Wireless Local Loop Comparison (Lee)

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n the ’70s fixed-to-fixedmicrowave links were developed and deployed. Wireless localloop (WLL) systems could also have been implemented in the’70s in U.S. rural areas, but were not. The cost justifications atthat time for WLL were:

For the Near Term — Using the ra dio link for WLL wasthe solution because th e cost of laying a long wire cable toconnect to a remote point was very high.

For the Long Term — Using the wire link was the solution.Although t he cost of laying a long wire cable was high in th ebeginning, adding more users along the wire link later onwould drive the cost per user down as the years went by. Nev-

ertheless, when the network layout rea ched its limit, WLL wasthe solution.

For an Urban Situation — In New York City, laying addi-tional wire cables was very costly. Therefore, WLL was thesolution.

In the meantime, in the ’70s Advanced Mobile Phone Sys-tem (AMPS) was developed and could have been applied inwireless local loop systems to gain two advantages:• It could have been easier to acquire the spectrum from the

FCC for WLL in the ’70s.• WLL services could have been deployed first; then we could

have gone into cellular mar kets using bundled services, thatis, providing wireless local loop and cellular together usingthe same spectrum and o perating the same wireless com-munication network. By taking this approach, both wireless

local loop and cellular operations would have taken placein the ’70s.Since telephone companies did not take advantage of this

opportun ity in the ’70s, wireless operators were busy developingcellular systems in the ’80s. Now, in the ’90s, we are starting tolook at the domestic and international markets for WLL.

The Advantage of WLLSince a WLL system is a fixed-to-fixed link system without thewireline connected, as shown in Fig. 1, the advantages are:• WLL can eliminate many problems and be cost inherent in

many areas and many countries.• WLL can shorten the time to deploy a telephone network.

• WLL can easily be moved around to accommodate new changes.• WLL is a quick start for startup telephone systems.

The attributes of a WLL system (Fig. 1) are as follows:• Coverage increase: the path loss is based on free space loss.• Capacity increases: the requ ired WLL carrierl-to-interference

ratio, (C/I)WLL, is much lower than the required (C/I) cellular.• High-gain directional antennas can be used. Thus, the inter-

ference decreases and t he frequ ency reuse distance isreduced (i.e., capacity increases).

Concepts of Deploying

Wireless Systems The General Key Drivers of

Deploying a Wireless System The gen eral key drivers of deploying a WLL system arecapacity, coverage, and quality. However, the three key driversare re lated amon g themselves. With an allocated spectralband as a given, then:

capacity ∝ (quality)–1

coverage ∝ (quality)–1

and they are all a function of C/I.

How to Measure Spectrum Efficiency To evaluate the spectrum efficiency in frequency reuse sys-

tems such as cellular, personal communication services (PCS),and WLL, we may use the radio capacity formula to expressradio capacity m as follows:

m = M/K (1)

where M is the total number of channels in 1 MHz and K isthe frequency reuse factor. K can be expressed as follows:

(2)

where D is the distance between two co-channel cells, and R isthe r adius of the cell.

In frequency-division multiple access (FDMA) or time-

K D

R=

1

3

2

IEE E Personal Communications • February 1998 1070-9916/97/$10.00 © 1998 IEEE

I

Spectrum and Technology of a Wireless Local Loop System

W i l l i a m C . Y. L e e , A i r To u c h Co mmu n i c a t i o n s , I n c .

AbstractThis article shows that the wireless local loop (WLL) may not necessarily provide more capacity than mobile cellular systems if an FDMA or

TDMA system is used. CDMA ha s proven to have higher capacity than t he othe r multiple access schemes due to the na ture of CD MA systems.

In industrial countries, the r equirements of WLL are qu ality and service features. In de veloped countries, the requirement is a low-cost high-

capacity system, even if voice quality must be sacrificed. Therefore, the system design aspects are different.



IEE E Personal Communications • February 199850

division multiple access (TDMA), the value of M is given, butthe value of K is a function of C/I.

In cod e division mu lt iple access systems (CD MA), Dalways equ als 2 R; thus, K is a constant, K = 1.33 from Eq. 2,but M is a function of C/I. Both K in FDMA (or TDMA) and

M in CDMA expressed as a function of C/I will be shown inthis article.

The Three Key Drivers Related to C/I The three key drivers (capacity, coverage, and quality) can beexpressed a s a function of C/I.

Quality, Q, is proportional to C/I:

Radio capacity, m, is inversed proportional to C/I:

(3)

Coverage, R, is inversed proportional to C/I:

(4)

Consideration of Deploying a WLL System

Impact of the Required C/I

The required C/I in each system is determined from the accept-ed voice qua lity or corresponds to the specific frame error r ate.

The required C/I of a WLL system under a nonfading fixed-to-fixed condition is always less than the required C/I of a cellu-lar system unde r a mobile radio multipath fading condition.

(5)

It shows that the WLL system can tolerate more Gaussian-like interferen ce than th e R ayleigh-like interference of thecellular system. Therefore, the frequency reuse distance forWLL is supposedly shorter than for cellular if the propagationpath losses for both systems are the same.

Impact of the Propagation Path Loss Coverage Increases in WLL — The coverage of a WLL isbased on a fixed-to-fixed pr opagation. The path loss of thef ixed- to-f ixed propaga t ion in a WLL i s based on 20dB/decade. However, the path loss of mobile radio propaga-

tion (fixed-to-mobile) is based on 40 dB/decade, which showshigh excessive loss. Therefore, the same wireless communica-tion system can cover mor e are a for WLL services than formobile ra dio services.

Capacity Decreases in WLL — (i f FDMA or TDMA isused). Based on the path loss of 20 dB/decade for WLL and40 dB/decade for cellular, the formu la of C/I and K of bothsystems can be obtained as follows:

WLL Systems —• Under a condition of six interferers (Fig. 2)

(6)

Substituting Eq. 6 into Eq. 2 yields

(7)

• Under a condition of one interferer (Fig. 3)

K D

R

C

I w

w w1

1 1

1

32

2

=

=

C

I

R

D

D

R

w

w

w

w

= ⋅ =

1

1

1

1

2

2

2

6 6

–

–

C

I

C

I w c

<

R DC

I ∝ ⋅

–1

mM

K

C

I

= ∝

–1

# channels / cell

QC

I ∝

C

I

C

I

↑=

↓ =

quality is improving

capacity or coverage is increasing

s Figure 1. The scenario of a WLL system.

Nonfadingfixed-to-fixed

radio link

Directionalantenna beam

s Figure 2. Six interferers (co-channel cells) m odel for frequencyreuse (wireless systems).

f 1

f 1

f 1

f 1

f 1

f 1

D

f 1

R

s Figure 3. Single interference from co-channel sites in WL Lusing directional antennas at both ends.

Co-channel sites

R

f 1

D

f 1



IEE E Personal Communications • February 1998 51

(8)

and

(9)

Cellular Systems —Always under a condition of six interfer-ers (Fig. 1),

(10)


(11)

Capacity Comparison —The ratio of K W /K C can be used tocompare the capacity of two systems; if

(12)

the capacity of WLL is greater t han t hat of cellular.The ratio of K W 1

/K C under a condition of six interferer s is

(13)

The ratio of K W 2 / K c under two different conditions, one

interferer for WLL and six interferers for cellular, is

(14)

Equations 13 and 14 are plotted in F ig. 4 with a variable a

(15)

where a is always greater than that shown in Eq. 5.Assume that the required (C/I)W of WLL is 6 dB or more;

then several observations can be stated from Fig. 4 as follows:• The region in which WLL capacity is greater th an cellular

capacity is below the line of

• A WLL system under the condition of six interferers cannothave a capacity greater th an tha t of cellular.

• The WLL system under the condition of one interferer canmost likely have a capacity greater than that of cellular.

• W h en t h e va lu e a becomes greater, the rat io of K W / K C

increases.

Capacity Is Independent of the Propagation Path Loss if CDMA Is Used

In a CDM A system, every cell operates the same radio chan -nels; therefore, D = 2 R has been stated pr eviously and K is aconstant, K = 1.33. However, M is known and is a function of C/I. The following deviation shows that the propagation pathloss does not impact capacity use. The scenar io in F ig. 5, withthe mo bile unit a t position A, would be the worst case. Thehome cell site and two close-in interference sites I 1 and I 2 areat the same distance R from the mobile unit. The C/I at posi-tion A can be expressed as

(16)

where I is the total interference, I s the self-interference, and I athe ad jacent interference. E b is the energy per bit and I 0 th einterference power per hertz. B is the allocated spectrum ba nd,

R the transmit rate per second. In Eq. 16, E b / I 0 and B / R are given.

In WLL systems, the 20 dB/dec loss is used, that is, R–2 with atransmit power at the base (Fig. 5). We assume that the domi-nant adjacent interferers are from I 1 and I 2.

(17)

where

I S = ( M W – 1) · Pt · R–2

I a = I 1 + I 2 + ∆1 = 2 M w · Pt ‚ R–2 + ∆1 ≈ 2 M W · Pt · R–2

Then from Eq. 17, the total number of traffic channels is

(18) M

C

I

w

w

=

+

1

3

11

C

I

P R

I I

P R

M P R M P R M

w

t

s a

t

w t w t w

=+

=⋅

( ) ⋅ ⋅ + ⋅ ⋅ +≈

–

–

– –– –

2

2

2 211 2

1

3 1

∆

C

I

C

I I

E

I

B

Rs a

b

o=

+

=

K

K

w

c

= 1.

C

I a

C

I c w

=

K

K

K

K

w

c

w

c

2 11

6=

K

K

C

I

C

I

C

I

C

I

w

c

w

c

w

c

1 1

2

2

3

2 45

2

=

=

.

K

K

w

c

< 1

K D

R

C

I c

c c

=

=

1

3

2

3

2

C

I

R

D

D

R

c

c

c

c

=⋅

=

–

–

4

4

4

6 6

K D

R

C

I w

w w2

2 2

1

3

1

3

2

=

=

C

I

R

D

D

Rw

w

w w

= =

2

2

2 2

2

2

2–

–

s Figure 4. Com parison of the ratio and the radio capacitiesbetween WLL and cellular.

0.1

0.2

0.3

0.4

0.5

0.6

0.70.80.91.0

(C/I)W

0 2 4 6 8 10 12 14

2

3

4

5

6

7

8910

Case A - bot h systems have six interferersCase B - cellular has six interferers but WLL has one

K W

K C

K W 1 K C

K W 2 K C

Region where WLLcapacity is greaterthan cellular

a = 1 5

. 8 ( 1 2

d B )

a = 1 0

( 1 0 d B )

a = 6. 3 ( 8

d B )

a = 4 (

6 d B )

a = 2 (

3 d B )

a = 1 (

0 d B )

a = 1 5

. 8 ( 1 2

d B )

a = 1 0

( 1 0 d B )

a = 6. 3

( 8 d B )

a = 4 (

6 d B )

a = 2 (

3 d B )

a = 1 (

0 d B )




In cellular systems, t he 40 dB/de c loss is used, tha t is, R–4

with a transmit power Pt the cell site (Fig. 5).

(19)

where

I s′ = ( M c – 1) · Pt · R–4

I a′ = 2 M c · Pt · R–4 + ∆2 ≈ 2 M c ·Pt R

–4

Then from Eq. 19, the total number of traffic channels is

(20)

Capacity Comparison — Comparing Eq. 18 with Eq. 20, thetwo formulas, one based on a path loss of 20dB/dec, one on apath loss of 40dB/dec, are identical. It shows that in CDMAsystems different prop agation path losses do not a ffect theradio capacity formula. Since (C/I)c > (C/I)W is always true inEq. 5, then

M W > M c (21)

Therefore , Eq. 21 is always true in CD MA systems.

Calculation of the

Capacity of WLL Systems We may compare the cap acity of CDM A with tha t of FDMA or TDMA. The C/I can be obtained from Eq. 16.Let us make some assumptions.

In CDMA systems (consider an s-sector cell with voiceactivity cycle)

Bc = 1.23 MHz R = 9.6 kb/ss = number of sectors = 3η = voice activity cycle = 0.4

M w′ = (s / η) · M W = the total channelsThen C/I from Eq. 16 becomes

(22)

Substituting Eq. 22 into Eq. 18, then into Eq. 1, the totalchannels per cell is

(23)

In FDM A or TDMA Systems — Bc = total spectrum = 1.23 MHz Bc = channel bandwidth or equivalent = 25 kHzthen

M = Bt /Bc = 49 channels• Under a condition of six interferers


(24)

• Under a condition of one interferer

Substitute E q. 9 into Eq. 1 yields

(25)

Compari ng the Capacity of CDMA with that of FDM A or TDM A in WLL Systems — The capacity of CDM A, m w′ inEq. 23, the capacity of FDMA o r TD MA, mw1

in Eq. 24 undera condition of six interferer s and the capa city of FD MA orTDMA, mw2

in Eq. 25 under a condition of one interferer areplotted in Fig. 6. Assume that the requ ired E b / I 0 of CDMA isequal to the required (C/I) w of FDMA or TD MA; then wemay conclude that the capacity of CDMA is always largerthan that of FDMA or TD MA. Also, the capacity of FDM A

or TDMA with one interferer is always greater than that withsix inter ferers.

Comparing the Capacity of WLL with the Capacity of Cellu- lar Using CDMA — We have shown that the capacity formulasfor both systems are identical by comparing Eq. 18 with Eq.20. Therefore, Eq. 23 can be used for both systems with a setof given assumptions.

mM

K C

I

C

I

ww

w w

2

2

2 2

49

1

3

147= =

=

mM

K C

I

C

I

ww

w w

1

1

1 1

49

2

24 5= =

=

.

′ =

′

= ⋅ +

= +

m

M

K

s

E

I

E

I

w

w

b

o

b

o

1

1 33

1

3

128

1 1 875

128

1. .η

C

I

E

I

b

o=128

M C

I

c

c

=

+

1

3

11

C

I

P R

I I

P R

M P R M P R M

c

t

s a

t

c t c t c

=′ + ′

=⋅

( ) ⋅ ⋅ + ⋅ ⋅ +≈

–

–

– –– –

4

4

4 421 2

1

3 1

∆

s Figure 5. CDMA system and its interference.

f 1

f 1

f 1

f 1

f 1

f 1

f 1

f 1

f 1

f 1

f 1

A

I

C

P t Home

I 4

I 3

I 2

I 5

I 1

s Figure 6. Trading voice quality with capacity in WL L system.

10 N o . o f c h a n n e l s / c e

l l / 1 . 2

3 M H z

20

30

40

5060708090

100

E b / I 0 for CDMA

C/I for FDMA or TDMA

1

200

2 3 4 5 6 7 8 9 10 11 12 13 14

(in dB)

F D M A o r T D M A f o r e q u i v a l e n t b a n d w i t h o f

3 0 k H z u s i n g d i r e c t i o n a l a n t e n n a s

C D M A f o r b a n d w i d t h o f 1 .2 3 M H z

Voice quali ty decreases

Capacity increases



IEE E Personal Communications • February 1998 53

We further make a reasonable assumption that

Eb / I 0 = 5dB for WLL

and

E b /I 0 = 8dB for cellular

Then

and

Comparing and m w′ and m c′, we may conclude th at thecapacity of WLL is double the capacity of cellular.

Advantage of Implementation

Due to the nature of the wireless communication medium andof WLL systems, there a re ad vantages in implementing aWLL system:

• The coverage of WLL is larger due to its low propagationpath loss (i.e., 20 dB/dec).

• The capacity of WLL can be larger than the capacity of cel-lular if the nu mber of interferers in WLL can be red ucedby multibeam d irectional antennas.

• Interference decrease: In a WLL, the frequency reuse dis-tance can be further reduced because the WLL fixed-to-fixed link uses directional antennas on both ends so that theinterference area becomes small. Reducing the frequen cyreuse distance more me ans that th e capaci ty is furtherincreased.

• In a WLL, no handoffs occur because it is a fixed-to-fixedlink. Furthermore, the air link from each building to thecell site can customarily be installed to reduce the interfer-ence. This link re mains unchanged a fter installation, and

the design of a WLL system is much simpler.• Since the WLL signal channel is a Gaussian noise channel

or strong Rician channel (not a Rayleigh fading channel),no interleaving of the data stream is needed. Thus, datacompression schemes and quadratu re amplitude modula-tion (Q AM) can be applied efficiently to genera te a highrate throughout, although the radio transmission rate is aslow as 14.4 kb/s.

Deployment of a WLL System for Industrialized Countries

The WLL system will be developed differently and will servein different marke ts, as described in this section.

The Requirements The requirement of a WLL system for those subscribers inindustrialized countr ies is good voice q uality. This req uire-ment is neede d to compete with the service from wirelinetelephones.

The r equirements of WLL systems for the system opera -tors in indu strial countr ies are high capacity and large cover-age. How can we meet these two requirements and still lowerthe cost of de ploying a WLL system an d utilize the spectrumefficiently? This is a big challenge.

Since the three key drivers — voice quality, coverage, andcapacity — are always competing among themselves, we may

have to deter mine an acceptable voice quality level first, andchoose a wireless communication system that can provide highcapacity and large coverage. The bottom line is to calculatethe d eployment cost per line and justify the potential of thebusiness in that area.

Why CDMA for Wireless Local Loop? The CDMA system can be used for high capacity because thefrequency reuse factor K approa ches one. All of the cell sitesuse the same radio channel. In deployment of WLL-CDMA,power control or adjustment is used to reduce near-far inter-ference. This power control will be set at the beginning of theservice, according t o all the fixed-to -fixed links, and leavethem unt ouched until the environment changes. No frequencyplanning is needed. The C DMA system uses up its processinggain for increasing capacity. Of course, due to the nature of aWLL environment , a high gain directional antenna can be usedto further reduce the adjacent interference. One 1.25 MHzradio channel can provide 40 traffic channels for mobile radio,but 77 traffic channels for WLL under the same conditions.

How to Build a WLL System in Industrialized Countries

CDMA can be used as a bundled service system. CDMA hasbeen chosen to be a standard system for cellular, PCS, and mobilesatellite (Globalstar) already. Therefore, it is a natural trend t ohave WLL using CDMA because CDMA would then becomea bundled service system; providing cellular a nd PCS services inurban and d ensely populated ar eas, and providing WLL andmobile satellite services in rural areas. They can all use CDMA.They share the same integrated network so that the investmentcost will be much lower. These bu ndled services can a lso evenout traffic density across the whole network; with cellular andPCS operating in urban areas, and WLL and mobile satelliteservices operating hea vily outside ur ban ar eas. The sameCDMA phone can be used for all the bundled services.

Deployment of WLL Systems in Developed Countries The Requirements

The radio capacity of WLL systems is a key issue. Voice qualityis not an issue in WLL systems in developed countries, in that acustomer who might not have a phone in this case already hasone, which adds great value. Because of this, a high level of spectrum e fficiency can be reached as long as voice qualitycan be sacrificed. This means that physical system capacity isnot a problem. System capacity versus voice quality is illustrat-ed in Fig. 6. In CD MA, when th e system’s req uired E b / I o(energy per bit/interference per hertz) is reduced (i.e., thevoice qua lity decreases), th e rad io capacity in terms of th enumber of traffic channels per cell increases; the calculation is

based on a CDMA radio channel of 1.23 MHz.Cost Considerations

Cost is a major issue in WLL systems in developed countries.Most people have to justify a telephone by the cost they canafford. Therefore, the business plan has to drive the cost down.

We have coined a new term, customer payment-to-investment ratio (C/I) in WLL, distinguished from the r egular acronym forthe carrier-to-interference ra tio (C/I), which is a system designparameter. C/I is a business planning parameter, where:C: customer payment in dollars in X monthsI: investment (including capital, operation, and maintenance)

in dollars per line over X months

′ = +

=mc 1 875128

6 31 40.

.

′ = +

= + =m E

I

wb

o

1 875 128 1 1 875 1283 16

1 77 76. ..

.




In a WLL system in a de veloped coun try, C is very low.Therefore, you need to have an I value that is much lowerthan C in order to make a profit. This is a big challenge.

How to Build a WLL System In Developed Countries On the Equipment Side — The cost of each piece of customerpremises equipment (CPE) needs to be lower. This meansthat a WLL has to:• Use an existing cellular system — The same phone can be

used in bo th WLL and cellular. This will lower the cost bysharing the same wireless communication network.

• Develop a low-cost WLL system with low-cost CPE and withthe capability to upgrade the system in the future — In devel-oped countries, the markets for WLL are huge; therefore, itmay be justified to choose a low-cost system solely for WLL.In a WLL system, the standard is not an issue. The WLL

links are fixed, not moving; therefore, the CPE does notrequire any standard.

In the Technology Area — In order to lower the cost of equip-ment per line, we may have to have more users share one line.The following examples can redu ce the cost:• Use multiplexing on one radio. We can use frequency-divi-

sion multiplexing (F DM ) o r t ime-division multiplexing

(TDM ) on on e radio to increase ta lk channels, but of course the voice quality may be reduced.

• Use the party l ine feature for those subscribers who arewilling share one subscribed line.

Conclusion

The WLL always provides large coverage but not necessarilymore cap aci ty than mobile cel lular systems if FDM A orTDMA is used. The WLL always provides more capacity thanthe cellular system if CD MA is used. In WLL systems, thecapacity of CDM A is always greater t han th at of FD MA orTDMA . Using CDM A schemes , the capac i ty of WLL i sroughly double that of cellular. The r equirement s of WLL sys-tems for industrial countries are the quality and service fea-

ture s. It is wise to choose a bundled services system to sharethe same wireless communication network in order to loweroperational cost and offer the subscriber, in the meantime,only a single handset to please the customers.

The WLL system of the future, in developed countries, is alow-cost system, even if voice quality has to be sacrificed. Of course, to adapt an existing cellular system, which can have bothservices, mobile cellular and WLL, share t he same spectrum ispossible. Many suggestions to lower the cost by developing low-cost WLL systems have also been addressed. WLL is a futurebusiness for both industrialized and developed countries.

References [1] W. C. Y. Lee, Mobile Cellular Telecommunication Systems, Analog and

Digital , McGraw-Hill, 1995.[2] W. C. Y. Lee, Mobile Communications Design Fundamentals , Wiley &

Sons, 1993.[3] W. C. Y. Lee, “ The Wireless Local Loop In t he Fut ure,” Telephony , Oct.

23, 1995, pp. 36–38.

Biography WILLIAM C. Y. LEE [F] received his Ph.D. from Ohio State University in 1963 inelectrical engineering. Chief Scientist and Vice President of Strategic Technologyat AirTouch Communications, Inc., he joined the company (for merly PacTelMobile Companies) in April 1985 to develop improvements to system perfor-mance and capacity. Prior to joining AirTouch Communications, he worked for

ITT Defence Communications Division (ITTDCD) engineering military mobile pro- jects. Previous to ITT, he was employed at Bell Laboratories, from 1964 to1979, where he studied wave propagation in an anistropic medium, antennatheory, mobile radio propagation and systems, millimeter and optical wavepropagation, switching systems, and satellite communications. He was thefounder and co-chair of CTIA’s subcommittee for Advanced Radio Technologiesin 1987 and was involved in digital cellular standard setting. He assisted in andsupported pursuing the CDMA system for cellular, and he also introduced theCDMA system to Korea where it later became the national standard system. Heinvented and patented a new microcell system in 1990 that reduced the fre-quency reuse factor from K = 7 to K = 3 and increased radio capacity by 2.5times over the conventional microcell. His UHF mobile radio propagation modelis known as the Lee Model. He conducts three three-day courses sponsored byGeorge Washington University. He is a Radio Club of America Fellow and a dis-tinguished alumnus of the Ohio State University. He has received the IEEE VTSAvant Garde Award, the Bell Laboratories Dedicated Service Award, and theITTDCD Technical Contribution Award. He has written more than 200 technicalpapers and three text books. Most recently in 1998, he revised his book Mobile Communications Engineering (McGraw-Hill).

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Wireless Local Loop Comparison (Lee)