Interference Analysis of a Total Frequency Hopping GSM Cordless Telephony System 1

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1 Interference Analysis of a Total Frequency Hopping GSM Cordless Telephony System 1 Jürgen Deißner, André Noll Barreto, Ulrich Barth*, and Gerhard Fettweis Endowed Chair for Mobile Communications Systems Dresden University of Technology, D Dresden, Germany Tel , Fax {deissner noll fettweis}@ifn.et.tu-dresden.de * Alcatel Corporate Research Center, D-0499 Stuttgart, Germany ubarth@rcs.sel.de ABSTRACT In this paper we present an interference analysis which reveals capacity reserves for a hierarchical GSM radio network. In the investigated concept a low-power Cordless Telephony System reuses the frequencies of a cellular GSM system based on Total Frequency Hopping (T-FH) multiple access. Based on a novel simulation method, which considers the dynamics of a T-FH cellular network, we present results for different cordless user densities and numbers of T-FH frequencies which exhibit the benefits of our CTS concept. Moreover, we show two possible ways to further improve the CTS downlink performance in case of difficult scenarios. I. INTRODUCTION Solutions for hierarchical radio networks where different layers share the same channels were already discussed earlier, e.g. for hot-spots. An application scenario for GSM radio networks providing low-power cordless indoor coverage was introduced as a home base station [1]. This culminated within ETSI into the current standardization efforts on CTS. A GSM Cordless Telephony System (CTS) [2,3,4] consists of a low-cost GSM base station, the CTS Fixed Part (CTS-FP) and a modified GSM mobile station, the CTS-MS which can work either in the conventional cellular or in the new cordless mode. The CTS-FP is connected to the fixed network like for example a DECT Fixed Part, however it uses GSM frequencies. In order to convince a GSM network operator to share his spectrum for GSM cellular operation with GSM cordless systems, our simulations have to reveal under which circumstances a GSM CTS system is able to perform well including: a dense GSM macrocell structure and a tight reuse scheme, different CTS user densities, different numbers of reused frequencies or a small number of separate frequencies for CTS use only (e.g. for an introduction and test scenario), and different GSM-to-CTS transmit power ratios. II. DISCUSSION OF PREVIOUS WORK ON TOTAL FREQUENCY HOPPING 1 In [1] a new multiple access method, called Total Frequency Hopping (T-FH), was presented for such a CTS system. It is a slow FH technique, which uses as much of the cellular GSM frequencies for FH in the CTS systems as agreed with the GSM operators and the regulator, ideally the whole GSM frequency range (e.g. 124 frequencies in GSM900). The more frequencies are used the more obvious is the effect of averaging the CTScaused interference over a lot of cellular GSM channels. In that paper it was concluded that cellular GSM and CTS systems can co-exist, if the GSM-to-CTS power ratio is well over or around 20 db for 25 or 5 FH frequencies, respectively. However, the assumed simulation environment does not represent a very dense urban residential area, which we expect as a worst case as well as one of the first deployment scenarios. Furthermore, the applied Monte-Carlo simulation method with SIR values for GSM bursts calculated for totally independent snapshots of the system model does not allow for an analysis of the time correlation of SIR values according to the dynamic environment. Especially, neither the ability of a FH algorithm to avoid sequences or accumulations of hits to the same frequency can be shown nor the ability of a GSM receiver to recover from poor SIR values in few bursts due to the deinterleaving and decoding gain. In [5] a FH algorithm is presented that is especially designed for the requirements of a T-FH GSM CTS system. Together with the GSM CTS concept proposed by Alcatel [4], which incorporates the T-FH technique, this is the basis for our simulations. III. SIMULATION ENVIRONMENT Our model of the cellular GSM radio network represents a dense urban environment with a high traffic load, because this is likely to be also an area of high GSM cordless user penetration. Our worst case assumptions are listed in the tables 1-6. As the result of the discussions within ETSI these assumptions were mainly agreed for their CTS simulation studies except for the BS-to-BS distance (ETSI: 500m), the portion of GSM-MSs being indoor 1 This work was supported by Alcatel.

2 (50%), the BCCH timeslot consideration (not modelled), and the CTS transmit power (1dBm, not varied) [6]. A. Radio network configuration cellular GSM regular hexagonal cells 3/9 reuse structure BS-to-BS distance D = 50 m cell radius R = 250 m all GSM-BS and GSM-MS are outdoor D R Table 1. System layout Table 2. Frequency usage and traffic load GSM 3 transceivers per cell result in 2 available frequencies 2 ; no frequency hopping BCCH modelled by the continuous occupation of the downlink timeslot 0 at one of the frequencies in every cell SDCCH not considered, but handled like a further TCH 23 available TCHs per cell 15.3 Erlang offered traffic (corresponds to 1.6% blocking probability and 65% mean load) New calls that encounter 23 occupied TCHs in their best-server-cell are blocked. exponentially distributed call interarrival time; mean call arrival rate per cell: 0.1/s exponentially distributed call hold time; mean value: 90 s Table 3. MS Movement GSM-MS MS s are generated for the duration of a call. The initial positions at the beginning of a call are uniformly distributed within the investigation area. The speeds are uniformly distributed within [0, vmax = 20m/s). The speed remains constant throughout the call. The directions are uniformly distributed within [0,2π). The direction remains also constant throughout the call. However, an MS will be reflected at the border of the investigation area. Initial assignment and handover (without a hysteresis) is carried out due to the bestserver-criterion including the 6 neighbouring cells. The update distance for the position and best- server-decision is 10 m. Table 4. CTS transmission characteristics CTS CTSs are generated within round buildings with a radius of 20 m, which are uniformly distributed over the investigation area. The number of occupied buildings corresponds to the CTS density. The CTS density [1/ km 2 ] is a simulation parameter. The position of a CTS-FP and a CTS-MS in its building upon generation is uniformly distributed over the building area. all CTS-MS and CTS-FP are indoor CTS Total Frequency Hopping is applied Every CTS user is provided with a unique T-FH sequence. The number of frequencies that are reused by the CTS systems for T-FH is a simulation parameter. Each CTS-FP supports only 1 CTS-MS. 0.1 Erlang per CTS-MS exponentially distributed call interarrival time; mean call arrival rate per cell: /s exponentially distributed call hold time; mean value: 120 s CTS-MS The initial positions at the beginning of a call are uniformly distributed within the building of its CTS-FP. The speeds are uniformly distributed within [0, vmax = 1.5m/s). The speed remains constant throughout the call. The directions are uniformly distributed within [0,2π). When reaching the update distance, a new direction is taken from that distribution. The update distance for the position and best- server-decision is 1 m. CTS-FP CTS-MS The CTS Fixed Part (CTS-FP) and CTS Mobile Station (CTS-MS) transmit power [dbm EIRP] is a simulation parameter. (Our reference scenario for comparisons uses 1 dbm.) Both, CTS-FP and CTS-MS, have an omnidirectional antenna. The CTS systems are modelled to be timeslot-asynchronous. Table 5. Cellular GSM transmission characteristics GSM-BS GSM-MS transmit power: 33 dbm transmit power: 33 dbm sector antenna with a horizontal pattern omnidirectional antenna having a 3dB-beamwidth of 60 o and a frontto-back ratio of 40 db 54 dbm EIRP due to 21 db max. gain (vertical pattern not considered) Furthermore, for cellular GSM and CTS we assume: Neither power control nor DTX is applied. The noise floor, including thermal noise and a noise figure, is assumed with -114 dbm. B. Propagation models 2 All propagation models are two-dimensional, statistical path loss models and based on the assumption of a heterogeneous environment. The shadowing S(σ) follows a log-normal distribution with standard deviation σ and is uncorrelated between locations, but does not change as long as the position remains the same 3 (Table 6). In particular, the shadowing value does not change during an interleaved block of 8 consecutive bursts in a certain channel (refer to IV.A). Table 6. Path loss equations GSM-BS - GSM-MS Walfish-Ikegami [03.30] with f = 900 MHz, hbs = 25 m, hms = 1.5 m, hroof = 15 m, w = 20 m, b = 50 m, ϕ = 90 o d m S db for d m LGG = lg( / ) + ( 8 ) lg( d / m) + S( 8dB) for d > 21m GSM-BS - CTS-MS LGC = LGG + Lindoor with Lindoor = 12dB CTS-FP - GSM-MS Walfish-Ikegami [03.30] with hbs = 4 m and Lindoor = 12 db L CG lg( d / m) + S( 8dB) + Lindoor for d 23. 5m = lg( d / m) + S( 8dB) + Lindoor for d > 235. m CTS-FP - CTS-MS linear attenuation indoor model [COST231] with attenuation coefficient α = 0.9 db/m up to the CTS building diameter of 40 m L CC lg( d / m) + S( 6dB) d / m for d 40m = lg( d / m) + S( 8dB) + 2 Lindoor for d > 40m IV. PRINCIPLES OF SIMULATION AND INTERFERENCE ANALYSIS A. Novel Simulation Method Our C++ simulation program combines the discrete event (DE) and random sampling simulation methods. The DE simulation provides signal-to-interference ratios (SIR) that are correlated in time due to the dynamics of the modelled cellular network caused by the user traffic, the movement of the users during calls, and the applied total FH multiple access technique in the CTS. The time basis for the DE simulation is the TDMA frame period T TDMA = ms. Through random sampling we select sequences of SIR values in one and the same GSM radio channel 2 The allocation of these 2 frequencies within the cluster is fixed. However, the order is not important because adjacent channel interference is not considered. 3 These and all further assumptions for the 4 propagation models comply with the definitions for the studies within ETSI [6].

3 (defined by frequency and timeslot) at random instants during an active call for post-processing in a Matlab program. We chose the TCH interleaving depth of 8 T TDMA as the length of such a sequence in order to evaluate the influence of the dynamics in the cellular radio network on the TCH performance with consideration of a decoding and deinterleaving gain in the receiver. In order to start the DE simulation in the stationary state, a random number of active GSM-MS and CTS-MS is already generated in the simulation setup according to the traffic values. B. Investigation area Three tiers of interfering clusters are taken into consideration (Fig. 1). The GSM BS and MS in all clusters cause interference, but only in the central cluster the GSM BS and MS interference is evaluated. CTSs which cause interference are generated within a radius of 1100 m, but the CTS-FP and CTS-MS interference is only evaluated within the inner radius of 00 m. 1 4 Fig. 1. Investigation area. C. Interference Analysis evaluated cluster 1st tier of interfering clusters 2nd tier 3rd tier CTS evaluation area CTS interference area investigation area Only co-channel interference is evaluated. Signal and interference power levels are mean values for the whole timeslot duration of T TS = 5 µs. The GSM BS and MS are timeslotsynchronous in our model. However, asynchronous CTS are modelled by a constant timeslot offset [ 0, T TS ) that is randomly chosen for each CTS-FP. Overlapping power portions from asynchronous interferers are averaged over the whole timeslot duration of the interfered receiver. The interference performance at all 4 network element types, GSM-BS, -MS, CTS-FP, -MS, can be studied in one and the same simulation. We are always able to distinguish the interference that is caused by the GSM cellular system from the total interference, which includes the CTS- caused interference. We apply the two following performance criteria to the SIR distributions that we get for each of the 4 network element types from our DE simulation: the outage probability P out = P[ SIR < 9 db ] as a model for signal detection, i.e. an outage appears if the SIR of a burst is smaller than the threshold of 9dB, and the frame erasure rate (FER), which we define as the probability that within a sequence of 8 bursts of consecutive TDMA frames 4 or more encounter an outage, as a model for the deinterleaving and decoding gain [9]. V. SIMULATION RESULTS For comparison we defined a reference scenario with 2 frequencies for Total Frequency Hopping (T-FH) in the CTS 4, 1 dbm CTS transmit power, and a density of 1500 CTS/km 2. Based on that (section V.A), we performed simulations with the parameter variations as follows: CTS density between 50 and 5000 CTS/km 2 in shared band operation with 2 T-FH frequencies as well as in separate band operation with 3 T-FH frequencies (section V.B), number of T-FH frequencies between 13 and 124 (sect. V.C), CTS-FP transmit power between 13 and 38 dbm (sect. V.D), a reduced frequency list with 24 and 6 shared T-FH frequencies (section V.E). A. The reference scenario for comparison Table summarizes the results for our reference scenario. We observe the strongest influence in the outage probability of the GSM uplink: a CTS- caused increase from 0.9% to 1.8%. The GSM uplink is the more sensitive GSM link due to the lower transmit power of a GSM-MS in comparison with the GSM-BS. The corresponding FER values indicate that the T-FH algorithm distributes the CTS interference in time and in frequency across all GSM connections in a way that the GSM receivers mostly are able to recover from single hits within interleaved blocks according to our rough model for the GSM receiver's deinterleaving and decoding gain. The CTS uplink also shows a reasonable performance, but the CTS downlink has a poor outage probability. In spite of an assumed average outdoor-to-indoor penetration loss of 12 db, the CTS-MS suffers seriously from GSM-BS-caused interference levels, which are considerably high in comparison with the signal level received from the CTS-FP. This is due to our model of the GSM-BS, which always transmits with its maximum power without power control and DTX for interference reduction. The CTS downlink is effected so strongly 4 In the case of a cellular GSM 3/9 cluster with 3 frequencies per cell, we consider the reuse of all those 2 GSM frequencies in the co-existing CTS as a worst case because the mutual interference between the cellular and the cordless systems is the most.

4 Table. Performance results for 1 dbm CTS transmit power, 1500 CTS/km2, and 2 shared T-FH frequencies Outage probability Frame Erasure Rate (FER) total, with CTS GSM-caused part total, with CTS GSM-caused part GSM uplink 1.8 % 0.9 % 0.9 % 0.9 % GSM downlink 0.9 % 0. % 0. % 0. % CTS uplink 2.9 % 2.6 % 0.2 % 0.1 % CTS downlink 25.5 % 25.3 % 22.8 % 22. % because of the different power ratios of the base stations and of the handsets of both systems; the GSM-BS-to-CTS-FP ratio is 25 db depending on the angle to the main lobe of a GSM-BS antenna, whereas the GSM-MS-to-CTS-MS ratio is only 16 db. B. Different CTS densities Table 8 shows the dependence of the performance on the CTS density in the case of 1 dbm CTS transmit power and shared band operation with 2 T-FH frequencies. The results for zero density represent the GSM-caused part of interference. In the range of 50 to 5000 CTS/km 2, which already covers a very optimistic maximum CTS user penetration, the CTS influence is not strong and only visible in the GSM uplink. The GSM uplink outage probability only varies in the range of 0.9% to 4.4%, the FER only in the range of 0.9% to 1.2%. Table 8. Outage probability (P out) and frame erasure rate (FER) without CTS and for densities between 50 and 5000 CTS/km 2 CTS density [1/km 2 ] GSM uplink Pout 0.9 % 1.4 % 1.8 % 2.8 % 4.4 % GSM uplink FER 0.9 % 0.9 % 0.9 % 0.9 % 1.2 % GSM downlink Pout 0.8 % 0.9 % 1.0 % 1.2 % 1.5 % GSM downlink FER 0. % 0.8 % 0. % 0. % 0.8 % CTS uplink Pout 2.5 % 2.6 % 2.9 % 3.2 % 3.6 % CTS uplink FER 0.1 % 0.1 % 0.2 % 0.2 % 0.4 % CTS downlink Pout 25.3 % 25.4 % 25.5 % 26.0 % 26.2 % CTS downlink FER 22. % 23.0 % 22.8 % 23.4 % 23. % small set of frequencies. However, it can be a solution for the CTS introduction and for medium CTS densities. C. Different numbers of frequencies for T-FH in the CTS Fig. 2 illustrates the relation between the GSM performance and the number of T-FH frequencies for 1 dbm CTS transmit power and 1500 CTS/km 2. As expected, the results reveal the increase in the effect of interference averaging, when considerably more than 2 GSM frequencies are used for T-FH in the CTS. Outage Probability GSM uplink, total interference GSM uplink, GSM caused interf. GSM downlink, total interference GSM downlink, GSM caused interf No. of frequencies for T FH Fig. 2. GSM outage probability for T-FH frequencies. D. Different CTS-FP transmit powers In this scenario with increased CTS-FP transmit power we expect similar results like in a scenario with interference reduction features at the GSM-BS. Fig. 3 shows an exchange of performance between GSM-MS and CTS-MS for increasing transmit powers, where the reduction of outage probability and FER at the CTS-MS is considerably stronger than the increase at the GSM-MS. This indicates that we can reach a reasonable performance in the CTS downlink by means that reduce the GSM-BS transmit power in relation to the CTS-FP transmit power. The CTS results also show the poor CTS downlink performance. However, we can still learn from the CTS uplink performance, which shows only a CTS-caused change in the total outage probability from 2.6% to 3.6%, that the reserves in the usage of the GSM frequencies for CTS are not exhausted yet - also for the reason that we only modelled two-dimensional propagation and did not consider floor losses. For the same reasons, this principle may also be applied to GSM picocells with the advantage of avoiding frequency planning. Outage probability or Frame Erasure Rate GSM downlink outage probability GSM downlink FER CTS downlink outage probability CTS downlink FER We also investigated the influence of the CTS density in the case of 3 T-FH frequencies solely dedicated to CTS operation. The results exhibit a linear increase of the outage probability in both, CTS uplink and downlink, in the range of 1.3 % to 8.6 % at /km 2. Using only 3 frequencies shows that the main T-FH advantage of interference averaging is not gained at this CTS transmit power [dbm] Fig. 3. GSM downlink performance for CTS-FP transmit powers of dbm.

5 E. A reduced frequency list for T-FH A further way to improve the CTS performance is to exclude the frequencies of the strongest interferers from the list for T-FH and thus to prevent both, the generation and the reception of interference on these frequencies. However, the less frequencies are available for T-FH the smaller is the interference averaging effect. Possible strategies for a reduced frequency list are to exclude: all frequencies of the potential serving cell, all BCCH frequencies of the potential serving and their neighbor cells (expecting that the other frequencies are not occupied with the maximum BS transmit power due to downlink power control), all BCCH frequencies (as before) and all further frequencies of the potential serving cell, all frequencies of the potential serving and their neighbor cells, the frequencies of the strongest interferers due to measurements. For the latter two cases (18 from 2 available GSM frequencies excluded due to measurements and 21 frequencies excluded due to the frequency plan) we already carried out simulations for the scenario with 1 dbm CTS transmit power and 1500 CTS/km 2. The results presented in Table 9 indicate a considerable performance improvement already for reduced frequency lists. But as expected, the interference portion that is caused by other CTS is visible now, because the CTS have to share in only 6 or 9 frequencies within a certain area. Table 9. First results for a reduced frequency list (gray) in comparison with the respective results for our reference scenario and for separate band operation with 3 T-FH frequencies [GSM-caused part of interference in square brackets] Number of T-FH frequencies 2 shared 6 shared (ideal AFA) 9 shared (measurements) CTS-FP Pout 2.9 % [2.6 %] 1.3 % [0.1 %] 0.8 % [0.3 %] 2.8 % CTS-MS Pout 25.5 % [25.3 %] 18.5 % [1.2 %] 18.4 % [18.0 %] 2. % VI. SUMMARY 3 separate Our simulator applies a novel method that allows for the evaluation of the dynamics of the cellular environment and especially of the algorithm for Total Frequency Hopping in relation to the deinterleaving and decoding gain of a GSM receiver. Our simulations of the CTS density (up to 5000/km 2 ) indicate, that the increase of that parameter in the shared band scenario with 2 frequencies does not significantly deteriorate the interference performance neither in the GSM nor in the CTS system. The main advantage of the T-FH multiple access method, the effect of interference averaging, could be shown in the GSM results for the varying number of T-FH frequencies. Facing the poor CTS downlink performance in our reference scenario, we showed that the GSM-BS-to-CTS-FP power ratio should be taken into consideration. As a way for improvement, a reduced frequency list was proposed. First simulations with such a list indicate a considerable decrease in interference. Therefore we will further investigate the gain that can be achieved by a reduced frequency list. Results for the consideration of a GSM microcell layer can be found in [10]. VII. CONCLUSIONS We presented an interference analysis of a hierarchical GSM radio network which reveals capacity reserves in the GSM for CTS or GSM picocells. The investigated CTS concept benefits from the Total Frequency Hopping multiple access technique, which allows for low-cost CTS Fixed Parts that make use of GSM handset chipsets with only modified software and protocols. The simulation results revealed poor CTS downlink performance for the assumptions of our reference scenario due to not modelled interference reduction features. However, for improvement we proposed a separate consideration of the transmit power ratios in uplink and downlink and a reduced frequency list. It is reasonable to assume that these observations are not only GSM-specific. Therefore, a similar hierarchical concept should early be considered in the UMTS standardization efforts. REFERENCES [1] M.I. Silventoinen, M. Kuusela, and P.A. Ranta, Analysis of a New Channel Access Method for Home Base Station, in Proceedings of the ICUPC 96, 1996, pp [2] G. Zimmermann, W. Stahl, and R. Toy, GSM Based Cordless Telephone System in Cellular Environment, in Proceedings of the EPMCC 9, 199, pp [3] Digital cellular telecommunications system (Phase 2+); GSM Cordless Telephony System (CTS), Phase 1; Service Description, TDoc SMG1 509/9, ETSI 199. [4] Low interference GSM-Cordless Telephony System description, TDoc SMG2 WPB 121/9, ETSI 199. [5] A. Noll Barreto, J. Deißner, G.P. Fettweis, A Frequency Hopping Algorithm for Cordless Telephone Systems, to be presented at ICUPC 98. [6] Definition of the Interference Environment for the Evaluation of the Interference Performance of GSM-CTS Radio Interface Concepts, TDoc SMG2 WPB 69/98, ETSI [] Digital cellular telecommunications system; Radio network planning aspects (GSM v ), ETR 364, ETSI [8] Draft COST 231 Final Report, COST231 TD(96)042-D, [9] M. Mouly and M.B. Pautet, The GSM system for mobile communications, [10] Interference Performance of GSM-CTS: Simulation Results, TDoc SMG2 WPB 95/98, ETSI 1998.