Summarize in less than one page (font 11pt) the paper attached. Be concise but precise.
Overview of cellular CDMA IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 40, NO. 2, MAY 1991 29 I Overview of Cellular CDMA William C. Y. Lee, Fellow, IEEE Abstract-This paper is a general description of code division multiple access (CDMA). The analysis of power control schemes in CDMA is an original work. The wide-band wave propagation in the cellular environment presents an interesting result (the short-term fading reduction over the wide band-signal in cellu- lar). Also less fading in urban areas than in suburban areas. The advantages of using CDMA listed in this paper have excited the cellular industry. Radio capacity is the key issue in selecting CDMA and is carefully described in this paper. I. INTRODUCTION HE development of the code division multiple access T (CDMA) scheme is mainly for capacity reasons. Ever since the analog cellular system started to face its capacity limitation in 1987, the promotion of developing digital cellu- lar systems for increasing capacity has been carried out. In digital systems, there are three basic multiple access schemes, frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In theory, it does not matter whether the spectrum is divided into frequencies, time slots, or codes, the capacity provided from these three multiple access schemes is the same. However, in the cellular system, we might find that one may be better than the another. Especially in the North American Cellular System, no additional spectrum will be allocated for digital cellular. Therefore, the analog and digi- tal systems will co-exist in the same spectrum. Also, the problem of transition from analog to digital is another consideration. Although the CDMA has been used in satellite communications, the same CDMA system cannot be directly applied to the mobile cellular system. In order to design a cellular CDMA system, we first need to understand the mobile radio environment; then study whether the character- istics of CDMA are suitable for the mobile radio environment or not; and finally describe the natural beauty of applying CDMA in cellular systems. 11. MOBILE RADIO ENVIRONMENT The propagation of a narrow-band carrier signal is a conventional means of communication. However, in a CDMA system, the propagation of a wide-band carrier signal is used. Therefore, we first describe the propagation of the narrow- band wave, then of the wide-band wave. A. Narrow-Band (NB) Wave Propagation A signal transmitted from the cell-site and received by either a mobile unit or a portable unit would propagate over a Manuscript received August 1, 1990; revised October 1, 1990. This paper was presented at the 1990 IEEE GLOBECOM Conference, San Diego, CA. The author is with PacTel Cellular, Imine, CA 92714. IEEE Log Number 9 14447 1. MOBILE PATH Fig. 1 . Mobile radio environment. particular terrain configuration between two ends. Therefore, the effect of the terrain configuration generates a different long-term fading characteristic which follows a log-normal variation appearing on the envelope of the received signal, as shown in Fig. 1. Since the antenna height of a mobile or portable unit is close to the ground, three effects are observed [ 11. First, the signal received is not only from the direct path but also from the strong reflected path due to the fact that the antennas of the mobile units are close to the ground. These two paths create an excessive path loss which is 40 dB/dec (fourth power law applied), i.e., doubling the path loss in decibels of the free-space path loss. Second, under the low antenna height condition at the mobile units, the human-made structures surrounding them would generate the multipath fading on the received signal called Rayleigh fading, as shown in Fig. 1. The multipath fading causes the burst error in digital transmission. The average duration of fades ? as well as the level crossing rates 7i at 10 dB below the average power of a signal is a function of vehicle speed V and wavelength A. ? = 0.132( ;) s (2) For a frequency of 850 MHz and a speed of 15 m/h then i = 6 ms and ii = 16 crossings/s. Third, a time delay spread phenomenon exists due to the time dispersive medium. In a mobile radio environment a single symbol, transmitted from one end and received at the other end, receives not only its own symbol but also many echoes of its symbol. The time delay spread intervals are measured from the first symbol to the last detectable echo, which are different in human-made environments. The average time delay spread due to the local scatterers in suburban areas is 0.5 ps and in urban areas is 3 ps. These local scatterers are in the near-end region as illustrated in Fig. 2, and the time delay spread corresponding to this region is illustrated in Fig. 3. There are other types of 0018-9545/91/0500-0291$01.00 0 1991 IEEE 292 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 40, NO. 2, MAY 1991 \- el/-- - ANTENNA I\ (a) MOUNTAIN ''''\\\ & JOCAL %. .______ -, NEAR-IN REGION (b) Fig. 2. (a) A mobile radio environment-two parts: propagation loss and multiple fading. (b) Time-delay spread scenario. LINEAR SCALE LOG SCALE T k . &4.*1 Fig. 3. An illustration on time-delay spread. distribution [3] - [ 5 ] . An FDMA system always requires less transmission rate than a TDMA system if both systems offer the same radio capacity. Usually an FDMA system can get away from using an equalizer as long as its transmission rate does not exceed too much above 10 kilosamples per second. The CDMA system does not need an equalizer but a simpler device called a correlator will be used. It will be described later. B. Wide-Band Wave Propagation [6J 1 ) Path Loss: Suppose that a transmitted power P, in watts is used to send a wide-band signal with a bandwidth B in hertz along a mobile radio path r . The power spectrum over the bandwidth B is S,( f), then the P, can be expressed as B The received power D where C( r , f) = medium characteristic = k / ( r 2 f ) (7) A e ( f ) = effective aperture of the receiving antenna k is a constant factor, c is the speed of light, G, and G, are the gains of the transmitting and receiving antennas, respec- tively. Substituting (3, (7), and (8) into (6), we obtain time delay spreads as illustrated in Fig. 2 . One kind of delayed wave is due to the reflection of the high-rise build- ings (far-out region), and one kind of delayed wave is due to the reflection from the mountains. Their corresponding time delays are illustrated in Fig. 3. In certain mountain areas, the time delay spread can be up to 100 ps. These time delay spreads would cause intersymbol interference (ISI) for data transmission [2] . In order to avoid the ISI, the transmission For simplicity but without losing much generality, let s,( f) = constant, ( 10) for f , - B / 2 s f I f , + B / 2 . Then (9) becomes 1 kc2 G, GR ( 1 1 ) P, = ~ rate R, should not exceed the inverse value of the delay spread A if the mobile unit is at a standstill (nonfading case), R, C l / A ( 3 ) ( 4 r r 2 ) 2 [ ( 2 ; o ) 2 ] z f ; 1 - - or R, should not exceed the inverse value of 2 a ~ if the mobile unit is in motion (fading case) Equation ( 1 1 ) is a general formula. For a narrow-band signal, B < fo,="" then="" (="" 1="" 1="" )="" becomes="" k="" c="" 2="" gtgr="" p,="(narrow-band)" .="" (12)="" r,="">< 1/(27ra). (4) if the transmission rate r, is higher than (3) or (4) , both fdma and tdma need equalizers which are capable of reducing the is1 to a certain degree depending on the hard- ship of the time delay spread length and the wave arrival (4 ?r r2 ) 2 f : from ( 1 1 1 , we may find the b/f, ratio for the case of i-db difference in path loss between narrow-band and wide-band. p lee: overview of cellular cdma 293 (b) fig. 4. band-limited impulse. (a) spectrum. (b) waveshape. that means by solving the denominator of (1 1) as follows: l0log[1 - -1db we obtain b = 0.66 fo. in most wide-band applications, b will not be wider than fo 12. therefore, the narrow-band propagation path loss should be applied to the wide-band propagation path loss. 2) multipath fading characteristic on wide-band: the wide-band pulse signaling s,(t) can be expressed as [7] sin (?rbt) ?rt s o ( t ) = a where a is the pulse amplitude shown in fig. 4. the received signal can be represented as sin ?rb t - - ?r t - - * (14) i ( ‘if) ~ ( t ) = ( a / b ) 5 bm(t) m = - m the pulsewidth of 1/b is the time interval of the pulse occupied. count all b,,, that are not vanishing over a range of a finite number of m which is corresponding to a time delay spread a. then the effective number of diversity branches, m, can be approximated by 1 a + - 1 b (15) m = - - 1/(27ra).="" (4)="" if="" the="" transmission="" rate="" r,="" is="" higher="" than="" (3)="" or="" (4)="" ,="" both="" fdma="" and="" tdma="" need="" equalizers="" which="" are="" capable="" of="" reducing="" the="" is1="" to="" a="" certain="" degree="" depending="" on="" the="" hard-="" ship="" of="" the="" time="" delay="" spread="" length="" and="" the="" wave="" arrival="" (4="" r="" r2="" )="" 2="" f="" :="" from="" (="" 1="" 1="" 1="" ,="" we="" may="" find="" the="" b/f,="" ratio="" for="" the="" case="" of="" i-db="" difference="" in="" path="" loss="" between="" narrow-band="" and="" wide-band.="" p="" lee:="" overview="" of="" cellular="" cdma="" 293="" (b)="" fig.="" 4.="" band-limited="" impulse.="" (a)="" spectrum.="" (b)="" waveshape.="" that="" means="" by="" solving="" the="" denominator="" of="" (1="" 1)="" as="" follows:="" l0log[1="" -="" -1db="" we="" obtain="" b="0.66" fo.="" in="" most="" wide-band="" applications,="" b="" will="" not="" be="" wider="" than="" fo="" 12.="" therefore,="" the="" narrow-band="" propagation="" path="" loss="" should="" be="" applied="" to="" the="" wide-band="" propagation="" path="" loss.="" 2)="" multipath="" fading="" characteristic="" on="" wide-band:="" the="" wide-band="" pulse="" signaling="" s,(t)="" can="" be="" expressed="" as="" [7]="" sin="" (?rbt)="" rt="" s="" o="" (="" t="" )="A" where="" a="" is="" the="" pulse="" amplitude="" shown="" in="" fig.="" 4.="" the="" received="" signal="" can="" be="" represented="" as="" sin="" rb="" t="" -="" -="" r="" t="" -="" -="" *="" (14)="" i="" (="" ‘if)="" ~="" (="" t="" )="(" a="" b="" )="" 5="" bm(t)="" m="-" m="" the="" pulsewidth="" of="" 1/b="" is="" the="" time="" interval="" of="" the="" pulse="" occupied.="" count="" all="" b,,,="" that="" are="" not="" vanishing="" over="" a="" range="" of="" a="" finite="" number="" of="" m="" which="" is="" corresponding="" to="" a="" time="" delay="" spread="" a.="" then="" the="" effective="" number="" of="" diversity="" branches,="" m,="" can="" be="" approximated="" by="" 1="" a="" +="" -="" 1="" b="" (15)="" m="-">