RF Planning for Broadband Wireless Access Networks

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54 www.rfdesign.com September 2000

cov er story

W i t h t he g rowi ng i mpor t a nc e of the Inte rne t an d da ta commun i -

c a t i o n s f o r b u s i n e s s , d e m a n d f o rhigh-bandwidth data connect ions has

skyrocketed.O n e o f t h e m o s t v i a b l e w a y s t omeet this bandwidth demand, part ic-ularly for small and mid-sized busi-n e s s e s a n d r e s i d e n t i a l a r e a s — a n despecially for the local loop (or “lastmile” as it is often called)—is throughfixed wireless connectivity. Fixed wire-less makes it possible forcarriers to provide broad-b a n d w i r e l e s s w i t h o u tr e l y i n g o n e x i s t i n gtelecommunications infra-s t r u ct u r e . I t a l s o h a s alower cost of entry, withmuc h fa s t e r de p l oyme nttimes than wired systems.This i s important in se r-v i ci ng t h e f ina l de s t i na -t i ons o f t e l e commu ni c a -t i o n s i n f r a s t r u c t u r e swhere the cost of servicecan often be exorbitant .

There a re a number of a pproa c he s t o p rov i d i ngbroa dba nd wi re l e s s a c ce s s (BWA) ,and at least as many issues as well .One of the foremost issues is that of frequency r euse.

In the United States, the spectrumrange of 24 to 40 GHz is being used

for local mult ipoint distribut ion ser-v i c e ( L M D S ) , a n d m o r e b r o a d l y ,BWA. Two sys tem types a re beinge m p l o y e d t o b u i l d o u t t h e s e n e t -works: point-to-point (PTP) systemshistorically used for backha ul of tele-com traffic, and a new generation of po in t - t o -mul t i po in t (PMP) sys t e mst ha t c a n c ove r e n t i r e s e r v i ce a r e a swith a single centr al hub. Given thelong history of PTP radios, RF plan-n i n g f o r t h e s e d e p l o y m e n t s i s

s t r a i g h t f o r w a r d a n d w e l l - k n o w n ,e s p e c i a l l y g i v e n t h e n a r r o w b e a mant ennas used in th is technology.

Point to multipointP M P s y s t e m s u s e w i d e b e a m

ant ennas and are deployed in a cel lu-l a r pa t t e rn t o c ove r a n e n t i r e c i t y .Consi s t ing of hundr eds of cus tomer

p r e m i s e s e q u i p m e n t ( C P E s ) b e i n gserviced simultaneously by a singlec e n t ra l i z e d ra d i o (hub o r ba se s t a -t ion) , they present the RF engineerwith a greater chal lenge in planninga fixed wireless network deployment,a s s e e n i n F i gur e 1 . A PMP ra d i osys tem i s in cont ras t wi th point - to-

p o i n t ( P T P ) r a d i o s y s t e m s , w h i c hhave only two endpoint s wi th com-munica t ions occurr ing in dedica tedc h a n n e l s b e t w e e n t h o s e t w o e n d -po i n t s . For PMP sys t e ms , t wo-wa yc ommu ni c a t i ons occ ur be t we e n t he

h u b a n d e a c h C P E o n a b u r s t - b y -burst basis .

As the deployments of PMP broad-band wireless access systems evolvef r o m f i e l d t r i a l s t o c o m m e r c i a ldeployments, it is necessary to exam-ine RF planning an d frequency reuseissues associated with current BWAsys t e ms . De p l oy i ng se rv i c e on f r e -q u e n c ie s a l l o ca t e d f or L M D S i n area l-world scena rio is differen t froml a bora t o ry s e t t i ngs . Wi t h mul t i p l e

base stations creating multiple cells,there a re inte rfe rence i ssues amongbase stations resulting from complexinteractions. And the type of duplex-ing scheme used ha s been drawn intothi s deba te . Some common miscon-ceptions about t hese issues surroun d-ing duplexing have resul ted in mis-under stan dings about th e complexity

and robustness of each approach.

Duplexing time divisionand frequency division

G i v e n i t s h i s t o r y o f ca r r y i n gan alog traffic, wireless duplexing hasbe e n i mpl e me nt e d t r a d i t i ona l l y byd e d i c a t i n g t w o d i s t i n c t f r e q u e n c y

ba nds : one fo r ups t r e a mtransmissions and one ford o w n s t r e a m . T h i s t e c h -nique is commonly referredto as frequen cy-division du-p l e x i ng (FDD) . T he t wobands are typically of equalsize, given the symmetricalnature of telephone com-mun ications. The upstr eamand downstream bands ar es e p a r a t e d b y a “g u a r db a n d , ” a n u n u s e d fr e -quency block that can bela rge , re la t ive to the up-s t r e a m a n d d o w n s t r e a mband sizes. The guard band

provides the necessary up/down isola-t ion to make a FDD sys tem opera teproperly.

Dupl e x i ng t o p rov i de s i mul t a ne -ous , t wo-wa y c ommuni c a t i ons s e r -v i c e s c a n a l s o b e p e r f o r m e d i n

t ime—as in t ime-division duplexing( TD D )— r a t h e r t h a n f r e qu e n c y. I nthi s approach, t ransmi t and rece iveopera te on the same frequency, buta t d i ff ere n t t i me s . Be ca us e o f t h eability to buffer traffic and the rela-t ive speed of switching between thetwo functions, simultaneous two-waycommunicat ions are preserved.

T DD i s a n e s t a b l ishe d t e c hn i quethat has been successful ly deployedi n c o m m u n i c a t i o n s s y s t e m s

RF planning for broadband wirelessaccess networksTh e need for broadband connectivit y is serious. Before deciding on an approachto BWA, you need to know the issues.

By Ja y Klein

Figure 1. A next-generation BWA point-to-multipoint network architecture.




t h r o u g h o u t J a p a n a n dEu rope. As th e techn ologyb a s i s f or p e r s o n a l h a n d y -p h o n e s e r v i ce ( P H S ) i nJapan and digi tal European

c or d l e s s t e l e c om m u n i ca -t i o n s ( D E C T ) , T D D h a sbeen commercially proven togive service providers a low-cost and low-power wirelessplatform for both base sta-t ions and CPEs. Shown tope r form i n t h e r i gor s o f amobi le t e lephone envi ron-ment , TDD is now makingits way into fixed wirelessa c ce s s a s a m o r e f l e x i b leand cost-effective method of delivering data, voice, videoand commu nications.

The essent ia l di f fe rence be tweenTDD and FDD for RF planning con-siderat ions is t hat TDD rel ies on onechannel for both upstream andd o wn s t r e a m t r a n s m i s s io n s ,dividing tr ansm issions by t ime.FDD uses two channels, one foru p s t r e a m a n d o n e f o r d o w n -s t r e a m t r a n s m i s s i o n s , w i t h ag u a r d b a n d b e t w e en t h e fr e -q u e n c i e s t o p r o t e c t a g a i n s ti n t e r fe re nc e be t we e n t he t woc h a n n e l s . F D D i s a l e g a c yapproach based on ana log tech-n o l o g y , w h i l e T D D h a s b e e ndeveloped for digi tal t ransmis-sion from the ground up. Criticsh a v e d i s m i s s e d o r a t t a c k e dT DD ba se d on i t s c ompl e x i t yand i ts deployment chal lenges,b u t a d e e p e r a n a l y s i s s h o w sthat TDD has clear advantagesover F DD.

Since the inception of two-way, dig-ital LMDS systems in 1995 and, morerecently, the U.S. auctions of 28 and31 GHz spectru m, there ha ve been nomajor deployments of PMP networksby carr iers. The rea son fort h i s d e l a y i s t h e h i g h c os ta n d l o w f u n c t i on a l i t y c on -

tain ed in first-, an d even sec-o n d -, g e n e r a t i on s y s t e m s—carr ie rs could not jus t i fydeploying on a mass scale. Asa r e s u l t , B WA ca r r i e r s a r eu s i n g P T P s y s t e m s f o r t h emajori ty of their bui ld-outs.TDD is poised to increase th ea t t r a c t i ve ne s s o f PMP sys -tems, but the cons idera t ionof robust , widespread build-o u t s f o r P M P n e t w o r k s

requir es a look at th e RF plann ing forthese new sys tems and how i t com-par es with the legacy FDD approach.

T h e r e m a i n d e r o f t h i s a r t i c l eexplores the RF planning considera-t ions of deploying a BWA network,while considering the use of synchro-niza t ion to reduce inte rfe rence andimprove robustness.

Addressinginterference

T h e f o c u s o f R F p l a n -n i n g f or T D D a n d F D Dsys t e ms i s t he a vo i da nc e

of inte rfe rence–unwanteds i gna l s t h a t d i s rup t c om-mun icat ions. The resul t of inte rfe rence in BWA sys-tems is errors and dr oppedpackets. Ult imately, inter -ference reduces th e capa ci-t y o f a n e t w o r k b e c a u s eb a n d w i d t h m u s t b e u s e dto correct err ors.

T h e t w o m a i n c a t e -gories of interference area d j a ce n t - ch a n n e l i n t e r -f e r e n c e (A C I ) a n d c o -c h a n n e l i n t e r f e r e n c e

( C C I ). A C I i n v o l v e s i n t e r f e r e n c efrom signals at a different frequencyon a n earby channel . ACI is primar i-

l y a p ow e r a n d s e p a r a t i o nissue . By adjus t ing the powero u t p u t s o t h a t e a c h h u b a n dCPE sends signals only strongenough to reach the other, thisi n t e r f e r e n c e c a n b e r e d u c e d .L i ke wi se , spa t i a l s e pa ra t i on ,inc luding f ront - to-back i sola -tion, is used to reduce interfer-e nc e . As c o-c ha nne l i n t e r fe r -ence involves other channels atthe same frequency, i t can bemuch harder to el iminate. Duet o t h e n a t u r e o f P M P b u i l d -o u t s , t h e s e t y p e s o f i n t e r f e r -ence can be fel t from both theother channels in the same huba n d t h o s e fr o m o t h e r h u b s .This is why RF planning is so

critical t o a su ccessful deployment.One criticism often leveled aga inst

th e TDD approach is th at it allows foradditional sour ces of interference notf o u n d w i t h F D D d e p l o y m e n t s .Because the same channel is used for

b ot h u p s t r e a m a n d d o wn -s t r e a m t r a n s m i s s i o n s , i t ’spossible for CPE to pick u p a

s i g n a l i n t e n d e d f o r a b a s es t a t i on s e n t by o t he r CPE .It’s also possible for base sta-tions to receive signals fromother base stations meant forC P E ( s e e F i g u r e 2 ) . F D Ddoes not experience problemsw i t h i n t e r f e r e n c e b e t w e enb a s e s t a t i o n s o r b e t w e enC P E . B e c a u s e d i f f e r e n tradios use different channelsfor sending and receiving, a

Figure 2. TDD-only interference issues: if frame-synchronized, these scenarios areeliminated.

Figure 3. FDD and TDD interference scenarios: base station to CPE(downlink) and CPE to base station (uplink).

Figure 4. Frequency reuse of 2: Synchronizing frequencies to preventinterference.




CPE c a n on l y “ spe a k” t o ba se s t a -tions, and vice-versa. Therefore, TDDsystems can experience four sourcesof i n t e r fe re nc e (ba se s t a t i on-CPE ,CPE-base s ta t ion, base s t a t ion-base

s ta t ion, and CPE-CPE), whi le FDDsys t e ms on l y e xper i e nc e t wo t ype s(base stat ion-CPE an d CPE-base sta-tion) (See Figur e 3). In an y case, withth e implement ation of TDMA technol-

ogy (someth ing th at is tru e of all cur -rent generat ion BWA systems), onlyone source of interference is experi-enced at an y given time.

Unsynchronized TDDDuring ini t i a l PMP deployments ,

i n t e r fe re nc e o f a l l t ype s— for bo t hTDD and FDD ne tworks—is less of a n i s sue , s i mpl y be c a use ne t works

will not be us ed to full capacity. Giveneven al locat ions of 100 to 150 MHzper carr ier, available spectrum will beadequate to serve the customer basewithout the need for frequency re-use

(FR) within each base stat ion. As aresul t , the ease of compensat ing forinter ference of ACI will only be preva -l e n t w i t h i n e a c h ba se s t a t i on . CCIacross base stat ions can be minimizedthr ough proper chan nel placement.

With proper RF planning, based onthe requirements of TDD, these inter-ference scenarios are improbable. Sowith FR up to1 (FR=1 being definedas u sing all available frequency with-i n e a c h ba se s t a t i on) , i n t e r fe re nc eissues including t hose unique to TDDcan be resolved adequate ly throughcareful RF planning. Carefully select-

ing chann els to be used in each sectorof a base station deployment, as wella s phys i c a l ly l oc a t i ng ba se s t a t i ona n t e nna s some d i s t a nc e f rom e a c ho t he r— a s on se pa ra t e c orne r s o f arooftop—can e l imina te inte r fe renceconcern s complet ely.

T h e t i lt o f t h e h u b a n t e n n a t oadjust the elevation of the flat beamt a r g e t e d t o wa r d C P E ca n k e e p t h es i gn a l f r o m r e a ch i n g u n i n t e n d e dCPE, and the CPE antennas use nar-row beams and are t argeted preciselyt owa rd t he spe c i f i c hub a n t e nna t owhi c h t he y a re c ommuni c a t i ng . Inpart icular, CPE-to-CPE communica-t ion is unl ikely because CPE anten-nas a re highly direct ional and ta rget-e d p r e c i s e l y a t b a s e s t a t i o n s , a n db e c a u s e l i n e - o f - s i g h t c o n d i t i o n sbe t we e n CPE se l dom e x i s t . I f t he ydo, a sl ight adjustment in the place-me nt o f one of t he a n t e nn a s e l i m i -nat es this problem.

Polarization of the signal as a keytechnique for reducing interference canalso be used t o create furth er isolationbetween wanted a nd unwant ed signals.I f t h e b e a m i s s e n t i n t h e v e r t i ca lplane, only CPE configured to receive avertical wave will be able to receive it.

CPE configured for horizontal waveswill only pick up a small am ount of thesignal, even if the ba se sta tion is aimeddirectly at the CPE. Proper alignmentof base stat ion and CPE antennas inconjunction with polarization can elimi-na t e a lmost a l l CCI. Any rema iningi n t e r f e r e n c e c a n b e e l i m i n a t e d b ya t tenua t ing the power output of theCPE so it is just great enough to reachthe right base station.

Continued on page 60




TDD AdvantagesG i v e n t h a t t h e a d d i t i o n a l

s o u r c e s o f i n t e r f e r e n c e c a n b ee l i m i n a t e d t h r o u gh t h e s e R Fplanning techniques, where there

i s n o r e u s e o f c h a n n e l s w i t h i ne a c h b a s e s t a t i o n ( F R ) , T D Ddeployments ar e on par with F DDsolut ions interference-wise. Andy e t , T D D p r e s e n t s t h e c a r r i e rw it h cl ea r m a r k e t a d v a n t a g e sover FDD. First and foremost , thebenefi t is the abi l i ty for the sys-tem to adapt in rea l t ime to theup and down burs t s of users . Incontrast, FDD systems use fixed-c h a n n e l a l l oc a t i on s f o r u p a n ddown ban dwidth; they are a lwaysinefficient in their ut i l izat ion of equipment an d capacity.

Be ca use t he r a d i o f r e que ncy a n dm o d e m p a r t s o f t h e t r a n s m i t a n drece ive cha ins a re opera t ing on thesame frequency at different t imes, aTDD system reuses certain elementsfor both chains (such as filters, mix-ers , f requency sources , and synthe-sizers) and eliminates isolation com-plexity completely.

In a dd i t i on , w i t h T DD, no gua rdb a n d s a r e r e q u i r e d t o s e p a r a t eupstream and downstream frequencyt ra f f i c . Usua l l y a s muc h a s 200 t o3 0 0 M H z f r e q u e n c y s e p a r a t i on i sneeded between tr ansm it and r eceivefrequencies for cost-effective modemde s i gns i n FDD. T he re su l t fo r t heservice provider is either the loss of that spectrum to serve customers ori n c r e a s e d s y s t e m c os t s b e ca u s e o f expensive and inefficient duplexers.Subsequently, the u se of TDD resul tsin a substant ial addit ional frequencys a v i n g s t h a t i s m o s t n o t i ce a b l e inb l oc k a l l oc a t i ons suc h a s L MDS Aband with 850 MHz or LMCS bandsof 500 MHz. For allocations such asthe LMDS B Band wh ere there is notenough guar d ban d for F DD, TDD isth e only solution.

T he a bse nc e o f gua rd ba nds a l so

makes a TDD system flexible enoughto be applied to different frequencya l l oc a t i on s . F o r e x a m p l e , v e n d o r shave designed FDD systems for spe-c i f i c t r a n s m i t , r e c e i v e s p a c i n g o f 1008 MHz ( fo r CE PT ) o r 700 MHz(for 38 GHz) . Applying these radiosolutions to new allocations requiresa redesign of the system. A TDD sys-t e m doe s no t ha ve a ny t r a nsmi t o rreceive considerat ions in i ts design.To implement an FDD approach for

e a c h f r e q u e n c y a l l o c a t i o n w o u l drequire a subst ant ial amount of R&Dchanges for each allocation (and pos-s i b l y mul t i p l e sys t e ms fo r a g i ve nl i c e n s e s u c h a s i n t h e c a s e o f t h eLMDS A licensee’s three allocations).As a resul t , th is great ly increases theper-uni t cos t s of each FDD sys temover the more flexible TDD approacht h a t c a n s e r v e a l l o f t h e m a r k e t swi t h a s i ng l e de s i gn . Fur t he rmore ,because of TDD’s inherent flexibility,i t i s eas i ly appl ied to work in spl i tbands that had been previously al lo-cated with F DD in mind.

Synchronized TDDA s P M P n e t w o r k s a r e b u i l t ou t

and carriers obtain a larger mass of c u s t o m e r s , c h a n n e l s w i t h i n e a c hbase s ta t ion wi l l need to be reusedf or m a x i m u m c a p a c it y t o s e r v e ag r o w in g c u s t o m e r b a s e . O n c e t h esame frequency begins to be reusedi n a g i v e n b a s e s t a t i o n ( o r F Rgreater tha n 1), addit ional complexi-t ies for RF planning must be consid-ered. Al though both TDD and FDDsuffer greater interference issues int h i s m o r e b u i l t - ou t n e t w o r k , t h eunique pat tern for TDD of base sta-

t i o n - b a s e s t a t i o n i n t e r f e r e n c ebecomes much more acute. The solu-t i o n i s t o i m p l e m e n t i n t r a h u b a n dinterhub synchronizat ion.

Synchronizing communicat ions sot ha t a l l ba se s t a t i ons a re t r a nsmi t -t i ng a t t he s a me t i me a nd a l l CPEare l i s tening a t the same t ime, andvice versa el iminates the pat terns of interference that are unique to TDDdeployments (CPE to CPE and basestat ion to base stat ion). By synchro-

n i z ing t r a nsmi s s i ons , ne a rby ba sestat ions can use the same frequencya t the same t ime, and TDD deploy-ments are as robust as FDD deploy-ments (see Figure 3) . TDD deploy-ments are as robust as FDD deploy-ments (See Figure 4).

And wi t h T DD’s so f t wa re -ba se ddesign, i t is easy to implement syn-chronizat ion at the bi t level with thetime division in milliseconds. By thesimple installation of a GPS receiver ateach base station—as is used in everyCDMA base station deployment—thes y n c h r o n i z a t i o n c a p a b i l i t y ca n b ei mpl e me nt e d by a dd i ng on l y a bou t$200 to the cost of each base station.No change to the CPEs is required toimplement synchronization.

As a base stat ion serves a l imitedregion in a n a rea, tar get ing ident icala s y m m e t r y p r o f i l e f o r a l l t r a n s -ceivers ha s only a sma ll impa ct on itsability to offer var iable asymm etr y inreal time, and in any case still holdsa major performance advantage overFDD systems that use a fixed chan-nel profile. As mentioned previously,one key benefit of TDD is its abilityto offer the addit ional advantage of va r i a b l e a symme t ry i n t he c ha nne l

based on demand from users . In anunsynchronized scenar io , the inte r-v a l s c a n v a r y b y d e s t i n a t i o n a n dc h a n n e l . O n c e s y n c h r o n i z a t i o n i simplemented for FR>1, the upstr eamand downst ream asymmetry remainsflexible throughout the service area.

Frame synchronizationTo make BWA deployments more

robust , synchronizat ion can be usedto avoid almost al l interrupt ions du e

Figure 5. Uncorrelated rain problem: In scenario 1: In cases of rain fade, APC will boost terminal signal, whichbalances antenna discrimination on an upfaded link. In scenario II, nearby sectors use same frequency andpolarization, but are time discriminated. In fade, the power boost would be ignored by neighbor hub, because ithas not time associated with it.




to inte rfe rence . Frames can be syn-chronized on an odd-even basis chan-ne l by channel , burs t by burs t . Fore x a m p l e , a g i v e n C P E m a y b eins t ruc ted to only t ransmi t on even

f ra me s i n a g i ve n c ha nn e l , whi l e ane a rby CPE c ou l d be i ns t ruc t e d t ot ran smi t only on odd f rames in th a tsame channel . The resul t is that anypotential for CCI is eliminated as thereceiver in the base station only “lis-tens” for even frames, so any signalfr o m t h e b a s e s t a t i on t r a n s m i t t in gonly odd frames will not be recognized

CPE-to-base station interference isthe most difficult problem because of the wide antenna pat tern that al lowsbase stations to pick up signals frommany CPE, which also makes i t sus-ceptible to picking up signals from the

wrong CPE. Because CPE-to-base sta-tion is the worst-case interference sce-nario, odd-even frame synchronizationcan be employed only wher e it is need-ed most, in the upst ream pat h.

Two examples where interferencecan be so grea t tha t f rame synchro-nizat ion sh ould be employed ar e com-b i n a t o r i a l a n d u n c or r e l a t e d r a i ninterference. Combinat orial interfer-ence occurs when the same channelis being re-used in mult iple sectors.A s r e - u s e i n c r e a s e s , s i g n a l s f r o meach sector can combine to provide as i gn a l s t r on g e n o u g h t o i n t e r r u p tservice. If the si tuat ion reaches thepoint where any single signal has agreat er t ha n 50 % chan ce of a line-of-sight connection with a base station,i t can prevent t he proper signal frombeing received by the base sta tion.

Rain can create upstream interfer-e n c e a s w e l l . B e c a u s e t h e L M D Swavelength is disrupted by rain, theCPE terminal increases i ts power toboost the signal to its base station, buttha t power boos t can a l so a l low thes igna l to reach other base s ta t ions ,espec ia l ly i f there i s no ra in in thepath between the CPE and the otherbase stat ion (see Figures 5 an d 6).

Due to the bi t - l eve l synchroniza-t i o n u s e d t o r e d u c e T D D - s p e c i f i ci n t e r f e r e n c e s c e n a r i o s , t h e i m p l e -m e n t a t i o n o f od d - e ve n f r a m e s y n -chroniza t ion for synchronized TDDs y s t e m s c o m e s w i t h o u t a d d i t i o n a lcost. And TDD’s variable asymmet rycapabilities can be used to keep thistype of synchronizat ion from affect-ing upstream capaci ty because morec a p a c i t y c a n b e c r e a t e d i n t h eupstr eam signal to compensat e.

FDD, on the other hand, does notmaint ain frame synchronizat ion, an dt he re fore c a nno t t a ke a dva n t a ge of synchronization to reduce or eliminateinterference issues. In the field, inter-

fe rence problems be tween CPE andbase stations have been demonstratedwith FDD. Operators using FDD mustface complex deployment scenar iosthat often must sacrifice capaci ty inorder to pr event interference.

Final thoughtsObvious ly, the la rger the amount

of f requency an opera tor has ava i l -able, the more flexibility that opera-tor has in deploying a network. Witha smaller amount of frequency, i t ismore tempt ing to a t tempt to re -usefrequency aggressively, but this may

result in more problems th an benefits.W i t h a g g r e s s i ve f r e q u e n c y r e - u s e ,there is a greater chance for interfer-e n c e , r e s u l t i n g i n m o r e e r r o r s .Ba ndwi d t h m us t be use d t o c or re c tthose errors, so the end result is thatt he a mount o f a va i l a b le ba ndwi d t hremains t he same. This is the frequen-cy reu se dilemma I (See Figure 6).

Another approach for maximizingf re que nc y re -use i s t o c re a t e moresectors per cel l . But with a smallerchan nel available, ther e’s more sensi-t ivi ty to accuracy, requiring carefula im of each antenna to avoid inte r-f e re nc e i s sue s . D i v i d i ng t he a va i l -able “pipe” into smaller sectors alsoeffect ive ly decreases th e am ount of band width avai lable per sector, mak-ing i t difficul t for operators to reapthe benefi ts of having mult iple cus-t o m e r s s h a r e t h e s a m e b a n d w i d t h .This is the frequency reuse di lemmaII (See Figure 6).

I n s u m m a r y , r e a l - w o r l d d e p l o y -m e n t s w i l l p r o v i d e a t r u e t e s t o f B W A P M P s y s t e m s t h a t w i ll fu l l yi l lus t ra te the cha l lenges fac ing RFplanners. While all approaches suffers imi la r int e rfe rence pr oblems, TDDhas addi t iona l types of inte r ference

because of its method of using a sin-gle channel for opera t ion. Synchro-n i z a t i on o f ba se s t a t i ons i s a t e ch-nique tha t can be used to e l imina tethese additional types of interferencea n d p u t T D D on p a r w it h F D D i nterms of interference profi le . So thecarrier can experience the benefits of the TDD approach whi le s t i l l be ingable to plan the n etwork as i f it werea n y F D D n e t w o r k . F o r u l t i m a t er o b u s t n e s s i n t h e n e t w o r k , fr a m e

synchronizat ion can be implementedto deal with combinatorial and uncor-re la ted ra in inte rfe rence . The goodnews is that this capability comes forfree in TDD because synchronization

for u p a n d down t r a n smi s s ions h a sa l ready been implemented. In FDD,i m p l e m e n t a t i on o f f r a m e s y n ch r o -nization is m uch m ore difficult. Over-a l l , sync hron i z e d T DD be s t a l l owsservice providers to maximize the u seof the ava i lable f requency to se rvemore cus t ome rs a n d ge ne ra t e morere ve nue , whi l e s t i l l o f fe r i ng h i gh-quality broadband services.

About the authorJay Kle in i s co-founder and chie f t echn ology officer for Sa n Diego-based Ensemble Communica t ions .P r i o r t o f o u n d i n g E n s e m b l e , h eserved as principal and vice presidentof research and development for CTPSystems, a wireless systems develop-er and manufac turer . Before CTP,Klein served eight years in the elec-tronic R&D department of the gov-

ernment of Israel, leading a range of wireless communications initiatives.For his accomplishments and contri-butions, he received an award fromthe President of Isra el. Klein holds aBSEE degree with honors from TelAviv Universit y. He may be cont actedon the Web at:www.ensemblecom.com.

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RF Planning for Broadband Wireless Access Networks