System capacity is considered a very spficant pvameter in Mobile Communication Systems. The most important reason for changtng the current cellular system ...
System capacity is considered a very s p f i c a n t pvameter in Mobile Communication Systems. The most important reason for changtng the current cellular system from Analog FDMA to DigitdTDMA is that many of the service areas are running out of capacity. That is because of the ever inmclliing popuIvrily of mobik lrnd portable telephones. This paper is aimed 011 evalruting the system capacity in Erlangs b a e d on the No& American Digital TDMA standards. The paper presenls a Monte Carlo simulation sludy of two different frequency plans, namely, the 7-cell clus(ers with 3 sectorslccll rod the 4 - 4 clu!W.s with 6 scctdccU Cclls arc assumcd to bc quai in radius and the user density is assumed (0 be uniformly distributed among the cells. In the forward carrier (Base Stalion to Mobile Ud) a mobile unit is dowed to move uniformly in a target sector and the carrier to Co-Channel intuference ratio (U)from aII co-channel base stations is evaluated at every location of the target mob& unit. The Erlang load versus the blocking probability is evaluated. The research also sludii the effect of the TDMA lime slot assignment technique when a mobile unit requests service. The propagation path loss is assumed to be proportional to r* with log-normal shadowing.
The h e t of cellular mobile conununications has increased dramalically during the lasl years. Recently the cellular industry starled selting the s(andmis for the new digital cellular communication system. lbe most important motivation behind this movc was thc lack of capady to provide service f a the Minumber of subscribers. According to the North Am.rkan standards the one RF c& that serves one subscriber in the analog system. will be able to serve three subscribers and e v e n t d y six subscribers using Time Division Multiple Access O M A ) technique. One might think that the capacity of the system will increase by the above ratio, or by whatever extra lraffic load, in Erlangs, the new number of available channels can carry. However, due to the special communication environment the mobile communication signals face, the capwily increase cwhJ n d be as expecled. The purpose of this paper is to study the system capacity in Erfangs for the TDUA aital cellular communication
systeoa An arbilrary system is assumed and simulated. The case of 3 sectorSlcell(1200 sectorizalion) and 7 ceWcluster is compared to the case of 5 seclors/ceU (60" seclrization) and 4 .-ec Only the forwacd channel (base station to mobile station) capacity is studied in this paper. It has been rep& that the capacity of the forward and the reverse channel are. with a good approximation. identical [I]. In thc Norch Amcrican standads 8 forward Radio Frequency (RF) carrier will ttpnsmil on all irs TDMA time slots even if only one slot is occupied. Hence. the time slot assignment teciuque can have a @icant effect on the capacity of the sys*m. This paper will compare the performance of two assignment techniques. These are called here the random assignment technique and the
pxklng tecww. The propagation environment is assumed to be susceptible to CoChannel Intderence, Raylcigh faand LogNormal shadowing. The path loss is assumed to be proportional to rn 131. w h e r is the distance and n is an appropriate number. The paper is organized as follows: the following section describes the cellular system under study and the method of calculating the Canier to CoChannel Section . 111 presents the simulation Interference Ratio (a) algorithm and the assum+ons used in the simulation. The last section is devoted for presenting and analyzing the results of the simulation study.
CA Cakulatims The systems under sludy are shown in Figures 1 and 2 for the lz00 sectorization -7 celllcluskr and 600 sectorimtion 4 cellsl cluster, respectively. The two systems are approlcirmtely of the same number of cells so that their capacily can be compared. For system 1 there are 11 cluslcrs.Hcncc thm arc 11 sectors that rcusc thc samc frequency. This number is 19 in system 2. CoChannel Inkference is assumed to be coming from sectors corresponding to only the first tier around each cell. Any f d e r interference is assumed negligibie. The profiles are assumed ideal, that is. with d t y g& (he ham width and zero gain outside. Therefore. in System 1 evety sector receives interference from a of 2 o(her =[ws. In syslem 2 every secb receives
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interference from a maximum of one sector. It is a known fact for cellular mobile communications that the UI is the most imporlanl parameter that affects both the voice quality and system capacity. A h i h traffic load mcans htgh RF activity and htgh interference and hence IOWquaby. Thus a minimum acceptable voice quality sels a limil on the maximum capacity of the system. In analug cehlar comunications the voice quality used to decrease grxiually with the degradation in U,but in digital cellular it is expected that the voicc quality will be acceptable with the degradation of cfl up to sonw value &r which the qualily will suddenly decrease very rapidly t o w a d s an unrcceptabh level. That is because of the frequent loss of bit and frame s y n c h o d o n sad lugh error rah and the inability of the source and c b l e h to perfam in satisfactorily. In this paper we will take UI of 17 dB as the minimum q u i r a d value for axephble pclrormance [2]. This 17 dB include the fade margin due (0 the Raykigh fading environment.
According to (he North American new digital cellular s l d d s U forwanl clrrricr is d i v e in all TDMA rime slots even if only one of the three time slots is active. Therefore the techque used in rinse slot assignment for arriving calls can grcdy affect the UI. In this paper we study 2 time slot as ' nmcnt khnigues. namely, the random assignment a n d 2 packing assignment A)
B)
In lhe random assignment an incoming call is ass@ randomly with equal probability to any available time slot in the sector where call is connected. In &e packing technique a call will be assigned randomly to any avadable time slot on an RF carrier where the other 2 lime slots at both ahwdy occupied. If no such time slot is available assignment is made randomly to any time slot on an Rf carricr where one of lhc two other tune slots is a h d y occupied. If no such carrier is available the ce~luuqueof (A) above is used.
It is worthwhile to mention that it is assumed that lhere is no i n h d l hllndofl in this system. I[ a canier has its three lime slots occupied by calls, a d then one of the 3 calk is terminated, no other existing call will be handed-off to occupy this time slot.
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The path loss from all base stations is function of rn , wbve r is thc distance from the base station to the mobile unit and n is a coas(lmt correspoding Lo the prclpqidion envirmumd n will be taken io be 3.68 for an urban environmeot [3]. All the base antennas are of the same heiihl. Also the mobile antenna height are the same. AII base stations signals are affected by independent I . o g - N d shadowing. lhe shadowing loss in dB is assumcd Gaussian of ZCTO mean and standard deviation U = 6 [4]. All signals are affected by Rayleigh frrdiry. The minimumacceptable UI for the mobile units of 17 dB includes a fa$e margm for the Rayletgh fading. For system 1 the number of cerrierslsector is 19. This number is 16 for system 2. Every carrier has 3 TDMA
time slots. 9) Calls inter-arrival time is exponential of mean equal to the load per sector divided by the mean call duralron. The call duration is also exponential with mean 3 nunutPS.
IO) Simulation and capacity estimalion is done only for the forward link. Le.. base station to mobile unit. 11) The karwmi(led power from all base st&ons are equal. All base stations have the same height and.
The carrier to co-channel interference ratio for the forward carrier is given by:
i= 1 where P, is the transdted power from base stations, r, is the distance from the base d o n to a mobile unit receiving service from it (i.e., desired mobile unit). q is the distance between a mobile unit and a co-channel interferin% base station. n is a constant equal to 3.68 for urban environment, G is a (iwsslm random vuiabk with zero mean and standard dcviation U, whcrc U is thc standard &viation of he Log-Normal shadowing, and L is the number of interference sources. L = 2 for system 1 and L = 1 for system 2.
The following assumptions are made in the simulation study. most of these assumptions are similar to those in [ 11: 1)
tier only. Since the antennas are assumed &al. m system 1 every sector can receive interference from only two sectors, while in system 2 every sector can receive d e r e n c e from only one sector. The mterfaing signals at any sector are independent so that their power can be added. Mobiks are syatielly uniformly distributed in their
The systems consist of uniform hexagonal cells, as shown in F&W 1 and 2. The load in b 1 q 1s equally distributed among aIl cells. Interference is assumed to be coming from the first
the m i c load is increa& g-&&y, ~uring and for each value calls are generated in each sector. When a call hves it is to a he slot accding to 1071
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algorithms A or R above. If no time slot is availablc the call is blocked. If a time slot is foutid the call is tcmpordy assigned to it and thc CA vvd bc calculatcd for this carrier. If C/I < 17 dB the call is immediately blocked. In a real system this ClI calculation could be either through the mobile unit readings of bit error rate and reported to the base station by lhe Mobile Assisted Himd-off Message, or the mobile unit not responding at all to the base slation messages. In eithcr case there is no point in continuing a call that is not intehgible. Such an RF activity would only cause interference tu other mobile units. Looking at Figure 1, if a certain carrier IS active in both clusters 3 and 10 this will reduce the chance of this s u e carrier to be used h cluster 2. which in turn increases the chance for this same frequency to be used in clusters 7 and 9, which again increases the chance for this carrier to be used in cluster 8.
The capacity/sec!or is the value in Elanglsector that prcivides an acceptable iokd blocking probal,iliiy (i.e., due to both unavailability of time slots or the insufficient. C/I) l'hc sirnulation is an event driven one and It si~iiulates 50,000 cvent per sector. An event would be eithcr call generation or call termination. Data are collectcd only after the system s t a b h s after a change in load.
1;iguiss 5 and 6 repiesents tke results of system 2 in 1;igure 2. In system 2 we note k a t 6 out of the 19 cluslcrs do not receive any interference which make the results optimistic.
Figures 5 and 6 show that at high load, and with this low interfeicnce the blocking due to the unavailability of channels dominates the blocking drir to the IOW C/I. At lower load the blocking due to the insufficient CA dominies. Comparing Figures 5 and 6 we note Ihal when the blocking due to the low CA dominates Ihe packing slot assignment twhnique offers better perforniance. This advantage diminishes gradually as the blocking due the unavailability of channels dominates. Jlc abovc rcsults suggcst that at systcms Limited by high interference the time slot assignment kchnique is very imporlant. Moreover, imd &er noting the &h blockuy due to the low UI, it could be advantageous to make Cn nieasurements before time slot assignment. At least the values of the CII on the available TDMA time slots must have some influence on the assignment technique. In systems not suffering from high C/I the random and packing assignment technique arc to a grcat exlend similar.
References G .L. Stuber and L. i3. Yen. "Downhnk Outage Predictions for Cellular Radio Systems," IEEE Transactions on Vehicular Technology, Voiume 40, Number 3. August 1991.
Before discussing the simulation results it is iniporlant to note tha these results are specific for the given i-cllular syctem and can not be generalized to any cellular system. The paper rather presents a method for estimation of the capacity of any cellular system. Figures 3 aucl 4 shows thc results for syslem 1 of Figure 1. The results are collected from all the 11 freqilency reuse sectors in h e 11 cluslcrs. Note that in system 1 thcrc arc 2 cluslcrs that do not receive any interference. The figures show the blocking probability due to both the unavadabdity oi chamcis (i.e., time slots) and the insufficient CA. Also the figures include the told blocking probability. In both figures it is clear that most of the blocking is due the insulficient CA. ?hat is due to the fact ihaf blocking the calls with iow CII makcs time
I(-. Raith and J. Uddenfeldt, "Capacity of Digital Cellular TDMA Syslem," IEEE 'Transactions on Vehicular lechnology, Volume 40,Number 2, May
i991.
W. C. Y. L2e, Mobile Cellular Telecommunication Systems. New York: McCiraw Hill. 1989
W. C. Jakes, Microwave Mobile Communications. New York John Wiley & Sons. 1974.
slots available most of lhe the. Th~sphenomena suggests that C/l measuremenls prior io assigning a time slot could reduce the blocking probability. Note lhat neitber algorithm A or B nicntioned in this paper measures C/I prior to assipnent. As the load increases the effect of interfcrcncc saturates and the blocking probability due to unavailability oi channel gradually approaches the blocking probabdity duc to insufficicnt W. A Comparison of Figures 3 and 4 shows [he advantage of the Packing technique over the random o w in a system dominated by interference. If we kk.e the maximum allowable blocking to be 1(1% ihe packing (echnique suggests 35 Frlangs/sector while the Random technique suggests less than 25 filiukgdsectm.
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