Capacity Planning in Wideband Cellular Networks

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1 Volume, Issue, February 0 ISS - Abstract The performance of any cellular mobile networks is determined by the coverage and capacity. This paper deals with the coverage and capacity issues of upcoming G (Third Generation) UMTS (Universal Mobile Telecommunication Systems). UMTS uses CDMA (ideband Code Division Multiple Access) as the radio transmission technology. The CDMA is interference limited, and this leads to a scenario where more users are admitted into the network the cell size shrinks. Therefore coverage and capacity planning is done simultaneously. It is observed that the capacity depends on different parameters such as energy per bit noise spectral density ratio, voice activity, inter cell interference and intra cell interference, etc. the coverage depends on frequency, chip rate, bit rate, mobile maximum power, MS antenna gain, EIRP, interference margin, noise figure etc. Different parameters that influence the capacity and coverage of UMTS cellular network are simulated using MATLAB. The outputs of simulations shows that number of users decreases for higher bit rates, noise rise increases with increasing number of users( or throughput). And also for a given number of users (or throughput) the noise rise increases with interference factor. Keywords: UMTS, CDMA, coverage, capacity, load factor, noise rise. ITRODUCTIO Capacity Planning in ideband Cellular etworks Ch Usha Kumari Professor, Department of E.C.E, Gokarau Rangarau Institute Engineering and Technology, Hyderabad, India The ideband Code Division Multiple Access (CDMA) is a wide band spread spectrum channel access that utilizes the direct sequence spread spectrum method of asynchronous code division multiple access to achieve higher speeds and support more users. It is the third generation (G) mobile cellular communication system.[] It is the technology behind the G UMTS standard and is allied with the G GSM standard. It is base on radio access technique proposed by ETSI Alpha group and the specifications was finalized. The attractive feature of CDMA is the very high data rate (up to Mbps) to support multimedia services (voice, video, and internet) and the internetworking with Global System for Mobile Communications (GSM) and General Packet Radio Service (GPRS). In addition CDMA has some performance benefits, such as increased multipath diversity []. In network planning, parameter estimations like large area coverage, highest capacity per km, soft capacity (Enlarge capacity), tracking efficiency and blocking probability. A typical -CDMA system can either be coverage or capacity limited. This is due to the system been interference limited, in that as more users are allowed in the cell area, the noise level increases and therefore reduces the cell size. This work assumes that CDMA radio interface is uplink limited with the mobile station transmit power being the maor limitation, therefore only reverse link capacity is considered. Although recent trend has shown that CDMA can be downlink limited this is due to the asynchronous nature of the radio interface with subscribers downloading at very high data rate and lot of work has been done in this concept. In this paper the factors affecting the coverage area and capacity are estimated. There exist a relationship between a given noise rise, load factor and the associated number of users which may be calculated using a simple uplink capacity technique called load equation as given by [],[]. This is modified for imperfect power control for practical scenario resulting from Multipath propagation as described by [][]. In UMTS network, network coverage and system capacity are linked, because the same carrier frequency is used over the radio network and moreover other user s signals are seen as additional interference. Hence attention should be paid on coverage and capacity planning phases simultaneously in UMTS radio network planning []. Capacity determines the maximum number of users that a particular cell can accommodate without compromising in providing good quality of service for the end users [0]. The capacity of UMTS network is known to be interference limited. If the maximum capacity is limited by the amount of air interface, it is by definition a soft capacity, since there is no single fixed value for maximum capacity []. The approach used to determine capacity is modified load equation. Volume, Issue, February 0 Page

2 Volume, Issue, February 0 ISS -. PAGE SIZE AD LAYOUT A. The Link Budge. The link budget of CDMA (G) has some specific parameters that are not used in GSM (G). The most important are [],[],[] i. Interference-margin: is needed in the link budget because of the loading of the cell, the load factor affects the coverage, the more loading is allowed in the system the large is the interference margin needed in the uplink and the smaller is the coverage area. For coverage limited cases, smaller interference margin is used (- db) corresponds to 0-0% loading. hile in a capacity limited cases a larger interference margin is used. ii. Fast fading margin (= power control headroom): Some headroom is needed in the mobile station (MS) transmission power for maintaining closed loop fast power control to effectively compensate the fast fading []. Typical values of the fast fading margin are - db for slow moving mobiles. iii. Soft handover gain: Handover (soft or hard) gives gain against slow fading (Log normal fading) by reducing the required Log-normal margin. Since slow fading is partly uncorrelated between the Base Stations (BSs), by making a handover, the mobile can select a better BS. Soft handover gives additional macro diversity against fast fading by reducing the signal power to noise power (Eb/o) relative to a single radio link. The total soft handover gain is assumed to be between - db in the example below including the gain against slow and fast fading. Three examples of link budgets are given for typical universal mobile. Telecommunication Services (UMTS) which implements CDMA.. kbps voice service using adaptive multirate (AMR) speech codec. kbps real time data. kbps non-real time data. In the urban macro-cellular environment at the planned uplink a noise rise of db and interference margin of db is reserved for the uplink noise rise. B. Load Factors The load equation is commonly used to make semi-analytical estimation of the average capacity of the CDMA cell, without going into system level capacity simulations. There are a number of elements that influence on the UMTS network load, which define the maximum number of users per cell. Moreover, the number of users and their bit rates has influence on the total throughput of a cell []. Also the activity factor in speech and data services affects the load. However, the most important contributor to the load is the E b / 0 requirement, which depends at least on the service type, the data rate of the service, the propagation conditions, and the receiver performance. If a very good quality (i.e high E b / 0 requirement) is desired, more bits are needed for the error correction in order to guarantee the quality of the connection. Thus when more bits are used for the error correction, the air interface load and capacity for the traffic channels are decreased []. The own-cell and other-cell interference influences on load of a cell. If the total received noise is high, also high transmit powers are needed in order to guarantee the communication. Hence, it may happen that the transmit power runs out. The noise received from other cells depends also on the environment. In urban areas, cells are isolated better than that in the rural areas. However, the radio network plan has probably the most significant influence on the interference [], []. An accurate radio network plan guarantees low interference levels and enables good quality and appropriate capacity. The following assumptions are made while determining the capacity Each cell is completely isolated. That is, there is no inter-cell interference. There is perfect power control from the base station. That is, all signals arrive at the base station with equal power. There is no limit to the number of spreading codes available. C. Uplink Load Factor -CDMA radio interface offers enormous possibilities in obtaining large capacities however; it imposes many limits as regards the acceptable level of interference in the frequency channel []. In every cellular system with spreading spectrum, the radio interface capacity is drastically limited due to the occurrence of interference. The theoretical spectral efficiency of a CDMA can be calculated using equation () shown below. This is the total load for the uplink. Volume, Issue, February 0 Page

3 Volume, Issue, February 0 ISS - ( i ) Eb R v o () where is the load factor for uplink, i is the interference power, Eb is the signal energy per bit divided by noise spectral power density that is required to meet a predefined block error rate, is the number of users per cell, is the CDMA chip rate (chips/sec), R is the bit rate of user, v is the activity factor for user and is 0. for voice and for data[]. The term i is also defined as other cell interference over own cell interference.the i factor depends on propagation environment, overlapping cell areas, the number of sector in the base station, the traffic intensity and its distribution, as well as on the distance to the serving and interfering cells[]. For voice service network, where all users in the cell have low bit rate (R) i.e. and hence the above equation simplifies to, E b Eb Rv o D. Modification of load equation for imperfect power control: Due to the multipath propagation, the variation in 0.. v( i ) () R E b 0 o is according to log-normal distribution with standard deviation in the order of.db and.db as described by []. Therefore in the case of imperfect power control, the constant targeted value of E b 0 for users with service needs to replaced by its expected value. The uplink load equation can be modified as, ( i ) () E b ( c ) e R v o here (ln0)/ 0 c is the power control deviation. E. oise Rise Volume, Issue, February 0 Page

4 Volume, Issue, February 0 ISS - oise Rise can be calculated as described by []. oiserise oiserise 0log ( )db 0 () () The load equation () is used to make a semi-analytical prediction of the average capacity of CDMA without going into system level simulation. Equation () can be used to predict cell capacity and noise rise. From equation () it can be seen that as becomes close to one, the corresponding noise rise approaches infinity and the system has reached its pole capacity. In terms of nose rise, here nr is the noise rise. nr ( i ) E b o ( e c ) R v () For noise rise calculation, the value of Eb is different for different data rates as given in table. o Table : E b / 0 for different services. Voice Data E b / 0 kbps Kbps Kpbs Kbps.dB.dB 0.dB db The below table shows simulation parameters for calculating channel capacity, Table : Simulation parameters for channel capacity. Parameter Eb/0 Power control deviation (σ c ) CDMA chip rate (chips/sec) Bit rate of the user Voice.dB.dB.Mcps kbps Interference power(i) 0. Volume, Issue, February 0 Page

5 Volume, Issue, February 0 ISS -. SIMATIO RESTS Activity factor(v) 0. The analysis is done for capacity by considering the load equation under imperfect power control. The parameters are selected as shown in the table. Using MATLAB the simulated results are shown. umber of Users versus oise rise 0 oise Rise umber of. Kbps Users Firgure: number of users for. Kbps vs oise Rise Figures to shows the noise rise versus number of users for different data rates. It can be seen from the figures to that for a constant nois rise value, the number of users decreases for higher bit rates. Figure shows the oise rise vs number of users. From figure it is seen that noise increases for increase in number of users. umber of Users versus oise rise 0 oise Rise 0 0 umber of Kbps Users Firgure: number of users for Kbps vs oise Rise Volume, Issue, February 0 Page

6 Volume, Issue, February 0 ISS - umber of Users versus oise rise 0 oise Rise 0 umber of Kbps Users Firgure:number of users for Kbps vs oise Rise umber of Users versus oise rise 0 oise Rise oise RIse umber of Kbps Users Firgure:number of users for Kbps vs oise Rise x umber of USers Firgure:number of users for Kbps vs oise Rise Volume, Issue, February 0 Page

7 Volume, Issue, February 0 ISS -. COCLUSIOS In this paper the various design parameters are estimated for different data rates (.Kbps to Kbps for voice). The simulation results shows noise rise verses number of users for different data rates and we can conclude that number of users decreases for higher data rates. It is also seen that noise rise increases with increasing in number of users. And for constant noise rise the number of user s decreases for higher bit rates. References [] eimela J: Impact of base station site antenna configuration on capcity in CDMA cellular network, Master of Science thesis, University of Technology, Sweden, 00. [] Laiho J.F., Jasberg M., Sipila K., acker A., Kangas, Comparison of three diversity handover algorithms by using measured propagation data, IEEE Proceedings Vehicular Technology Conference, pp. 0-, Spring. [] Virterbi, A.J CDMA Principles of Spread Spectrum Communication. Addison-esley, [] awrocki, M. J., Dohler, M., Aghvami, A. H., Understanding UMTS Radio etwork", John iley & Sons, Ltd, 00, Chapter 0. [] Holma, H., Toskala, A., "CDMA for UMTS"., rd Edition, John iley & Sons, 00 [] E.H. Dinan and B. Jabbari, "Spreading codes for direct sequence CDMA and wideband CDMA cellular networks", IEEE Communications Magazine, vol., Issue, pp. -, Sept.. [] K.S. Gilhousen, I.M. Jacobs, R. Padovani, A.J. Viterbi, L.A. eaver and C.E. heatley, On the Capacity of Cellular CDMA System, IEEE Transaction on Vehicular Technology, vol.0, o., pp.0-, May,. [] T. Griparis and T. Lee, The Capacity of a CDMA etwork : A Case Study, Bechtel Telecommunications Technical Journal, vol., o., August, 00. [] Samuel C. Yang, CDMA RF System Engineering, Artech House, Boston- London. [0] Sharad Sambhwani, ei Zhang and ei Zeng, Uplink Interference Cancellation in HSPA: Principles and Practice, QUALCOMM Incorporated, 00. [] K. Ayyappan and R. Kumar, QoS based capacity enhancement for CDMA network with coding scheme, International Journal of VLSI design and communication systems (VLSICS), vol., o.,march, 00. [] B. Revision, Air Interference Cell Capacity of CDMA Systems, ESG Engineering Service Group, Qualcomm, May, 00. [] Kumari CU, Rao GS, Madhu R. Erlang Capacity Evaluation in Gsm And Cdma Cellular Systems. International Journal of Mobile etwork Communications & Telematics (Imnct) Vol. 0 Oct (). [] Kumari, Ch Usha, and M. Ramya Krishna. "HIGH PERFORMACE IRELESS COMMUICATIO CHAEL USIG LEACH PROTOCOLS. [] Kumari, Ch Usha, and G. Sasi Bhushana Rao. "Modelling of CDMA Base Station Signal in Multipath Environment." [].C. Jakes, Microwave Mobile Communications. IEEE Press,. [] S. Hamalainen, H. Holma, and A. Toskala, Capacity Evaluation of a Cellular CDMA Uplink with Multiuser Detection, IEEE Proceedings of the th International Symposium on Spread Spectrum Techniques and Applications, Vol., pp. -, September. [] B. Hagerman, F. Gunnarsson, H. Murai, M. Tadenuma, and J. Karlsson, CDMA Uplink Parallel Interference Cancellation - System Simulations and Prototype Field Trials, EURASIP Journal of Applied Signal Processing, vol., pp.-, 00. [] D. Staehle, K. Leibnitz, K. Heck, B. Schroder, A. eller, and P. Tran-Gia, Approximating the other cell interference distribution in inhomogeneous UMTS networks, IEEE th Vehicular Technology Conference,00. vol., pp.0, May 000 AUTHORE Ch.Usha Kumari, is a Professor in Gokarau Rangarau Institute of Engineering and Technology, Hyderabad. Completed Ph.D form Jawaharlal ehru Technological University, Hyderabad. She has completed M.Tech in Radar and Microwave from Andhra University College of Engineering, Visakhapatnam. She has years of teaching experience in Engineering Colleges. She published various research papers in various national and international conferences. Volume, Issue, February 0 Page