Intelligent PCS Cells

The author describes how system capacity can be achieved in wireless personalcommunication systems (PCS) by applying the intelligent cell concept, by which the cellreduces interference either by intelligently delivering the signal to the mobile unit, ortolerating a great deal of interference while receiving the signal.

William C. Y. LeeAirTouch CommunicationsWalnut Creek, California

An increase in system capacity can be achievedin wireless PCS systems by applying theintelligent cell concept. There are two means

by which intelligence in a cel1 can reduce interference.Either it can use intelligence to deliver the signal to themobile unit, or else it can tolerate a great deal ofinterference while receiving the signal. By the firstapproach the cell isolates the signal with multiple zones.In the second, the signal resides with the interferencebut retains its processing gain. The philosophy forachieving greater PCS capacity through the intelligentcell concept is described in this paper

The intelligent PCS cell can be defined in two ways.First, it intelligently monitors the mobile unit’s orportable unit’s location and finds a way to deliverconfined power to that mobile unit. Second, the signalsreside comfortably and indestructibly while embeddedwithin the interference found in a cell. In the firstdefinition, the intelligent cell is called the power-deliveryintelligent cell , and in the second, the intelligent cell iscalled the processing-gain intelligent cell.

24 APPLIED MICROWAVE & WIRELESS SPRING 1995

The intelligent cel1 may be a large-sized cell such as a When all the lights of house A and all the lights ofmacrocell or a small-sized cell such as a microcell. The house B arc on, the houses must be separated adequatelyintelligent cel1 increases capacity and improves in order to avoid light from one house being visible fromperformance of voice and data transmission. Since PCS the other. However, if only one room in house A andrequires vast capacity and high quality, the intelligent one in house B are illuminated, the houses might becell concept is very suitable to PCS in the future. much closer together before risk of illumination from

one spilling into the other becomes great..Power-Deliverv Intelligent Cells

As long as the cel1 site can deliver power to the locationof the mobile unit, various embodiments of theintelligence at the cel1 site arc possible. The situation isanalogous to that of a man entering a house (Figure 1).In a conventional macrocell or microcell, when a mobileunit enters a cell or a sector the cell site blankets theentire cell or sector with power to ensure thecommunication. Such a cell site dots not know wherethe mobile unit is within the cel1 or sector. This, in theman in the house analogy, is equivalent to turning on allof the lights in the house as its owner enters the frontdoor.

Conventional Macrocell/Microcell

Similarly, for a cellular system the frequency reusescheme is implemented to increase spectrum efficiency.If two cochannel cells (cells that use the same frequency)can be placed much closer to each other, the samefrequency channel can be reused more frequently in agiven geographical area, and so can a finite number offrequency channels provide many more traffic channels.Thereby both system capacity and spectrum efficiencyarc increased.

In order to reduce the separation between two cochannclcells, the power employed in each cel1 must be reduceduntil it is just sufficient to cover only one ofnumcrouslocal areas within the cell. This scheme presupposes thatthe cell operator has intelligence of the location of themobile unit or handset down to the local level.. Thisoperation poses two requirements.

intelligent MicrocellFirst, the cel1 operator must know the location of the

g$g @$J :I_-$$

mobile unit. Different resolution methods can be usedto locate it Second, the cell operator needs the ability todeliver power to the locality of that mobile unit withoutthe need to illuminate other localities simultaneously. lf,the transmitted power the cell site to the mobile unit canbe confined to a sufficiently small area (analogy of the

Figure 1. In the intelligent microcell philosophy - energy follows light turning on in a small room when someone enters),the mobile unit, analogous to lights in a house following a person cochannel interference is reduced substantially, and thefrom room to room. system capacity is correspondingly increased.

Intelligent Power D e l i v e r y : In the limit, interference can be reduced altogether byinterconnecting the base transmitter to the mobile

Just as the light in only one room is turned on as areceiver with a wire, but this reduces the system to one

person enters a house, similarly in an intelligentof wircline communication. As this limit is approached

macrocell or microcell, when a mobile unit enters a cellthe wireless communication link behaves more like

or a sector, the cell site blankets only a local area inwireline communication. Interference is reduced to a

which the recipient is known to be. As the person in theminimum, as shown in Figure. 2.

house travels to a second room, the first room’s lightsare extinguished and those in the second roomilluminated. For that single house occupant, only thelight of a single room at a time need bc activated, not allthe lights of the whole house simultaneously.

26 APPLIED MICROWAVE & WIRELESS SPRING 1995

O-Antenna Beamwidth

0, > e2> es> 04 tL--\O

Figure 2. The best case of delivering the power to the mobile inspace is by a highly directive antenna, thereby simulating theinterference reduction of wired communication links.

Radio Capacity:

In a frequency-reuse system, such as the cellular system,we always use the term radio capacity to measure thetraffic capacity. The radio capacity m is defined as:[ 1]

Mm = -

K (1)

where m is the number of channels/cell in anomnidirectional radiation cell.or

Mm=-

KxS (2)

where m is the number of channels/sector in an antennasectored cell, M is the total number of frequencychannels, K is the cell reuse factor, and S is the numberof sectors. K can be expressed as:[2]

K= f(D/R)2 (3)

in which D is the cochannel-cell separation and R is thecell radius. According to Figure 3(a), the relationshipbetween the carrier-to-interference ratio (C/I) and D/Rcan be expressed[3] as:

C/I = CDW46

(4)

Figure 3. The D/R relationship in different carrier to inference (C/I) ratio conditions..

Equation (4) is obtained based on the propagationpathloss of 40dB/decade and omni cells. The radiocapacity of omni-cell systems is:

Mm = channels/cell

(5)

Other parameters such as Erlangs/cell, Erlangs/Km2,Calls/Km2, and so forth,., can be derived from the radiocapacity.

Normalized radio capacity:

channel/cell/spectra1 band (6)

where BT is the total spectral band. When two systemsoperate at two different spectral bands such as, B,, andthen B, the radio capacities ml and m2 must benormalized first using Equation (6) as m,, /B,, andm2 / B, before comparing their radio capacities,

rk1 and I&.

Power-Delivery Intelligent Cells Applications.

In the following sections, different types of intelligentcells are described and their radio capacities compared.

APPLIED MICROWAVE & WIRELESS SPRING 1995 3 1

Zone-Divided Cells:

In general there are three kinds of zone-divided-cellsystems:

1. Sectorial Cells - applied in the existing cellularsystem (see Figure 3(b) and (c))

2. Intelligent Microcells

When dividing a cell into many zones[4] as shown inFigure 4, the cell operator knows which zone the mobileunit is in and delivers the radio signal to just that zone.

Microcell Utilizes D, /R, ~4.6for Active Zone Separation

This Provides a D= 3R forMicrocell Cell Separation

This Yields a K = 3

This is a 2 to 2.5 CapacityIncrease

fi=f;+f;

Figure 5. Reuse of sectorial beams with directional antennas.

Figure 4. Intelligent microcell capacity application.

3. Reuse of Sectorial Beams with DirectionalAntennas

We use antenna beams to confine the energy toindividual mobile units in the cell as shown in Figure 5.Of course, the directional antenna beam front-to-backratio should be considered in this implementation.

These adaptive antenna patterns provide a means ofgenerating multiple cochannel mobile calls on the reverselinks, and with the identical antenna patterns the callsare conducted on the forward links as well. Thereciprocity principle works based on Lee’s Model[6],where the active region around the mobile unit is anapproximate radius of 100-200 wavelengths[7]. Thebeam angle a received at the cell site is a function ofdistance R (Figure 6).

2X200Xa=

RAdaptive Antenna Array.

The antenna pattern can be formed by tracking themobile unit and nulling the interference[5].

Cochannel interference is reduced because the adaptiveantenna beam follows the mobile units.

where h is the wavelength. Usually operating at a UHFfrequency, the antenna beamwidth 0 is always largerthan a . The isolation between the two cochannel mobilecalls will be measured by 0 not a. Also, the definitionof the antenna beamwidth 0 is based on an 18 dBbeamwidth, not on a 3 dB beamwidth. A handoff isinitiated when the two cochannel mobile units movewithin one 8 angle. In some proposed systems, the beamnulling is formed between two mobile units. In this case,the nulling angle measured between two mobile unitscannot be less than a.

32 APPLIED MICROWAVE & WIRELESS SPRING 1995

Figure 6. Intelligent cell with aduptive untenna-array beams.

Inbuilding Communication.

Within a building, the number of traffic channels canbe increased by treating each floor of a building as acell. Therefore, let a group of frequency channels, Ml,be assigned to inbuilding use. Due to the naturalshielding afforded by the building’s walls and floors,the inbuilding signal enjoys isolation from the outsidecellular signals. Those same Ml channels will be reusedon every floor of the building and also in every floor ofneighboring buildings as shown in Figure 7. The radiocapacity Ml of inbuilding communications can beobtained by setting K = 1.

m =MI- = Ml number of channels/floorK

Implementing Processing-Gain Intelligent Cells

A processing-gain intelligent cell is analogous to theconduct of many simultaneous conversations takingplace in a large hall (Figure S), the hall being an analogof one big radio channel serving all the traffic in anintelligent cell, and the conversations of the differentparties are traffic channels. The processing gain is equalto the number of conversations all using the samemedium. The necessary gain is related to the size of thehall, which limits the number of people, and therefore,the number of conversations.

20-30 dB Ifi1Wireline

Wireline

Figure 7. Within a building frequencies are reused on each floor,and again in adjacent buildings.

Because all the conversations take place in the hall, thespeaker level (power control) of each conversation is thekey element which keeps the interference level down toacceptable levels for each traffic channel in thisintelligent cell. Also, if the interference level of eachindividual conversation can be controlled intelligently,then we can maintain the total interference level and addmore conversations. If the power control is not working,the cell is not an intelligent cell and faces a cocktail-party syndrome; no party can talk except by raising hisor her voice to unacceptable levels. If everyone does thesame, no one can hear the conversation.

Processing-Gain Intelligent Cells (KI’ 1 Systems)

A. Direct-sequence CDMAIn this CDMA system [8-10] the broadband frequency

channel can be reused in every adjacent cell so that K isclose to 1, as shown in Figure 9. In a practical sense, Dequals 2R, i.e., all the same available frequencies areused in each cell. From the previous definitions ofEquation (3) we obtain:

34 APPLIED MICROWAVE & WIRELESS SPRING 1995

Figure 8. Multiple conversations in one hall are made possible bythe processing-gain in an intelligent cell.

K= ww2 = 1.333 .

Figure 9. A CDMA System and its interference.

In DS-CDMA, since P.G. is greater than 1, C is alwaysRadio capacity is also based on Equation (1). However, smaller than I in Equation (7) even if there is only one

in direct sequence DS-CDMA, K is fixed but M (the active user. The processing gain is used to overcome Itotal number of available channels) is a variable and and determine the number of traffic channels that can bedepends on the interference situation. created.

Using the scenario shown in Figure 9, the interference Assume that in a DS-CDMA system[8], B = 1.23 MHzcomes from the home cell and the adjacent cells; the and Rb = 9.6 kbps, then PG. = 1.23 MHz/9.6 Kbps (=)value of C/I is expressed as: 21 dB.

c % R, Eb/lo In this system, voice quality is accepted at a frame error---x-=I - I, B P.G. (7) rate of FER = 10-2 which typically corresponds to Eb/Io

= 7 dB. Knowing the values of PG. and Eb/Io, C/I ofEquation (7) can be obtained:

where

Eb is the energy per bitIO is the interference per hertz

C/I = 7 - 21 = -14 dB (=) 0.03981

(To >>No, where No is thermal noise per hertz)

Rb is the information rateB is the bandwidth per channelB/Rb is the processing gain (P.G.)

Find the number of traffic channels mi in each cell fromC/I by:

mi and a i are the number of traffic channels and thepower level respectively in each i cell. Solving Equation(9) we obtain:

P.G. (Processing Gain) = lOlog

APPLIED MICROWAVE & WIRELESS SPRING 1995 39

CII=

a, w4& - i) R-4 + (a2m2 + cx3m3)(R)-4 + p (ZRq4 + y(?h3H)-4

own cell

= 0.03981= &

where

y= &xmi=7

i i

ml = 26.1- T2qa:a3m3]

--&(2r4 -$(2.63r4

(9)

(10)

Case A - a single-cell case

a . =Ofori#l

Then from Equation (10)

Iml = c + 1 = 25.1+ 1 = 26.1 traffic channels/cell

Case B - identical-cell case

All the cells have the same power and the same numberof traffic channels:

(mi = rni and ai = CX~)

We may substitute mi = m and ai = a into Equation(9) or Equation (10) and solve for m as follows:

26.1=m 3+3.(2)-”[

+ 6. (2.633)-‘1

thenm = 7.88 traffic channels/cell

Both traffic channels appearing in Case A and Case Binclude the overhead channels for sync and setup, butdo not take into consideration the voice activity cycle orsector-reuse factor as used in real commercial systems[8].

An alternative to CDMA is frequency hopping.. Thehopping pattern can be controlled by a code sequence.Frequency hopping has been used in the past to overcomeenemy jammers in military applications. There are twokinds of hopping, fast frequency hopping and slowfrequency hopping.

Slow Frequency Hopping (SFH) is defined as sendingmultiple bits on a single hop[l The hopping rate isadjusted relative to the degree of quickness of theenemy’s reaction. Fast Frequency Hopping (FFH) isdefined as sending a bit on a pseudo random pattern offrequency channels, then sending the next bit on anotherdifferent pseudo random pattern of frequency channels.The multiple frequency channels form a code for onebit, which can be sent out simultaneously andsequentially.

The bandwidth of the channel depends on thetransmission bit rate. The scheme of simultaneouslysending out the same bit on different frequency channelsrequires a larger bandwidth for sending each bit. This isanother variant of wideband CDMA. Sending bitssequentially over frequency channels is the conventionalFH CDMA system. The fast frequency hopping (FFH)CDMA system requires a larger bandwidth than the slowfrequency hopping (SFH) CDMA system, because inFFH one bit requires multiple frequency hoppingchannels to be sent out sequentially.

There are two types of processing gains in an FHsystem. One kind of processing gain is used to measurethe power necessary to defeat enemy jammers. It focuseson minimizing collisions of two carriers occupying afrequency channel at the same time. If the desired signalhas 1,000 channels available among which it can hop toavoid the jammer, the processing gain is 30 dB. We canderive this anti-jamming processing gain from (C/I)Jusing Equation (7).

(c/r), = Eb.RbI, x(Bx N) (11)

40 APPLIED MICROWAVE & WIRELESS SPRING 1995

where B is the bandwidth for sending a data rate of Rhthrough a single channel and assuming that B = R,, N isthe number of available frequency channels among whichthe hopping is to occur. Each channel has the samebandwidth B. The processing gain is:

BxNP.G. = - = N

R, (12)

From Equation (11) we conclude that FFH and SFHare equivalent in achieving processing gain.Another kind of processing gain is used to gain radio

capacity. It focuses on spreading energy channels so thatthe interference seen by each bit is near the minimumacceptable performance threshold. The carrier-to-interference ratio (C/ I)F of a frequency hoppingsystem can be expressed differently from Equation (7)as:

(C/& = Eb .RbI, .(B.F)

where

Eb is the energy per bit

R, is the bits per second

F is the number of frequency

channels per bit (Fh 2 1)

B is the bandwidth of sending a

signal of R, stream

(13)

In a non-FH system or an SFH system, F is alwaysequal to 1 because there is no spread spectrum (PN)coding of the data bits. Then Equation 13 becomesEquation (7). It has been shown that SFH does notexperience processing gain in order to increase radiocapacity. The SFH system is more like the Aloha multipleaccess scheme[ 14].

In an FFH system, the processing gain for radio capacitydepends on F, the number of frequency channels per bit,as

B.FP.G. = - = F

% (14)

Assume that an information rate Rt, is 10 lbps and onebit hops among 100 frequencies, F = 100, then the totalbandwidth required is BF = 1 MHz.

Impulses in Time Domain

We can create a spread spectrum system based onimpulse positioning modulation in time domain[ 12, 13].The Cd1 can be obtained as:

G Ep . Pb . R,--1 - I,.B

whereCp - carrier power of data streamEp - energy per pulseP, - number of pulses/bitRs - bits/set.

(15)

If P, = 1, then E . P, = E, and Equation (15) becomesEquation (7). If $ ~1, the impulse positions modulationsystem becomes a spread spectrum system.

The same principle of using DS-CDMA to increasecellular capacity can be applied to the impulse positionmodulation. When the pulse width of impulse is less than1 ns, the advantage of applying this impulse positionmodulation becomes apparent.

The bottom line of the intelligent cell is either to preventinterference at signal reception, or tolerate a great dealof interference while receiving the signal. This can beaccomplished by either delivering the signal intelligentlyto only the intended recipient or having the signal resideindestructibly in the presence of the interference. Themethods described for implementing intelligent cells arevery important for PCS.

Among the methods of forming a power-deliveryintelligent cell, the sectorial cells may increase capacityif the cell reuse factor K is reduced. Alternatively,additional transmitters (see typical PCS transmitter inFigure 10) might be installed within high density areasas frequently as power transformers on poles.

APPLIED MICROWAVE & WIRELESS SPRING 1995 41

Figure 10. The compact size of PCS transmitters (unit in photo)would permit their installation as frequently as power transform-ers on light poles, making the power level control of intelligentPCS cells practical.

Frequency reuse with multiple antenna beams and themicrocell with multiple zones using the intelligent cellconcept can also increase capacity. Among the methodsof forming a processing-gain cell, both direct-sequenceCDMA and FFH can increase capacity due to theirprocessing gain, but SHF cannot.

References

1.

2.

3.

4.

5.

6.

7.

8.

W. C. Y. Lee, Mobile Cellular Telecommunications Systems,McGraw Hill, 1989, p. 379.

V. H. MacDonald “The Cellular Concept,” Bell SystemTechnical Journal, Vol. 58, January 1979, pp. 15-42.W. C. Y. Lee, “Spectrum Efficiency in Cellular,” IEEETransactions on Vehicular Technology, May 1989, pp. 69-75.

W. C. Y. Lee, “Smaller Cell for Greater Performance,” IEEECommunication Magazine, November 199 1, pp. 19-23.

M. Cooper and R. Roy, “SDMA Technology - Overview andDevelopment Status,” ArrayComm-ID-010, ArrayComm, Inc.,CA.

W. C. Y. Lee, “Lee’s Model,” IEEE VTS Conference Record1992, Denver, Colorado, May 11, 1992, pp. 343-348.

W. C. Y. Lee, Mobile Communications Engineering, McGrawHill, 1982, p. 202.

K. Gilhousen, I. Jacobs, R. Padovani, A. Viterbi, L. Weaver, C.Wheatley, “On the Capacity of a Cellular CDMA System,” IEEETransactions on Vehicular Technology, Vol. 40, May 1991,pp. 303-312.

9. R. Pickholtz, L. Milstein, D. Schilling, “Spread Spectrum forMobile Communications,” IEEE Transactions on VehicularTechnology, Vol. 40, May 199 1, pp. 3 13-322.

10. W. C. Y. Lee, “Overview of Cellular CDMA,” IEEETransactions on Vehicular Technology, Vol. 40, May 199 1,pp. 291-302.

1 1. ETSI/TC “Recommendation GSM 01.02,” ETSI/PT12,January 1990.

12. F. Anderson, W. Christensen, L. Fullerton, B. Kortegaard,“Ultra-Wideband Beamforming in Sparse Arrays,” IEEEProceedings - H, Vol. 138, No. 4, August 1991, p. 342-346.

13. R. A. Scholtz, “Multiple Access with Time-Hopping ImpulseModulation,” MILCOM 1993, Boston, MA, October 11-14,1993.

14. R. A. Comroe and D. J. Costello, Jr., “ARQ Schemes for DataTransmission in Mobile Radio Systems,” IEEE Transactions onVehicular Technology, Vol. VT-33, August 1984, pp. 88-97.

15. W. C. Y. Lee, “Increasing System Capacity in PCS,” Personal,Indoor and Mobile Radio Communications (PIMRC’94).

Dr. Lee was awarded the Ph.D. degree fromOhio State University in 1963.

From 1964 to 1979 he was with BellTelephone Laboratories, engaged in thestudy of wave propagation, antenna theoryand mobile systems. Subsequently he waswith ITT Defense Communications Division,and later with PacTel.

Currently he is the Chief Scientist and VicePresident of Applied Research and Science for AirTouch Commu-nications (formerly PacTel Corporation).

Dr. Lee has written more than 150 technical papers and 3technical books. He is an IEEE Fellow, a Radio Club of AmericaFellow, a distinguished alumni of the Ohio State University,recipient o f the IEEE VTS Avant Garde Award, recipient of the BellLabs Dedicated Service Award, and a recipient o f the ITTDCDTechnical Contribution Award.

42 APPLIED MICROWAVE & WIRELESS SPRING 1995