Repeater in WCDMA/UMTS

Transcription

1 Repeater in WCDMA/UMTS by Md. Masudur Rahman A master s thesis at the department of Signals and Systems, Chalmers University of Technology, Sweden Conducted at Telia Research AB. Examiner and Supervisor: Prof. Arne Svensson; Supervisor at Telia Research AB: Per Ernstrom Ph. D. EX001/2002 January 2002

2 Abstract In this thesis, the use of repeaters in a WCDMA/UMTS system containing hotspots (highly loaded areas within an otherwise homogeneously loaded network) for different hotspot loads and repeater losses is studied. The results are achieved through Static Monte Carlo network simulations. The thesis shows that repeaters are very beneficial for the downlink. The system outage can be lowered to levels comparable to when there are no hotspots in the system at all, even when the hotspot loads are as large as half of the background cell load. In the uplink, repeaters are somewhat less effective but still quite beneficial for the network. When the hotspots are located at the cell border, the system outage can be lowered by about 35% compared to when no repeaters are used. For hotspots located halfway between the base stations and the cell border, repeaters can lower the system outage by about 25%. The results thus indicate that repeaters are very well suited for hotspot scenarios in WCDMA/UMTS systems.

3 Table of Contents 1 Introduction Previous work Problem definition Background Wideband CDMA Spreading and Despreading for WCDMA Systems A Short Theoretical Review of WCDMA Network Traffic Model for WCDMA Outage of the WCDMA system Repeater setting in the system with base-stations Antenna Isolation Horizontal and vertical Antenna patterns Vertical separation Antenna Environment Time delay Simulation Model and Results Repeater model Some important parameters for the simulations Result Analysis Simulation scenarios Downlink Uplink Conclusion Further work...26 References...27 Appendix A...29 RAKE Receiver...29 Maximal Ratio Combining (MRC)...30 Appendix B...31 Relation with repeater dominated distance and it s loss...31 Increased coverage and repeater loss...33

4 1 Introduction A repeater is a device that is situated in between a base station and a mobile station. It has two antennas, one directed towards the base station (typically line of sight) and one directed towards a service area. It amplifies received signals and retransmits them in both up and downlink with a delay of a few microseconds. Repeaters may be used for different purposes. The most classical application is to increase the coverage area for a base station, e.g. for coverage in a tunnel. It can, however, also be used to decrease interference in the system, since mobiles communicating via the repeater need a lower output power than without the repeater. This is of particular interest in interference-limited systems, such as CDMA based systems, where reduced interference results in increased capacity. It is well known that repeaters are very cost effective for specific applications such as for coverage in tunnels. This work is focused on repeater applications for WCDMA cellular systems, quantifying the capacity and coverage gains that can be achieved. The results will be used to evaluate if repeaters could be cost effective also for these applications. The scenario that is studied is the use of repeaters to increase capacity at hot spots (areas with much traffic). There is a significant difference between the use of repeaters to cover dead spots (valley, tunnels and buildings) and the use of repeaters in the hotspot to cover the same area or extend some area. Mobile stations in the dead spot receive only one signal, i.e., the signal from the coverage antenna or the signal from the base-station that is blocked by the aforementioned obstacles. When a repeater is used in the hotspot, there are no obstacles between the base-station and the mobile. So, in this case a mobile receives the sum of two signals. For the study static system level simulations were performed. A simulator developed at Telia was used, but work has been done to adopt it for the study of repeaters. 2

5 1.1 Previous work There are some work about repeaters in WCDMA in the literature. Most of them are mentioned in references [6], [10], [11]. In paper [6] repeaters are discussed as means for effectively enlarging service coverage in areas with weak signal strengths. But in urban environments with lots of interference among sites, heavy traffic and frequent soft handoff, the repeater will act as another interference source to neighbour cells although it eliminates a poor call quality. In [10] a repeater is considered as an Automatic on off switching device for dead spots. If an active user is within the coverage area, the repeater will work normally, otherwise the repeater will be off. Using a repeater like this reduces additional interference to the system compared to if the repeater is always on, which is good for an interference limited system. In paper [11], repeaters are considered as a solution for coverage in CDMA systems. However, no work concerning repeaters in a usual base station system with circular hotspot (high loaded area) within the normal cell have been found. In such a scenario, users will receive signals both from the base station and the repeater. In this thesis, the use of repeaters for this purpose is studied and the impact of repeaters on the system is analysed. 1.2 Problem definition As was mentioned in the previous section that there are some works on repeaters for various purposes, but nothing on repeaters used in usual base station systems when there are some circular hotspot within the normal cell. The task of the thesis work is to introduce various hotspots in a network and compare results with and without repeaters for different traffic loads and repeater losses. The comparison will be done in terms of the user outage and the total interference present in the system. To make progress, we have to made some assumptions. They are as follows. We have a regular network of base stations, i.e. a hexagonal shaped cellular system with base stations in every hexagon. There is a homogenous background traffic, i.e. users are uniformly distributed in the entire network. 3

6 Some circular areas (hot spots) with increased traffic loads are introduced within the system. This is also where the repeaters are positioned. Comparison of scenarios with the same traffic load but with and without repeaters. The considered scenario is illustrated in figure 1. Repeater Hotspot BaseStation Figure 1 A hexagonal cell with increased traffic load in a certain circular area 4

7 2 Background 2.1 Wideband CDMA WCDMA is a broadband direct sequence code division multiple access (DS-CDMA) system. It means that user information bits are spread over a wide bandwidth by multiplying them with random bits. Multicode transmissions are supported along with variable spreading factors. The maximum user bit rate will be about 2Mbps. The chip rate of 3.84 Mcps used leads to a carrier bandwidth of approximately 5 MHz. As WCDMA is wide carrier bandwidth it supports high user data rates and also has certain performance benefits, such as increased multipath diversity. It supports highly variable data rates, in other words the concept of obtaining Bandwidth on demand is well supported. It also supports two main modes of operation: Frequency Division Duplex (FDD) and Time Division Duplex (TDD). In the FDD mode, separate 5 MHz carrier frequencies are used for the uplink and downlink respectively, whereas in TDD only one 5 MHz is time shared between uplink and downlink. WCDMA supports the operation of asynchronous base stations, so that unlike in the synchronous IS-95 system there is no need for a global time reference, such as GPS. It employs coherent detection on uplink and downlink based on the use of pilot symbols or common pilot. While already used on the downlink in IS-95. The use of coherent detection on the uplink cause in an overall increase of coverage and capacity on the uplink [1]. The lower bandwidth for all 3G proposals is 5 MHz and there are some real reasons for choosing this bandwidth. They are as follows. Data rates of 144 kbps and 384 kbps, which are the main targets of 3G systems, are achievable within the 5 MHz bandwidth with a reasonable capacity. Even under some limited conditions a 2 Mbps peak rate can be provided. Narrower bandwidths cannot separate more multipaths but 5MHz bandwidth can do it easily and thereby increase the diversity, which improves the performance. Even though larger bandwidths of 10, 15 and 20 MHz have been proposed in order to support higher data rates, the main trend is to evolve the standard within the original 5 MHz bandwidth. 5

8 2.1.1 Spreading and Despreading for WCDMA Systems A BPSK modulated bit sequence is used for the user data (user data bits are assumed to take the values of ±1). The spreading operation, which is shown in the figure 2 is a multiplication of user data bits with random sequence code bits. We see that the resulting spread data is at a rate of 8 R. The resulting spreaded data has the same random appearance as the spreading code itself. In the figure a spreading factor of 8 is used. After spreading, the broadband signal can be transmitted across a wireless channel to the receiving end. The despreading operation is just the opposite of spreading. The spreaded data/chip sequence is multiplied, bit duration by bit duration, with the 8 code chips used during spreading. CDMA systems are usually called spread spectrum system since the increase of the signalling rate by a factor of 8 corresponds to a widening of the occupied spectrum of the spread user data signal. The increase of the signalling rate by a factor of 8 corresponds to a widening (by a factor of 8) of the occupied spectrum of the spread user data signal. Due to this reason, CDMA system are more generally called spread spectrum systems. All spread spectrum systems has a good effect is called processing gain. The processing gain is what gives CDMA systems the robustness against self-interference that is necessary in order to reuse the available 5 MHz carrier frequencies over geographically close distances. For example speech service with a bit rate of 12.2 kbps has a processing gain of 25dB = 10 log10 (3.84e6 /12.2e3). After dispreading, the signal power needs to be typically a few decibels above the interference and noise power. 6

9 Symbol Data +1-1 Chip Spreading Spreading code +1-1 Spread signal= Data*code +1-1 Despreading Spreading code +1-1 Data= Spread signal* code +1-1 Figure 2 Spreading and Despreading in WCDMA(source:ref.1) 7

10 2.2 A Short Theoretical Review of WCDMA Network Traffic Model for WCDMA The traffic in each cell approximately accords with an independent M / G / queue, see [13]. If the mean time between call arrivals is 1/ ψ s and the mean call holding time is 1/ µ s, the traffic of the system is A =ψ / µ Erlangs. Let K be the random variable representing the number of active calls in the cell at steady state. Thus, K is Poisson distributed, with k A A P( K = k) = e, k = 0,1,2... (1) k! In order to take the voice activity effect into account, we assume that the call is active with probability ρ ( 0 < ρ 1) or inactive with probability 1 ρ, once a mobile user is connected to the network. K is still Poisson distributed according to the formula about compound probability and equation (1) can be written as k ρa ( ρa) P( K = k) = e, (2) k! but the traffic is reduced by ρ Outage of the WCDMA system The outage probability is defined as the probability that a mobile achieves an insufficient SIR. In the uplink, the SIR for user j is defined as SIR j = W R n W + 0 λ p j j, bs( j) k j p λ k k, bs( j), (3) where j, bs( j) p j denotes the transmitted signal power from user number j, λ is the total loss between the base station and user equipment j, bs ( j) denoted the base station to which the user j is connected, λ j, bs( j) is the total loss between the base station and user equipments k excluding user equipment j for the base station bs ( j), R information bit rate for the user. The parameter W is the carrier bandwidth, while n 0 denotes the noise level. 8

11 One of the important quantities of a CDMA system is the processing gain G, which is the ability to suppress noise and interference relative to the desired signal. It is defined as W G = (4) R and using this together with equation (3), we can write p j G λ j, bs( j) SIR j =. (5) pk n0w + λ k j k, bs( j) A user in the uplink suffers an outage whenever the power he needs to transmit with exceeds the maximum available output power of the UE. This corresponds to that his signal, when received at the base station, does not satisfy the uplink SIR requirements. The downlink SIR is defined as SIR j = n W k j p λ k j, bs( k ) q j j λ R j W α λ p k j, bs( j) bs( k ) = bs( j) k j where q j is the output power to user number j from the base station, and α is the orthogonality factor. If α =1, the user channels are perfectly orthogonal and if α =0, they will add to the interference in the same way as noise. The orthogonality is set up for different users by different orthogoinal codes. But in reality it never be 1 or 0. So, in our case we assume that orthogonality is ½. The third part of the denominator is the contribution from users that are connected to the same base station as user j. Again using equations (4) and (6) together we can write SIR j = n W + 0 k j p λ k q λ j, bs( k ) j j G α λ p k j, bs( j) bs( k ) = bs( j) k j (6). (7) A user in the downlink suffers an outage whenever his SIR requirement cannot be met without exceeding power limitations in the base station. 9

12 3 Repeater setting in the system with base-stations 3.1 Antenna Isolation In order to get a good performance, repeater antenna isolation is an important issues, see [8]. As stated previously, a repeater receives signals from a base station and amplifies it. Under certain circumstances, the repeater may act as an oscillator (see [18] chapter Amplifiers and Oscillators p251, paragraph-oscillation and gain conditions), with the coverage and donor antennas as the feedback path in the amplifier system. For the prevention of oscillations in the system, the feedback must be lower than the amplifier gain. Otherwise the repeater will work as an oscillator, which will lead to a collapse of the whole system. The loss, which one is cause to keep working our repeater as an amplifier is called the Antenna Isolation. There are some influential factors on the antenna isolation. They are described subsequent sections Horizontal and vertical Antenna patterns The optimum way of antenna isolation is a combination of donor and coverage antennas that are mounted such that there is a null in the antenna pattern in the direction pointing towards the other antenna. A null means minimum antenna gain in the specified direction [8]. As both antennas are usually mounted in opposite directions, it is useful to choose both donor and coverage antenna types that have a high front-to-back ratio [8] Vertical separation Usually antennas used for repeaters have a narrower aperture in the vertical antenna pattern since the vertical distance of the antenna influences the isolation of the antenna system. Normally, when both antennas are mounted on a pole, there is a null in the antenna pattern pointing vertically up and down from the antenna s feeding point. Additional lobes in the vertical antenna pattern have to be taken into account when horizontal separation exists between the antennas. 10

13 3.1.3 Antenna Environment Time delay One of the most important factors is the antenna environment. The reflection and attenuation properties of all material near the antenna can influence the pole with the antenna isolation drastically [8]. The waves transmitted by antennas are reflected by surfaces, depending on the materials. If there is a reflection from a building towards the pole with the mounted antennas, this can decrease the antenna isolation by more than 10dB [8]. The material of the tower itself has also an effect on the isolation: If both antennas are mounted on a tower made of concrete, this improves the antenna isolation, since signals are attenuated and reflected by the material of the tower. A steel grid tower, however, might not increase antenna isolation particularly, since the distances between the single elements might be larger than a half wavelength, which means that radiated power can pass the tower almost unattenuated. In this case, antenna isolation is more dependent on the antenna pattern. Shielding grids mounted near the antennas also have an effect on the isolation. Generally, isolation can be improved using a shielding grid by approximately 5dB [8], depending on the shape of the shielding grid. Grids that are shaped according to the antenna are better than simple ones. The UTRA BS and UE can handle a 20µs time delay between two signals path [8]. One signal comes directly to the user equipment from the base station and the other via repeater (will be shown in the next paragraph). The repeater introduces a time delay of 5-6µs. The signal path introduced through the repeater will be longer than the direct path, both due to the extra travelling distance required for the signal (approximately 5µs per 1.5 km [8]) and due to the group delay in the repeater itself. For the repeater coverage, where the area can be substantial, it is a rule of thumb that the repeater site should be placed between the repeater service area and the donor base station. 11

14 In our simulation we do not consider this time delay of different paths of the signal. For further studies of repeaters in WCDMA, one can consider this delay in the simulations 4 Simulation Model and Results 4.1 Repeater model There is a significant difference between the use of repeaters to cover dead spots (valley, tunnels and buildings) and the use of repeaters in the hotspot to cover the same area or extend. Mobile stations in the dead spot receive only one signal, i.e., the signal from the coverage antenna or the signal from the base-station that is blocked by the aforementioned obstacles. When a repeater is used in the hotspot, there are no obstacles between the base-station and the mobile. So, in this case a mobile receives the sum of two signals as illustrated in figure Repeater Mobilestation BaseStation Figure 3 Mobilestation connection with Repeater and Basestation 12

15 The two signals are combined in the rake receiver. We assume an ideal maximum ratio combining (MRC), modelled by summing the two signal powers as- P r = Pt Pt 1 gbs Ar g rep λ + l η z l η z (8) 1 c 2 2 b 3 3 Here, A r is the repeater amplifier gain, g rep the combined antenna gain for the two repeater antennas, g bs the base station antenna gain, l c the cable and connector loss, l b the body loss, η 2 the path loss from base station to repeater, and η 3 the path loss from repeater to the mobile station. But using MRC model the problem is that, we still don t know the perfect result in reality because of we don t have link level simulation. Anyway, for the simplicity we will consider here only this model. From the above equation we find that Pt Pt Pr = + (9) λ1 lλ 3 In equation (9) the second part of the right side is due to the signal propagation from the base station to the UE via the repeater. Also, λ i denotes the total propagation loss i = 1, 3 ; and lclb ziηi λ i = gbs Here z i is the shadow fading and the repeater loss is denoted by l. It follows that 1 g rep Ar = (10) l η z 2 2 η z2 or, l = 2 g (11) rep A r The repeater functions as a bi-directional amplifier of RF signals from base stations in the downlink and from UEs in the uplink. Thus, the repeater has an output noise density that can be calculated as N _ r( f ) = N _ thermal( f ) + NF + g r (12) where f, is the frequency in which the repeater amplifies. In our case, noise in the repeaters is neglected. When the repeater loss is large, it does not affect the system that much. Our simulation considers an urban macro cellular system. Scenarios with repeaters and without repeaters cases are compared as 13

16 follows. The network consists of 16 cells, each containing a base station. Among these 16 cells, 4 cells contains hotspots (areas with a higher traffic load). Each hotspot is situated at equal distance from the base stations in respective cells. Repeaters are situated at the centre of the hotspots and various hotspot loads are analysed. Simulations are also done for various repeater losses and the system behaviour is studied. The total system is explained by figure 4. Cells marked by letter A contain hotspots and repeaters (circles marks the hotspots). The letter B identifies cells close to a repeater and the letter C identifies cells far from a repeater. B A B A C B C B B A B A C B C B Figure 4 System configuration 14

17 4.2 Some important parameters for the simulations Main Parameters Initial Value Antenna type Omni-directional Pilot Power W Sync Power W Prediction model Okumura Hata urban Voice Activity 1 Processing Gain 25 db Cell Radius 390 m Hotspot Radius 50 m SIR_uplink 6.1 db SIR_downlink 7.9 db Max. Mobile output 21 dbm power Max Base-station output power 45 dbm 4.3 Result Analysis Simulation scenarios In the simulations we study two traffic distribution cases. In the first case the hotspots are located at a distance of 340m from the mother base-station (the hotspots are close to the cell border) and in the second case the hotspots are located at a distance of 195m from the mother base-stations (the hotspots are halfway from the mother basestation to the cell boarder). In the graphs, results for the first traffic case with repeaters located close to the cell boarder are indicated by circles and blue coloured lines, while results for second traffic case are indicated by squares and green coloured lines. Both traffic distribution cases are studied with and without repeaters. The repeaters are always situated in the centre of the hotspots. In all graphs results with repeaters are indicated with solid lines, while results without repeaters are indicated by dashed lines Downlink Without repeaters We first study the system without repeaters. In figure 5 we see that the outage of the system increases with increased hotspot load. In figure 6 we see that at a hotspot load corresponding to half the 15

18 With repeater background cell load and at a background cell load of calls/cell the outage increases by a factor 10. The effect is worst for a hotspot close to the cell border. The increase in outage is about twice as large for the border hotspot case compared to the halfway hotspot case. This is due to the fact that neighbour cell interference is larger close to the cell border and thus the calls in the border hotspot require more power from the BS. After introducing a repeater into the system we can see from figure 7 that outage is dependent on repeater loss and at a loss of 9-15dB we have the best performance. The repeater shows worse performance at low loss due to mother cell over loading and worse performance at high loss since the area for which the repeater has any effect shrinks, see figure 14a and 14b in Appendix B. At a loss of 9.54dB repeater performance is remarkably good for both hotspot positions. At this loss the repeater reduces the outage almost to the level without hotspots even when the hotspot load is as large as half of the background cell load. We also note that by using repeaters, we have a small gain in outage even without hotspot load Outage vs Hotspot load Repeater and hotspot position is 340m Without repeater and hotspot position is 340m Repeater and hotspot position is 195m Without repeater and hotspot position is 195m 0.07 Outage Average nr of calls per hotspot(hotspot load) Figure 5 (Downlink) Outage VS Hotspot load, repeater loss=9.54db, Average nr of calls per cell (Background cell load)=

19 Outage vs Hotspot load Repeater and hotspot position is 340m Without repeater and hotspot position is 340m Repeater and hotspot position is 195m Without repeater and hotspot position is 195m Outage Average nr of calls per hotspot(hotspot load) Figure 6 (Downlink) Outage VS Hotspot load, repeater loss=9.54db, Average nr of calls per cell (Background cell load) = Outage vs Repeater loss 0.03 Repeater and hotspot position is 340m Without repeater and hotspot position is 340m Repeater and hotspot position is 195m Without repeater and hotspot position is 195m Outage Repeater loss[db] Figure 7 (Downlink) Outage VS repeater loss with fixed Hotspot load=18 and Average nr of calls per cell (Background cell load) =

20 4.3.3 Uplink Without repeater In figure 9a we see that the hotspots increase the outage of the system. At a hotspot load of almost half of the background cell load the outage increases from close to zero at zero hotspot load to about 10% at a hotspot load to about a 50% increase in cell load. The effect is worst for hotspots close to the cell border but the difference is not as large as for the downlink case. The increase in outage is about 25% higher than for the other case (Hotspot in halfway of the base station and cell boarder). The difference is due to the higher path loss to terminals in the border hotspot, which requires a higher terminal output power. Since the downlink is the bottleneck for the system capacity and has been used to set the background load we see almost no outage in the uplink without hotspot load With repeaters In figure 8a we can see that also for the uplink case outage depends strongly on the repeater loss. At intermediate repeater losses we can see clear gains from the repeaters in terms of decreased outage. At low loss, however, outage is higher than without repeaters This is due to the fact that at small repeater loss the coverage area of the mother cell will increase, and thus more mobiles will connect to the mother base station, increasing the load of the base station. The repeater will also pump in more interference to the mother base station from mobiles that are still connected to the neighbour cell base station (see figure 9c and 9d), but are also close to the repeater. At very high repeater loss the area where the hotspot has any effect shrinks to nothing and asymptotically the outage and interference levels reach the levels without repeaters. In our cases the repeaters show best performance at 17-23dB losses. Taking a closer look in figure 8b and 8c at interference levels separately in mother cells and neighbour cells, we can see that the interference is highest in the mother cell (cell type A), in line with our interpretation of the results in terms of increased coverage of the mother cell. In fact, for the case with the hot spot closer to the cell boarder (figure 8c) the mother cell interference is larger than without repeaters for all losses. The system gains come from interference reduction in the neighbour cells (cell type B and C). In our case repeaters show best performance at 17-23dB losses. 18

21 We now study the repeater performance at an optimal repeater loss of 20dB. The repeater performance is not as good as for the downlink. The outage decrease but not down to the level without hotspots (see figure 10a). Even interference in the mother cell less than without repeater case (see figure 10c and 10d). The repeaters have the best effect for the cell boarder hotspot case Outage vs Repeater loss Repeater and hotspot position is 340m Without repeater and hotspot position is 340m Repeater and hotspot position is 195m Without repeater and hotspot position is 195m Outage Repeater loss[db] Figure 8a (Uplink) Outage VS repeater loss with fixed Hotspot load = 18, Average nr of calls per cell (Background cell load) =

22 Average BS Interference VS Repeater loss Repeater and hotspot position is 340m Without repeater and hotspot position is 340m Repeater and hotspot position is 195m Without repeater and hotspot position is 195m 145 Average BS inteference[dbm] Repeater loss [db] Figure 8b (Uplink) Average BS interference VS repeater loss with fixed Hotspot load=18, Average nr of calls per cell (Background cell load) = Average interference comparison for 3 kind of cell mode, repeater position 340m Average BS interference with repeater for cell type A Average BS interference with repeater for cell type B Average BS interference with repeater for cell type C Average BS interference without repeater for cell type A Average BS interference without repeater for cell type B Average BS interference without repeater for cell type C Average BSinterference[dBm] Repeater loss[db] Figure 8c (Uplink) Average BS interference for same BS type VS repeater loss, repeater position is 340m and Hotspot load=18, Background cell load=

23 Average interference comparison for 3 kind of cell mode, repeater position 195m Average BS interference with repeater for cell type A Average BS interference with repeater for cell type B Average BS interference with repeater for cell type C Average BS interference without repeater for cell type A Average BS interference without repeater for cell type B Average BS interference without repeater for cell type C Average BSinterference[dBm] Repeater loss[db] Figure 8d (Uplink) Average BS interference for same BS type VS repeater loss, repeater position is 195m and Hotspot load=18, Background cell load= Outage vs Hotspot load Repeater and hotspot position is 340m Without repeater and hotspot position is 340m Repeater and hotspot position is 195m Without repeater and hotspot position is 195m 0.08 Outage Average nr of calls per hotspot (hotspot load) Figure 9a (Uplink) Outage VS Hotspot load when repeater loss is 9.54dB, Average nr of calls per cell (Background cell load) =

24 Average BS interference vs Hotspot load Repeater and hotspot position is 340m Without repeater and hotspot position is 340m Repeater and hotspot position is 195m Without repeater and hotspot position is 195m 148 Average BSinteference[dBm] Average nr of calls per hotspot (hotspot load) Figure 9b (Uplink) Average BS interference VS Hotspot load, repeater loss 9.54dB, Average nr of calls per cell (Background cell load) = Average interference comparison for 3 kind of cell mode, repeater position 340m Average BS interference with repeater for cell type A Average BS interference with repeater for cell type B Average BS interference with repeater for cell type C Average BS interference without repeater for cell type A Average BS interference without repeater for cell type B Average BS interference without repeater for cell type C Average BSinterference[dBm] Average nr of calls per hotspot (hotspot load) Figure 9c (Uplink) Average BS interference for same BS type VS Hotspot load, repeater position 340m and repeater loss 9.54dB, Average nr of calls per cell (Background cell load)=

25 Average interference comparison for 3 kind of cell mode, repeater position 195m Average BS interference with repeater for cell type A Average BS interference with repeater for cell type B Average BS interference with repeater for cell type C Average BS interference without repeater for cell type A Average BS interference without repeater for cell type B Average BS interference without repeater for cell type C Average BSinterference[dBm] Average nr of calls per hotspot (hotspot load) Figure 9d (Uplink) Average BS interference for same BS type VS Hotspot load, repeater position 195m and repeater loss 9.54dB, Average nr of calls per cell (Background cell load)= Outage vs Hotspot load Repeater and hotspot position is 340m Without repeater and hotspot position is 340m Repeater and hotspot position is 195m Without repeater and hotspot position is 195m 0.08 Outage Average nr of calls per hotspot (Hotspot load Figure 10a (Uplink) Outage VS Hotspot load when repeater loss is 20dB, Average nr of calls per cell (Background cell load) =

26 Average BS interference vs Hotspot load Repeater and hotspot position is 340m Without repeater and hotspot position is 340m Repeater and hotspot position is 195m Without repeater and hotspot position is 195m 149 Average BSinteference[dBm] Average nr of calls per hotspot (hotspot load) Figure 10b (Uplink) Average BS interference VS Hotspot load, repeater loss 20dB, Average nr of calls per cell (Background cell load) = Average interference comparison for 3 kind of cell mode, repeater position 340m Average BS interference with repeater for cell type A Average BS interference with repeater for cell type B Average BS interference with repeater for cell type C Average BS interference without repeater for cell type A Average BS interference without repeater for cell type B Average BS interference without repeater for cell type C Average BSinterference[dBm] Average nr of calls per hotspot (hotspot load) Figure 10c (Uplink) Average BS interference for same BS type VS Hotspot load, repeater position 340m and repeater loss 20dB, Average nr of calls per cell (Background cell load)=

27 Average interference comparison for 3 kind of cell mode, repeater position 195m Average BS interference with repeater for cell type A Average BS interference with repeater for cell type B Average BS interference with repeater for cell type C Average BS interference without repeater for cell type A Average BS interference without repeater for cell type B Average BS interference without repeater for cell type C Average BSinterference[dBm] Average nr of calls per hotspot (hotspot load) Figure 10d (Uplink) Average BS interference for same BS type VS Hotspot load, repeater position 195m and repeater loss 20dB, Average nr of calls per cell (Background cell load)=

28 5 Conclusion In this report, WCDMA systems with hotspots were studied. The network performance with and without repeaters located in the hotspots were compared in order to investigate whether repeaters are useful in such scenarios or not. Different hotspot locations within cells were also considered. The comparison factors were the outage and interference of the system and were obtained by static Monte Carlo simulations. The results show that repeaters have very good effect in downlink case. They improve the system outage almost up to without hotspot system outage even in hotspot loads half of background cell load. They are less effective in the uplink case but still reduce effect by 35% for border and 25% for halfway hotspot case. But it is very important to set proper repeater loss otherwise with wrong repeater gain settings, outage might increase instead of decrease. 6 Further work In this work, repeaters with omni directional antennas were used. By using sectorised antennas one can reduce interference and thus improve capacity. Therefore sectorised antennas may result in lower outage for the same load compared to using omni directional antennas. So, a study including sectorised antennas would be very interesting to perform. In this study the combination of the direct signal and the repeater signal was modelled as ideal maximum ratio combining. A study based on a more realistic model would be of interest. Such a model could be based on measurements or link level simulations. 26

29 References 1. WCDMA for UMTS, Harri Holma and Antti Toskala, 2000 by John Wiley & Sons, Ltd Baffins Lane, Chichester, West Sussex, PO19 1UD,England. 2. Wireless Communications Principle and Practice, Theodore S. Rappaport, Prentice Hall PTR Upper Saddle River, New Jersey The coverage and capacity of a UMTS network-a first study, Sommer M and Almers P, 77/ /FCPA. 4. Erlang Capacity of a Power Controlled CDMA System with Antenna Array, Guocong Song, Ke Gong, State Key Laboratory on Microwave & Digital Communications Tsinghua University, Beijing , China. 5. Erlang Capacity of a Power Controlled CDMA System, Audrey M. Viterbi and Andrew J. Viterbi, IEEE journal on selected areas in communications VOL. 11, NO.6 August 1993, pp An Analysis of Effect of Wireless Network by a Repeater in CDMA System, Sang-Jin Park, Whan Woo Kim, Bum Kwon, IEEE / , pp The coverage and capacity of a UMTS network II, Ulf S Nilsson and Magnus Sommer, Wireless Solutions Telia Research AB rd Generation Parnership Project; Technical Specification Group Raio Access Networks; UTRA Repeater; Planning Guidelines and System Analysis (Release 4), 3 GPP TR v4.0.0( ). 9. On the Capacity of Cellular CDMA system UP-link with Multiple Base Station Diversity, J. Orris, S. K. Barton, Department of Computer Science University of Manchester, United Kingdom, IEEE2000, pp Automatic On-Off Switching Repeater for DS/CDMA Reverse Link Capacity Improvement, Wan Choi, Bong Youl Cho, and Tao Won Ban, IEEE communication letters, vol.5 NO. 4, April 2001, pp Repeater for CDMA Systems, Moussa R. Bavafa Howard H. Xia,, AirTouch Communications, Inc Oak Road, MS 900, Walnut Creek, CA 94598, IEEE1998, pp On the Capacity of a Cellular CDMA System, Klein S. Gilhousen, Irwin M. Jacobs, Roberto Padovani, Andrew J. Viterbi, Lindsay A. Weaver, Jr and Charles E. Wheatly IIIm 27

30 IEEE transactions on vehicular technology, vol. 40, no. 2, May 1991, pp On the Teletraffic Capacity of CDMA Cellular Networks, Jamie S. Evans and David Everitt, IEEE transection vehicular technology, vol. 48, no. 1 January 1999, pp Effective Bandwidth-Based Admission Control for Multiservice CDMA Cellular Networks, Jamie S. Evans and David Everitt, IEEE transactions on vehicular technology, vol.48 no. 1 January 1999, pp Call Admission control in multiple service DS-CDMA Cellular Networks, Jamie Evans and David Everitt, The authors are with the Department of Electrical and Electronic Engineering, University of Melbourbe, Parkville 3052 Victoria, Australia, IEEE 1996, pp Modeling a Homogeneous WCDMA network, Ulf S Nilsson and Magnur Sommer, Wireless Solutions Telia Research AB. 17. WCDMA: Towards IP Mobility and Mobile Internet, Tero Ojapera, Ramjee Prasad, Artech House, Boston, London Microwave Engineering with Wireless Applications, S.R. Pennock, P.R. Shepherd, Macmillan Press, ISBN ,

31 Appendix A RAKE Receiver In a multipath channel, the original signal reflects from obstacles such as buildings and mountains, and the receiver receives several copies of the signal with different delays. Receiver can resolve the signals if they arrive more than one chip apart from each other. In fact, from each multipath signal point of view, other multipath signals can be regarded as interference and they are suppressed by the processing gain. A further benefit is obtained if the resolved multipath signals are combined using rake receiver. Rake receiver consists of correlators, each receiving a multipath signal. After dispreading by correlators, the signals are combined using, for example, maximal ratio combining. Because the received multipath signals are fading independently, diversity order and thus performance are improved. In figure describe the principle of RAKE receiver. In figure we can see that there are three multipath components with different delays t, t, ) and attenuation factors ( 1 2 t3 ( a 1, a2, a3 ), each corresponding to a different propagation path. The rake receiver has a receiver finger for each multipath component. In each finger, the received signal is correlated by a spreading code, which is time-aligned with the delay of the multipath signals. After dispreading, the signals are weighted and combined. In figure 11 maximal ratio combining is used (see the next paragraph about MRC). ( 1 c t t ) Modulator t 1 t 2 a 1 a 2 Demodulator c( t t 2 ) a 1 a 2 t 3 a 3 a 3 Code generator Multipath channel c( t t 3 ) RAKE receiver Figure 11 Principle of RAKE receiver (Source [17]). 29

32 Maximal Ratio Combining (MRC) Figure 12 illustrates points 2 and 3 by depicting modulation symbols (BPSK or QPSK) as well as the instantaneous channel state as weighted complex phasors. To facilitate point 2, WCDMA uses known pilot symbols that are used to sound the channel and provide an estimate of the momentary channel state (value of the weighted phasor) for a particular finger. Then the received symbol is rotated back, so as to undo the phase rotation caused by the channel. Such channelcompensated symbols can then be simply summed together to recover the energy across all delay positions. This processing is called Maximal Ration Combining (MRC). Transmitted symbol received signal at each time delay modified with the channel estimation combined symbol Fingure 1 Fingure 2 Fingure 3 Figure 12 The principle of maximal ratio combining within the CDMA Rake receiver (Source[1]). 30

33 Appendix B Relation with repeater dominated distance and it s loss It is very important to set the repeater loss to cover the essential (identified circular area) area. Otherwise the system might behave worse. It means the system interference might increase instead of decrease then outage will be higher than without repeater used in the hotspot. Repeater loss=l d Repeater S Figure 13 Configuration of Repeater loss and coverage Here we will try to find a mathematical relation between repeater loss l and it s dominated distance d (distance from repeater to it s covered boundary). l ) n n * d = ( S d ± (15) Here n is path loss exponent. After some manipulation with above equation we can write the final relation as bellow: S d ± = (16) 1 n l 1 We will show the dependence of distance (d - ) on repeater loss by figure 14a and 14b bellow and how the repeater loss will effect to the system will be shown in simulation results. 31

34 100 Repeater dominated distance VS repeater loss Repeater dominated distance d Repeater loss l Figure 14a Relation of repeater dominated distance with its loss at 195m of repeater and hotspot position. 180 Repeater dominated distance VS repeater loss Repeater dominated distance d Repeater loss l Figure 14b Relation of repeater dominated distance with its loss at 340m of repeater and hotspot position. 32

35 Increased coverage and repeater loss With repeater implementation we have arise an another important parameter. That is the increased coverage (d) (see in figure 15) of the mother cells. From this increased coverage one can understand approximately how many users (as users are homogeneously distributed) from neighbouring cells are connected to the mother base-station via repeater and from this interference behaviour of the system. That is why we will find how the coverage increase with repeater loss. If in the real cell boarder (increased cell boarder) we have user then for that user we can write following equation. Cell boundary before repeater implementation Real Cell boundary with repeater implementation Repeater loss=l d R-d x Repeater Base-Station R R Base-Station Figure 15 Configuration of Repeater loss and increased coverage = (17) PLbsA l * PLr PLbsB Where l = repeater loss, PL bsa = Pathloss from user to BS A, PL r = Pathloss from user to repeater and PL bsb = Pathloss from user to BS B. PL bsa bsb l = * (18) PL bsa * PL PL bsb 1 PL r converting pathloss to the distance and pathloss exponential we can write the following equation (from figure 15): l = n n ( R + d ) *( R d ) 1 * ( R + d ) n ( R d ) n ( d + x) n The relation between pathloss and increased coverage is shown with the nest two graphs for two repeater locations. (19) 33

36 150 Increased coverage with repeater implementation VS repeater loss Increased coverage d with repeater implementation Repeater loss l Figure 16a Relation of increased coverage with reepater loss at 195m of repeater and hotspot position. 150 Increased coverage with repeater implementation VS repeater loss Increased coverage d with repeater implementation Repeater loss l Figure 16b Relation of increased coverage with repeater loss at 340m of repeater and hotspot position. 34