Mbps improvement) in the measured indoor locations. The average throughput was seen to saturate to a level of 3 Mbps in both locations measured with repeater. Furthermore in open area case, the impact of repeater gain on the HSDPA performance was smaller compared with the corridor area. In the corridor, much lower average RSCP level (−90 dBm) was required compared with the open area (−74 dBm) to raise the average throughput to the maximum level. In addition, the average throughput was not following the average RSCP levels in the two locations. The measured CQI values were clearly affected by other parameters than the actual radio channel measurements (RSCP and EC /I0 ). The further increase in the HSDPA performance was limited by the uplink noise rise detected at the mother base station. Acknowledgement The authors would like to thank European Communications Engineering Ltd, Anite Finland, Nokia Siemens Networks, Nokia, and Elisa corporation for providing equipment and support for the measurements. References [1] T. Isotalo and J. Lempi¨ ainen. HSDPA Measurements for Indoor DAS. 65th IEEE Vehicular Technology Conference, 2007. [2] J. Borkowski, J. Niemel¨ a, T. Isotalo, P. L¨ ahdekorpi, and J. Lempi¨ ainen. Utilization of an Indoor DAS for Repeater Deployment in WCDMA. 63rd IEEE Vehicular Technology Conference, 2006. [3] J. Lempi¨ ainen and M. Manninen. UMTS Radio Network Planning, Opimization and QoS Management. Kluwer Academic Publishers, 2003. [4] H. Holma and A. Toskala. HSDPA/HSUPA for UMTS: High speed Radio Access for Mobile Communications. John Wiley & Sons, 2006. [5] R. Anderson, B. Arend, and K. Baker. Power Controlled Repeaters for Indoor CDMA Networks. white paper, Qualcomm Inc., 2003.