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IT Strategy

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http://www.tenet.res.in/Papers/wll/iete1.html

Ashok Jhunjhunwala, Devendra Jalihal, K.GiridharTelecommunications and Networks (TeNeT) Group

Dept. of Electrical Engineering, IIT MadrasChennai 600 036, INDIA

Abstract

The enhancements in Internet technology and mobile access technology over the lastdecade can be leveraged effectively to build Wireless in Local Loop (WLL) systems, whichcan enable rapid expansion of telecom and Internet access in developing countries.However, the design of a WLL system requires one to understand some fundamentalsconcerning the Access Network and its connectivity to backbone network as well as thetraffic requirement for a voice and Internet connection. The requirements of WLL, incontrast to that of a Mobile cellular system need to be clearly understood. Equally important

is the concern for capacity and spectral efficiency, especially as higher bit-rate Internetsystems becomes a must for developing countries to get a fair share of the economicadvantages that telecom technologies provide. This paper looks at these fundamentalissues in context of GSM, IS-95 and DECT technologies. The paper further takes a brieflook at some recent technological developments, which are likely to impact the Wireless inLocal Loop systems.

The paper concludes with a discussion on the emerging third generation (3G) wirelessstandards, and the new technologies, which are being introduced into the network, andwhat will be their impact on Internet and multimedia bit-rates and services.

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Project:Wireless Local Loop

Version:01

Date:9/10/2010

Status:Preliminary

Reason forIssue

Initial IssueRevision No. (include description of revision below)
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KNUT HAAKON FLOTTORP: VERSION: 01 DATE: 10/09/10

Table of Content

1. Introduction.....................................................................................................................................12 The Internet Tangle..........................................................................................................................23 Wireless in Local Loop Vs Mobile Wireless Access System.............................................................3

3.1 Mobile Telephone System.......................................................................................................3

3.2 Wireless in Local Loop System................................................................................................3

4 Capacity and Spectral Efficiency......................................................................................................4

4.1 Channel Pay Load...................................................................................................................4

4.2 Signalling overhead.................................................................................................................5

4.3 Modulation Efficiency...............................................................................................................5

4.4 Cell radius...............................................................................................................................5

4.6 Interference Reduction Techniques..........................................................................................8

4.6.1 Sectorisation.........................................................................................................................9

Other Interference Reduction Techniques .....................................................................................9

4.7 Capacity................................................................................................................................10

5 Cellular and Wireless Standards....................................................................................................11

5.1 AMPS and NMT....................................................................................................................11

5.2 GSM and D-AMPS................................................................................................................11

5.3 IS-95......................................................................................................................................11

5.4 DECT....................................................................................................................................11

5.5 Capacity provided by GSM, IS-95 and DECT system.............................................................12

5.5.1 Modulation and multi-access efficiency...............................................................................12

5.5.2 Subscriber Densities for Various Systems...........................................................................13

5.5.3 Capacity Comparisons for Providing 64 kbps Data Connections .........................................14

6 The Future......................................................................................................................................15

Space-Time processing ..............................................................................................................15

6.1.2 Transmit Diversity...............................................................................................................15

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6.1.3 Spatial Multiplexing.............................................................................................................15

6.1.4 Space-Time Coding............................................................................................................16

6.2 3G Terrestrial Wireless Standards.........................................................................................16

6.2.1 Brief Background on the New Techniques...........................................................................17

6.2.2 DECT evolution towards 3G................................................................................................17

6.3 Internet Access using Wireless Networks...............................................................................17

7 Summary and Conclusion..............................................................................................................18References.......................................................................................................................................18

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1. Introduction

Till around mid 80s' in India, a local loop or an Access Network (AN) used to consist of a pairof copper wires connecting the subscribers at home or office to the nearest exchange. Thelocal loop length in urban areas would be typically as large as 6 to 8 kms and the coppergauge used was 0.5 mm to 0.6 mm. The loop was designed to carry 4 kHz voice and was

difficult to maintain, with almost 85% of all faults found in the local loop. Above all, it wasexpensive, difficult, and time-consuming to deploy. With the rising cost of copper and cost ofdigging increasing every year, if one had continued with such an approach, the per line localloop cost itself would have been Rs.40,000 to Rs.50,000 and would have amounted to over85% of the total cost of putting a telecom network.

Fortunately, an uncelebrated but major technological innovation changed the AccessNetwork from mid-eighties onward. As shown in Fig. 1, the Access Network now consists ofa fibre from an exchange to a RLU/RSU and a typically 3-4 km copper loop from theRLU/RSU to the subscriber premises. The signals carried on fibre is time multiplexed digitalvoice and signalling. A RLU typically serves 1000 to 4000 subscribers, and the signal fromRLU to exchange consists of 4 to 16 E1 [1]. The copper used now is only 0.4 mm and thecosts are down considerably.

The rising cost of copper however continues to make even this solution expensive. Todaythe per line 3-4 km coppercost (including layingcharges) would beRs.13,000 to Rs.16,000,the shared fibre cost wouldtypically be Rs.1000 perline, and RLU cost wouldbe Rs.3000 to Rs.4000.

The Rs.20,000 plus cost ofthe Access Network todayagain amounts to almost2/3 of the total per linecost.

The signalling protocol onthe Access Network (in the signalling slots on E1 link between RLUs and exchange)remained proprietary for about a decade, forcing an operator to purchase Access RLU andexchange from the same manufacturer. In mid 90s', however, access signalling protocolswere finally standardised internationally in the form of V5.1 and V5.2 protocols [2].

This separated AN from anexchange and AN could now beindependently deployed. TheAccess Network could now usenew innovative technologies likefibre, wireless, DSL on copper,hybrid fibre-coaxial cable or evenpower-lines. As accessdominated the cost, was mostfault-prone, and was most timeconsuming to deploy, availability

of new access solutions became the key to expanding the telecom network, especially in

developing countries.

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This paper focuses on some of the fundamental issues involved in the choice of wirelessAccess Networks and their interconnection to the PSTN and Internet network from the pointof view of developing countries. Obviously, wireless ANs, just like any other access networkof today, must connect to an exchange using V5.2 access protocols. Today, a telecomnetwork can no longer focus on providing telephone service, but must integrate Internetservices. Section 2 of this paper therefore looks at some of the issues involved in use ofInternet on existing telecom networks and the lessons for emerging wireless Access

Network. We then proceed to discuss in Section 3, the distinction between wireless in localloop and mobile communication system and the requirements of the two. In Section 4, welook at some important issues governing choice of wireless access technology, the issuesthat determine capacity and spectral efficiency. In Section 5, we briefly describe some of thekey wireless standards that have emerged over the last twenty years and discuss theirsuitability for use in wireless in local loop. We conclude with a refocus on cost, as reductionof cost is key to expansion of telephone and Internet network in developing countries.

2 The Internet Tangle

Internet has emerged as second only to telephone in connecting people and may tomorrow

subsume the telephone network [3]. But today, the Internet access at homes and offices isbased on the telephone network. The Internet access today appears to be simple; just takea telephone line, connect a modem and a computer and dial an Internet Service Provider(ISP). The ISP has a bunch of telephone lines and an equal number of modems connectingthe users to a Router as shown in Fig.2. This router is connected to other routers on Internet.A dial-up connection to an ISP router gives a user access to everyone and everything onInternet.

This straightforward looking access to Internet, however, has problems. The telephonenetwork is designed to handle 0.1E traffic per subscriber. This is generally adequate fortelephony. However, Internet sessions are usually of long duration, very often evenexceeding an hour. As a significant percentage of telephone users start using Internet, the

load on the telecom network would far exceed 0.1E per subscriber, resulting into severecongestion and eventual collapse. If this has not happened so far, it is only because a smallpercentage of telephone users have started using Internet.

The second problem isassociated with thelocal call chargesassociated with usingInternet in this manner.The telephone call forInternet costs Rs.26 perhour in Indian citiestoday, in addition to thecharges payable toISPs. Thirdly, the

analogue modem-to-modem link between the subscriber and the ISP is unreliable. Onedoes get 33.6 kbps connectivity sometimes, but connection could go down to 9.6 kbps andeven 4.8 kbps at times. Further, the connection often drops. Finally, an ISP with N telephonelines, N modems and a N port router could serve at most N subscribers at time. Aconnection drop may not get a reconnection during busy hours.

This Internet tangle requires a different approach in order to support future growth.Fortunately, even though an Internet connection is kept on for long hours, a peculiarity of

computer-to-computer communication is that the use of the connection is not continuous butbursty. Packets are transmitted to and from Internet in bursts, with the communicationalmost silent most of the time. A circuit switched connection on telephone network, however,



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is unable to take advantage of this and occupies resources throughout the connection, andthereby congesting the network.

An ideal solution to this problem is to have totally shared, packet-switched access. But localloop is usually a separate physical line to each subscriber and packet access on thisunshared line gives no advantage, as no one else can use this resource. In such a situation,it is advisable to separate the Internet data at a point nearest to the subscriber, where datafrom multiple subscribers can be multiplexed. This is shown in Fig.3, where separation ofInternet data and voice data takes place at the Access Unit (AU), located typically at a street-corner.

As shown in the figure, both wired and wireless interfaces to the AU are possible. High-speed digital subscriber loop technology (HDSL) and narrow band ISDN equipment canprovide high speed, simultaneous reliable voice and Internet access on a single copper pair.In wired access there is strictly no restriction of the bit-rate between the subscriber and theAU.

However, wireless access makes use of an important shared resource, the frequencyspectrum. It is this resource, which limits the capacity of a wireless system. Therefore

medium access strategies which assign channels to a subscriber only when he/she wishesto transmit a packet would significantly enhance capacity for Internet access. Wirelessaccess networks which can share the frequency spectrum and utilise it during packet burstare obviously very attractive candidates for rapid expansion of Internet access in the future.

3 Wireless in Local Loop Vs Mobile Wireless Access System

There is little doubt that wireless access systems deployed at the turn of century wouldprovide digital access. Wireless connectivity to subscribers today is provided by mobilecommunication systems as well as wireless in local loop systems. These two appear to besimilar and are often confused with each other. However, the requirements for the two

systems are significantly different.

3.1 Mobile Telephone System

Mobile Telephone systems are primarily meant to provide telephony for people on the move.The telephone is meant to keep the person connected while he/she is away from home andoffice. The key here is universal coverage. The mobile telephone must be reachablewherever the subscriber is, in the car, on the street, or in a shopping mall. Otherrequirements are less severe. A modest voice quality is acceptable as the user may mostlybe speaking from a location with high ambient noise. Data communication is not veryimportant, and wherever required, low-bit rate data communication will be acceptable. Faxcommunication is unlikely to be used. Furthermore, the traffic per subscriber will not be veryhigh, since the user is unlikely to make long calls. One is typically looking at 0.01 to 0.02Erlang traffic per subscriber. Some airtime charges for such premium service are generallyacceptable.

3.2 Wireless in Local Loop System

Wireless in Local Loop (WLL), on the other hand, is meant to serve subscribers at homes oroffices. The telephone provided must be at least as good as wired phone. Its voice qualitymust be high -- a subscriber carrying out long conversation must not be irritated with quality;

one must be able to use speakerphones, cordless phones and parallel phones. Thetelephone must support fax and modem communications and should be connectable to aPublic Call Office. Ability to provide atlas medium rate Internet access is a must. Further, the



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traffic supported should be reasonably high at least as high as 0.1E per subscriber.Besides, ability to support a large number of subscribers in an urban area (large teledensity)with a limited frequency spectrum is required. Finally, for the systems to be of use indeveloping countries, the cost of providing this wireless access should be less than thatrequired for wired telephone. Airtime charges are totally unacceptable.

Therefore, even though the mobile communication systems and wireless in local loopsystems may appear to be similar, and sometimes even used interchanging, therequirements are quite distinct. Let us now take a more detailed look at some of the issuesgoverning the choice of wireless access technology.

4 Capacity and Spectral Efficiency

Having looked at the way to interconnect a WLL system to the PSTN and the requirementsthat a WLL system has to fulfil, let us now take up the most important issue that governs thechoice of a WLL technology. A wireless communication system has to recognize that thefrequency spectrum available will always be limited. Obviously, since the telephone as wellas a Internet connection is not used continuously, the channels must be assigned to a

subscriber on demand. But this is not enough. The key focus has to be efficient use andreuse of the spectrum.

What governs use and reuse of spectrum?

The use and reuse of spectrum is governed by multiple factors including:

I. Channel pay load (bit rate)

II. Signalling overhead

III. Modulation efficiency

IV. Cell-radius (range)

V. Choice of multiple access

VI. Interference reduction techniques

VII. Spatial diversity and space-time processing

We will discuss spatial diversity and space-time processing in section 6. Let us discuss theother factors here.

4.1 Channel Pay Load

It is obvious that higher bit-rate payload will require larger frequency resources as comparedto a lower bit-rate payload. Therefore for voice communication on wireless systems, it maybe desirable to have efficient voice compression and lower bit-rate voice codecs. Theresulting slightly inferior quality is quite acceptable for mobile communications. But fortelephones at homes and office, toll quality voice communications at 32 kbps / 64 kbps may

often be desirable. Besides PCM and 32 kbps ADPCM give large degree of transparency forother communication services like fax communications. Even then, lower bit-rate voicecommunications may sometimes be acceptable. However, when one wishes to also use theline for Internet communications, higher bit-rate communication will obviously be desirable.



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As the frequency resource used per channel is directly proportional to the payload bit-rate,medium to high bit-rate Internet implies higher use of frequency resource.

4.2 Signalling overhead

As signalling is key to setting up, monitoring and tearing down of a call, signalling

communications need to be carried out on air between subscriber equipment and the basestations. The signalling channels may be dedicated for each user or may be shared. Usually,more sophisticated the system, more is the signalling requirement. The signalling becomesan overhead that takes away certain frequency resources and plays a role in overallefficiency of spectrum usage.

4.3 Modulation Efficiency

The modulation technique employed has a direct bearing on efficient use of spectrum.Highly spectrum efficient techniques have been developed over the years. For example, 16-

QAM technique is more spectrally efficient compared to 8-QAM technique, which in itself ismore efficient than QPSK and MSK modulation techniques. But more efficient techniquesare usually expensive to implement and may sometime require larger power margins. Thesetechniques are used commonly with systems such as high bit-rate point to point microwavelinks as the number of such systems required are small and each of these systems is sharedby a number of subscribers. But for wireless in local loop, one would require the technique tobe implemented in each subscribers equipment. Therefore cost in an importantconsideration. Further, often the power margins available are not large. Therefore QPSK,MSK or even BFSK techniques are often used even though their spectral efficiency ismoderate.

4.4 Cell radius

Cell radius is perhaps the most important factor governing the spectrum utilisation in awireless system. Let us take a simple example. Let there be Nindependent channelsavailable for use in a cell of radius r. It is the reuse efficiency discussed later insection 4.6, which would determine the reuse of channels in neighbouring cells. Leaving thisissue for a later discussion, let us concentrate on the Nchannels available for a cell. Let usalso assume that the traffic per subscriber is e Erlang. Therefore the number of subscribersthat can be served in the cell works out to N/e and Subscriber Density (SD) that can beserved in this cell is approximately,

Thus, subscriber density is inversely proportional to the square of cell radius. The implicationof this can be seen by a example: For e.g., ifN= 200, e = 0.1 Erlang, the capacity(subscriber density) varies with cell radius as follows:

r = 25 km, SD 1 per sq. km





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r = 500 m, SD 2550 per sq. km

Therefore, cell radius plays the dominant role in determining the subscriber density given acertain frequency spectrum. In other words, a smaller cell radius is the key to efficient use ofspectrum and one may have to use cell radius as small as 500m, if one desires areasonable subscriber density.

4.5 Choice of Multiple Access

A key parameter determining the efficient reuse of spectrum is governed bymultiple-access technique used. The access technique defines how the frequency spectrumis divided into channels and affects reuse of the channels.

4.5.1 FDMA

The oldest technique used in wireless access, especially in mobile communications, isFrequency Division Multiple Access (FDMA). Here the available frequency spectrum isdivided in a number of orthogonal frequency channels and these channels are assigned tothe user on demand. FDMA can be used both for analogue as well as a digitalcommunications. This simple technique used extensively in first generation analogue mobilesystem, however, had poor reuse and the same channels can be reused only once in 14 or21 cells. One way to increase re-use efficiency is by employing sectored or directionalantennas at the cell site. (A brief discussion on sectorisation will follow in section 4.6.1) Evenwith sectorisation, say 3 sectors per cell the best planning gives a typical reuse of once in 7cells [4], implying reuse factor of 1/7 = 0.143 per cell.

4.5.2 TDMAThe most widely used multi-access technique today, both for mobile as well as in wirelesslocal loop, is Time-Division Multiple Access (TDMA). Here the frequency spectrum availableis again divided, but into a few (wide) bandwidth channels or carriers. Each carrier is usedfor transmission of multiple time-multiplexed channels. Each such orthogonal channel (ortime-slot as is commonly referred to) could be assigned to a user on demand. The techniquecan be used only for digital communication, and the ability to work with smaller signal tointerference ratio in digital domain, gives this technique better reuse factor as compared tothe analogue FDMA. For example, with three sectors, a cell reuse factor of 1/4 or even 1/3 isachievable [4].

4.5.3 CDMA

Late in the eighties emerged a multiple-access technique referred to as Direct Sequence,Code Division Multiple Access (DS-CDMA). Based on spread spectrum techniques usedextensively in defence applications for over twenty years, this technique enables definition ofnear-orthogonal channels in code-space. CDMA enables multiple channels to use the samefrequency and time slots. Each bit to be transmitted by or for a user is uniquely coded byspreading the bit into 64 or 256 or even 1024 chips. The receiver separates the data of auser by a decoder which correlates the receive signal with the code vector associated withthat user. On correlation, the interference from other users would become nearly zero andadd only a small amount of noise, where as the desired signal will be enhancedconsiderably. The technique is useful in exploiting the inherent time-diversity from multipath

delay-spread, especially if the spreading is significant (Chip time of 0.1 sec to 1 sec. The

only problem with the technique is that as completely orthogonal codes are not possible,especially on the uplink, the total bit-rate supportable from all users using this technique is



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significantly less than the total bit-rate supportable with TDMA and FDMA technique usingthe same frequency spectrum.

This disadvantage in the CDMA system is made up by better reuse efficiency, as the samespectrum with different set of codes can almost totally be reused in every cell. Thetheoretical reuse efficiency could be as high as 1.0, but in practice less. With sectoredantennas, it is possible to reuse the spectrum in each sector, with a 3-sector cell siteresulting in a reuse efficiency of nearly 0.5 per sector.

An issue that is as important as reuse offrequency spectrum is fine power controlso that more or less equal power fromeach subscriber set reaches a basestation. Such a control mechanism wasdifficult to implement and delayedwidespread use or CDMA for sometime.Fortunately, the problem has been largelyovercome today.

4.5.4 MC-TDMA

One of the latest access techniques thathas emerged is the Multi-Carrier-TimeDivision Multiple Access or MC-TDMA withDynamic Channel Selection (DCS). MC-TDMA is a variation of TDMA. A time

frame is divided into time slots, as in TDMA; however, in each time slot, a subscriberequipment or Base Station can use any of the several frequencies available. Therefore as inTDMA, the spectrum available is divided into a set of frequencies. Each frequency or carriercan be used in anytime slot by communicating equipment. The key is that no frequency ortime-slot is assigned to any subscriber equipment. Nearly all channels are available as apool for every one to choose from. A technique known as Dynamic Channel Selection

(discussed in more detail in section 4.5.5) governs the choice of the channel and is key tohigh reuse efficiency. A reuse efficiency of 0.7 to 0.8 may be possible from cell to cell andwith a 3 sector cell, one may get a reuse efficiency of almost 0.7 per sector.

4.5.5 Fixed Channel Allocation (FCA) Versus Dynamic Channel Selection(DCS)

Most wireless access systems till recently used Fixed Channel Allocation which required aprior allocation or assignment of certain number of channels (carrier frequencies) to a sectorin a cell using an exercise generally referred to as frequency planning. The planning had tobe carried out using a worst case scenario assuming the nearest possible distance betweenthe interfering signals. Having carried out this worst case planning, and having assigned thechannel pool to the base station serving the sector, it was up to these base stations (or fixedpart) to assign channels to subscribers in the sector on demand.

A totally different approach emerged recently [5], initially as a theoretical concept and assoon implemented in a number of systems. This approach, called Dynamic ChannelSelection (DCS), does no assignment of channels to any base station or subscriberequipment. All channels are available to every one. The radio equipment is designed tomeasure the signal strength that it receives on all channels (using something akin to aspectrum analyser) and thus determines the actualradio environment in its vicinity. It carriesout this measurement on a continuous basis, whether it is using a channel or not. Thus thecomplete knowledge of the radio environment enables it to select a channel in which it cancommunicate best. The key is that even while it is communicating on one channel, it is

measuring the radio-environment in all other channels. If it finds that another channel canprovide it better communication, it switches to this channel seamlessly.



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Thus the radio equipment (usually associated with the subscriber equipment) continuouslymonitors all the channels, and selects the best channel dynamically. It tries to establishcommunication on this seemingly best channel. If it succeeds, so be it; otherwise it tries thenext best channel.

The DCS is thus based not on worst-case scenario, but on actual radio environment. It isthus possible sometimes to reuse a channel even 25 m from the other. One such case isshown in Fig.5 (a) and 5(b). In this case two base stations, B1 and B2, are located only 25 mapart. In scenario (a), the two subscriber equipments (handset or HS) referred to as HS1and HS2, are located close to each other. The HS1 is communicating to BS1 and HS2 withBS2. The two communications cannot reuse same channel and use different duplexchannels C1 and C2. However, in scenario (b), the HS1 and HS2 can communicate onsame duplex channel C1. This is because HS1 is about 2 m from BS1 but 25 m from BS2.The interference that it receives from BS2 (on the same channel) is approximately (25/2)2 ornearly 22 dB less than the signal it receives from BS1. This interference level causes noproblems. Same is the situation for reception by HS2 from the two base stations.

This reuse even 25m apart is possible because of DCS. No FCA with worst case planningcan come close to this in reuse. DCS gives a factor of 2 to 4 in reuse advantage comparedto FCA [6].

It is the DCS, which gives techniques like MC-TDMA an edge over other multiple-accesssystems that, makes its reuse efficiency on the average very high. Of course, DCS requiresfairly sophisticated radio environment measurement techniques in each radio equipmentincluding subscriber equipment. However, such sophistications can easily be included intoday's integrated circuits and digital signal processors.

4.6 Interference Reduction Techniques

The target Signal to Interference Ratio (SIR) determines the re-use distance primarily. Thetarget SIR is based on the minimum sensitivity required at the receiver input in order toobtain a particular Bit Error Rate (BER). The required BER is typically 10-3 for voiceapplications (and 10-6or higher for data applications by using error control coding and/orARQ). Depending on the choice of multiple access, the modulation scheme and theparticular application (mobile or fixed wireless), the target SIR set point will differ.

Interference reduction techniques are widely used in wireless systems to increase re-useefficiency while retaining the target SIR requirement [7]. These techniques include:

A. Sectorisation



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B. Voice Activity Detection

C. Power Control

D. Rate Control

E. Frequency Hopping

4.6.1 Sectorisation

It is possible to sectorise a cell by using base stations with directional antennas, such thatthe base station serves subscribers only in that sector as shown in Fig. 6. Suchnon-overlapping sectors can not only reduce the interference power, but also increaserange. It is also possible to use overlapping sectors in order to improve trunking efficiency,and thereby, support more users in an area. Overlapping sectors are more suitable for fixedwireless applications.

Directional antennas providing 600sectors have been used in GSM and IS-136 deploymentsto increase the re-use ratio from 1/7 to 1/4 and even 1/3 (i.e. an increase from 7-cell reuse to4cell and 3cell reuse). A capacity increase of nearly 4.5 times over omni cell capacity hasbeen obtained using 600 sectoring in IS-95 deployments. The reuse is better if the antennaat the base station has high loss outside the sector(s) that it is supposed to serve. Thesubscriber end antenna can also be directional in case of fixed (non-mobile) subscriberinstallations. This helps immensely to reduce interference power especially when thesubscriber is near the edge of the cell. Of course, directional antennas are not preferable onthe mobile site since they affect handoff performance.

Other Interference Reduction Techniques

In continuously transmitting wireless systems like DS-CDMA, Voice Activity Detection (VAD)is very useful to reduce power consumption, and also increase user capacity. In VAD, notonly is the presence or absence of speech energy monitored, but also the regionscorresponding to unvoiced speech and the transition regions between voiced to/fromunvoiced speech. IS-95 exploits VAD to reduce its bit-rate from 8kbps to 4/2/1kbpscorresponding to unvoiced speech, transition regions, and comfort noise regions. While the13kbps GSM codes also incorporates VAD, it is not explicitly exploited in reducing power orbit-rate.

VAD information can be used to define rate control and/or power control operations. Forexample, in IS-95, when VAD returns a no-speech activity flag, the bit-rate for that user can

be dropped from 8kbps to 1kbps (rate control) by lengthening the bit period by a factor ofeight (from Tb to 8Tb). Simultaneously, the transmit power of the user can be reduced by afactor of 82 = 64. Now, the integrator in the receive will integrate over 8Tb (and not Tb), whichwill give back the same energy per bit for that signal. Therefore, rate and power control donot charge the effective energy-per-bit or SNR for that signal.

Of course, sophisticated power control is also done (both on the base and mobile stations) inIS-95 to mitigate the near-far problem of direct sequence spectrum communications. EvenTDMA/FDMA systems can employ rather simple power-control mechanisms to reduce co-channel and adjacent channel interference. For example, all low range (near base station)duplex channels can lower their peak powers and still meet the target SIR in GSM systems.

Another way to reduce co-channel interference in FCA applications is to have a pre-set slowfrequency hopping between the base stations in the region. Thus slow frequency hopping,



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typically once every second or so, is synchronised by the mobile switching centre whichcontrols these base stations. The effort of this slow hopping of the frequency allocation is torandomise the geographical location of the co-channel signal(s), and thereby, on theaverage reduce interference and increase re-use efficiency.

4.7 Capacity

Thus, there are several factors, which determine the efficient use of a radio spectrum in awireless in local loop system. In this section, we will attempt to determine an expression forthe capacity, C, of a WLL system. The capacity expression will be in terms of the number ofsubscribers per sqkm that can be served given an available spectrum, Erlang traffic used,cell radius and number of sectors used and the payload that has to be provided to eachsubscriber. Some of the popular access techniques will be compared using this capacityexpression. Let us start with a few definitions:

Capacity = C = Numbers of subscribers served per sq km

Multi access and modulation efficiency and overheads = Mbps/hz: This factor combinesmodulation efficiency, effect of overhead and multi-access efficiency and gives the number

of bits of payload delivered per Hz of spectrum.

r = radius in km for each cell

ns = number of non-overlapping sectors used per cell

R= reuse efficiency (depends on access technique and ns, and governs fraction of totalspectrum that can be effectively used in each cell and each sector)

e = Erlang traffic per subscriber

Te = trunking efficiency (a factor which depends on the Erlang traffic per subscriber, e,

and number of channels available in each sector/cell)

S = total spectrum 9available in Hz

x= payload in bps required per subscriber.

Therefore, total number channels ofxbps available is total spectrum available multiplied bymodulation and multi-access efficiency and divided by payload required per subscriber or(SM/x).

Since Rfraction of these channels can be utilised in each sector, the numberxbpschannels available per sector(SMR/x).

Since traffic per subscriber is e Erlang, if the number of channels available in each sector islarge, the total number of subscribers in each sector would have simply been the number ofchannels available divided by e. However, if the number of channels per sector is not large,the trunking efficiency, Te, will reduce the number of subscribers that can be served in eachsector.

Therefore, number of subscribers in each sector

Since there are ns non-overlapping sectors per cell, and ris the radius of the cell, the

capacity or subscriber density Cthat can be served is



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subscribers/sq.km (2)

Note that not all variables on the right hand side of the equation are independent as Ris afunction of access technique and ns, whereas Te is a factor depending on (SMR/x) as wellas to some extent on the non-ideality of the sectored antennas.

5 Cellular and Wireless Standards

Having discussed various parameters, which affects the efficient use and reuse of frequencyspectrum, let us take a brief look at some of cellular and wireless standards that haveemerged in the last twenty years. In section 5, we will look at the capacity provided by somethem.

5.1 AMPS and NMT

First generation cellular system, which promised wide-scale mobile communications,emerged in early eighties in the form of NMT-450 (Europe, 1981), NTT (Japan, 1978), andAMPS (USA, 1983). These were analogue FDMA systems and used 400 and/or 900 MHzfrequency spectrums to provide analogue voice connection to mobile users [7].

5.2 GSM and D-AMPS

In late 80's emerged the second-generation mobile systems. These systems were digitaland mostly used TDMA. GSM [8], [9] was the most prominent amongst these and used 13.6kbps voice coding. Initially designed for 900 MHz operation, the systems are now availablein 1800 MHz and 1900 MHz in the name of DCS1800 or DCS1900. The GSM system is by

far the most dominant system used in the world today.

A second TDMA system that emerged in the USA at about the same time is the DigitalAMPS or D-AMPS [10] and was standardised as IS-54. The system was designed tomaintain compatibility with analogue AMPS system and used three 8 kbps time slots on a 30kHz band used in AMPS. In mid-nineties, IS-54 standard was further refined andgeneralised and is today known as ASI-136. This has a proposal to incorporate micro-cellarchitecture and Dynamic Channel Selection.

5.3 IS-95

In early 90's, a CDMA standard emerged for mobile communication [11] in the form of IS-95.The standard used a 8 kbps (and later, a 13 kbps) voice coder, and was initially designed tooperate in 900 MHz band. The IS-95 mobile system is widely used in South Korea and partsof North America, where GSM system was initially not allowed to operate. The IS-95 systemis now proposed to be used as a Wireless Local Loop solution in some countries. An 1800MHz version IS-95 has also been proposed [12].

5.4 DECT

In early 90's another wireless standard emerged in Europe, called DECT [13]. This MC-

TDMA system, initially proposed for home cordless and office PBX market, was lateradopted for wireless in local loop system. The system is defined for the 1800 MHz band, andemploys 32 kbps ADPCM coding for voice and allows 64 kbps and even higher-rate data



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communication. The system uses Dynamic Channel Selection [14] to enhance frequencyreuse. Similarly, the Micro-cellular system PHS [15] was standardised in Japan and PACS[16] was standardised in USA at about the same time. These systems were also proposedto provide communications with limited mobility in areas with very high teledensity.

5.5 Capacity provided by GSM, IS-95 and DECT system

In what follows, an attempt is made to determine capacity (as defined in section 4.7) ofGSM, IS-95 and DECT systems. A comparison of the capacity is difficult because thesesystems were designed to provide communications in different environments. Even then, anattempt is made in this direction.

5.5.1 Modulation and multi-access efficiency

Lets begin with comparison of the factorM, the modulation and multi-access efficiency factordefined in section 4.7. Note that this also takes account the signalling overhead anddetermines the number of bps of payload per Hz of spectrum delivered by each technique.

GSM: Enables 8 channels each with 13 kbps payload using 200 kHz of spectrum.Obviously,

M32(GSM) = 8 x 13 kbps = 0.52bps/Hz2000kHz

IS-95: 25 voice channels of 8 kbps without Voice Activity Detection (no VAD) in 1.25 MHzspectrum [11]. Therefore,

M8,noVAD(IS-95) = 25 x 8 kbps = 0.16bps/Hz

1250kHz

40 voice channels of 8 kbps with VAD and silence suppression in 1.25 MHz spectrum.Therefore,

M8,VAD(IS-95) = 45 x 8 kbps = 0.256bps/Hz1250kHz

25 voice channels of 13 kbps with VAD and silence suppression in 1.25 MHz spectrum.Therefore,

M13,VAD(IS-95) = 25 x 13 kbps = 0.26 bps/Hz1250 kHz

DECT: 120 channels of 32 kbps full duplex in 20 MHz band.

M32(DECT) = 120 x 32 x 2 kbps = 0.384bps/Hz20,000 kHz

It is obvious that modulation and multi-access efficiency is higher for GSM than for DECT orIS-95. But IS-95 and DECT perform better compared to GSM when it comes to reuseefficiency, as discussed in section 5.5.2. DECT uses less efficient modulation (GMSK with

BT = 0.5), which can be implemented at a very low cost. It also uses a large signallingoverhead. However, the dynamic channel selection in DECT, more than compensates forthese deficiencies.



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5.5.2 Subscriber Densities for Various Systems

Let us now look at the capacity of each system. For comparison purposes, let us assume 20MHz spectrum (10 MHz for uplink and 10 MHz for downlink) is available for each system.Let us take Erlang traffic per subscriber to 0.15 E/subscriber, taking into account that WLLtoday is not only used for telephone, but also for Internet service. What does each type ofsystem enable us to do?

5.5.2.1 GSM Capacity

GSM uses 13 kbps voice communication. In 20MHz of total spectrum (paired spectrum of10MHz each way), 400 channels are available. When using non-sectorised cells, at best areuse efficiency of 0.33 is possible. This implies, about 400 x 0.33 or 135 channels per cellimplying a trunking efficiency of about 0.85. Thus the number of subscribers per cell worksout to be approximately 766. For a cell radius of 10 kms, 3 kms and 1 km, of the subscriber

density served given by 766/( r2) will be 2.4, 27, and 245 subscribers/sqkm, respectively.

Using three-sector deployment with 120o sectors, the reuse efficiency will be closer to 0.2

per sector per cell. This would give 400 x 0.2 or 80 channels per sector giving a trunkingefficiency of 0.8. This works out to be 80/0.15 or 425 subscribers in each sector of a cell.Therefore for cell radius of 10 kms, 3 kms and 1 kms, the subscriber density supported is

425/( r2) or 4, 45, and 410 subscribers/sqkm, respectively.

5.5.2.2 IS-95 Capacity

IS-95 uses either 13 kbps or 8 kbps voice communications with or without VAD. For use of 8kbps with VAD, 20 MHz spectrums (for two way communications) would support 10,000 x0.25/8 or 320 voice communications (using the fact that M8, VAD (IS-95) = 0.256 bps/Hz)simultaneously. For a non-sectorised cell, with reuse efficiency R= 0.7, nearly 225 channelsare available. The trunking efficiency for these many channels would be 0.9 implyingnumber of subscribers per cell would be 225 x 0.4/0.15 or 1350 and for cell radius of 10 km,

3 km and 1 km , subscriber density would be 4.3, 47 and 429 subscribers/sqkmrespectively. For 3 sectors cell, reuse efficiency is approximately 0.5 for each sector. Thetrunking efficiency would again be 0.9 and the subscriber density served wouldapproximately be 9.2, 102, and 920 subscriber/sqkm, respectively.

For 8 kbps communications without VAD, 20 MHz spectrums would support 200 voicecommunications simultaneously. The trunking efficiency would now reduce to 0.85 and 0.82for non sectorised and 3 sector deployment and the subscriber density supportable for 10km, 3 km and 1 km would be 2.5, 28, 252 subscribers per sqkm respectively for non-sectorised deployment and 5.2, 58, and 522 subscribers/sqkm for 3 sector deployment.

For 13 kbps communication with VAD, 20 MHz spectrums would again support 200 voicechannels and subscriber density supported would be approximately same as that in 8 kbpswithout VAD case.

5.5.2.3 DECT capacity

DECT supports 120 full duplex channels of 32 kbps in 20 MHz spectrums. The reuse factoris about 0.7 for non-sectorised deployment, implying 85 channels in a cell.

For 85 channels, the trunking efficiency should have been 0.8. However, due to dynamicchannel selection and due to all 120 channels being available to every sector/cell, an

average trunking efficiency of 0.9 is more likely to be achieved. The number of subscribersper cell would therefore be 85 x 0.9/0.15 or 510, and the subscriber density supportable for10 km, 3 km, 1 km and 500 m radius would be 1.6, 18, 162 and 650 subscribers/sq km. Cell



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radius of 500 m has been taken as this is possible in DECT, whereas it is very difficult andexpensive to implement it in GSM or IS-95.

For a three-sector deployment, reuse efficiency is 0.5 for each sector implying 60 channelsin a cell. Again, trunking efficiency would remain close to 0.9. The subscriber densitysupportable would therefore be 3.4, 38, 343 and 1375 subscribers per sqkm. Tables 5.1 and5.2 tabulates these results for GSM, IS-95 and DECT.

Cell-radius km GSM (13) IS-95(8, noVAD) and

IS-95 (13, VAD)IS-95(8,VAD)

DECT (32)

10km 2.4 2.5 4.3 1.6

3km 27 28 47 18

1km 245 252 429 162

500 m -- -- -- 650

Table 5.1: Subscriber density (subscriber/sqkm) supportable in single sector deployablespectrum = 20 MHz, Erlang traffic = 0.15 E/subscriber

Cell-radiuskm

GSM(13)

IS-95(8, noVAD)and

IS-95 (13, VAD)

IS-95(8,

VAD)

DECT(32)

10km 4 5.2 9.2 3.4

3 km 45 58 102 38

1 km 410 522 420 343

500 m -- -- -- 1375

Table 5.2: Subscriber density (subscriber/sqkm) supportable in 3-sector deployment

(Spectrum = 20 MHz, Erlang traffic = 0.15E/subscriber)

From the results in Table 5.1 and Table 5.2, one can conclude

A. Micro cell is a must to support high subscriber density. Only cells with 1 km orless can give the type of subscriber densities required in urban areas.

B. DECT with 32 kbps payload supports only slightly lower subscriber density ascompared IS-95 with 13 kbps (with VAD). This implies DECT has higher spectralefficiency.

5.5.3 Capacity Comparisons for Providing 64 kbps Data Connections

To compare apples with apples, let us take a look at a situation, where both DECT and IS-95 is used to provide 64 kbps full duplex data communication using 20 MHz BW. Without



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VAD, IS-95 would support only about 3 channels of 64 kbps in 1.25 MHz (each way)spectrum, implying 25 (64 kbps) channels for 20 MHz bandwidth. The trunking efficiencywould now be approximately 0.6. Thus only 15 channels would be available per sector orcell, even assuming 100% reuse. On the other hand, DECT supports 60 channels of 64kbps. The trunking efficiency would be close to 0.8 implying that 48 channels would beavailable per sector of a cell assuming once again 100% reuse.

6 The Future

Having looked at the fundamental issues of importance for WLL systems and a brief look atavailable technological options, an obvious question that emerges is what lies in the future?We would like to conclude the article with a brief pointer towards some interesting on-goingdevelopment. The first is in terms of incorporating space-time processing and second in theemergence of 2.5G and 3G wireless technologies and its impact on future WLL systems.

Space-Time processing

The last decade has seen the emergence of many theoretical and practical techniques,which exploit the spatial dimension in a more effective way than more diversity combining[17]. One may classify these new techniques broadly under four heads, namely:

(i) Smart Antenna Technology,(ii) Transmit Diversity Schemes,(iii) Spatial Multiplexing and(iv) Space-Time Coding.

6.1.1 Smart Antennas

Smart antennas enable focus of beams from base station towards the subscriber terminaland vice-versa. Such electronic focussing using adaptive, phased-array antennas not only

enables improvement in link margins, thus enabling longer reach with same power, but alsohas the capability of significantly improving spectrum reuse. The latter is because; twofocussed pencil beams may use the same channel and still could produce very littleinterference to each other. Key is to have smart antennas provide such pencil beams foreach user. The concept is extremely interesting and it has the potential of significantlyimproving spectral efficiency especially in fixed wireless applications. However, the work inthis area is still in early stages with some commercial products incorporating some very earlyversions of smart antennas. It will take a few years for products providing significant spacediversity to emerge.

6.1.2 Transmit Diversity

A signal is transmitted simultaneously from two antennas at the base station, on the samecarrier. By appropriate coding of the data streams put on the two antennas, the receiver canreduce the fade margin significantly. It requires feedback from the mobile/portable to thebase station to obtain the channel state information and optimise performance.

6.1.3 Spatial Multiplexing

Use of multiple antennas at a (fixed) subscriber terminal as well as at the base station allowsone to spatially multiplex different data streams on the same carrier by transmitting thestreams through different antennas. Use ofL antennas can give (nearly) an L-fold increase

in data rate. This technique is also called MIMO (multi-input multi-output) processing, onepopular version of spatial multiplexing, called V-Blast [18] has been shown to yield spectralefficiencies as high as 30 bits/sec/Hz in short-range, fixed wireless application.



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6.1.4 Space-Time Coding

In conjunction with MIMO processing, error-control coding can further increase range and/orbit-rate. This coding across spatial channels use ideas similar to Trellis Coded Modulation(TCM) to define mappings of symbols to antenna ports [19]. Some of these techniques arealso being currently used to provide broadband wireless access to homes at rates up to2Mbps.

6.2 3G Terrestrial Wireless Standards

The third-generation (3G)-standardisation activity was started by various groups after ITUannounced the release of new spectrum for the International Mobile Telephone (IMT)application, and invited proposals for the same. This spectrum, in the 1.9-2.1 GHz band,could be utilised only by those standards that satisfy the IMT-2000 requirements. (See forexample, [20], [21], and [22], for good overview articles on IMT-2000 and 3G technologies).

For the IMT-2000 terrestrial radio transmission technology, the main specification was thatany compliant standard should provide data rates of 144kbps for mobile applications, 384

kbps data for pedestrian applications, and up to 2Mbps speed for fixed applications. Inaddition, all 3G systems are likely to support Generalised Packet Radio Service (GPRS),and interface with the GSM core network. Greater capacity and higher spectral efficienciessupport for multimedia services, incorporation of 2G services, interconnection with mobilesatellite communication systems, and international roaming are a few other importantrequirements of IMT-2000.

By end 1998, about 11 proposals were submitted to ITU, of which 9 of them were directsequence CDMA based and the other two were TDMA based (one which was a DECTevolution, and the other was a GSM evolution). However, by end 1999, many of theseproposals were either withdrawn or merged to form 4 proposals, namely: (i) 3G PartnershipProgram (3GPP) Wideband CDMA (W-CDMA) proposal from Europe and Japan, (ii)

Enhanced Data Rates for Global Evolution (EDGE) from USA and Europe, and (iii)Multicarrier CDMA or CDMA2000 from USA, (iv) EP-DECT from Europe.

All the 3G standards incorporate features to support higher peak data rates, multiple rateservices, and multimode capabilities. For example, the 3G radio will be a coherent (ordifferentially coherent) one. It will support the new W-CDMA (3GPP) and/or CDMA2000standard, as well as a fallback to either GSM or IS-136/IS-95. Support of EDGE is also apossibility, using multiband functionality. The W-CDMA standard has a channel spacing of 5MHz and a chip rate of 3.84Mcps, while CDMA2000 has a chip rate in multiples of1.2288Mcps is bandwidth multiples of 1.25MHz. EDGE, on the other hand, has exactly thesame slot and frame structure as GSM, but by changing the modulation from GMSK to 8-PSK, will have a gross bit-rate of 812.49 Kbps.

While W-CDMA will use the idea of Orthogonal Variable Spreading Factor (OVSF) codes toprovide multiple rate services, EDGE would use a link adaptation procedure to providevariable bit rates. It is expected that the 3G systems will provide about twice the capacity ofthe current 2G systems, where most of the gain accrues from more efficient modulation andbetter interference reduction approaches. In addition, the 3G standard provides the requisitesupport for a number of new techniques that were invented in the last decade.

These include:

1. Turbo codes

2. Multi-user detection



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3. Space-time processing

6.2.1 Brief Background on the New Techniques

We have already discussed space-time processing in section 6.1, and therefore we willconfine here our description to the other two techniques.

Turbo Codes: These new error correcting codes are compute intensive at the receiver. Theidea of parallel concatenation of simple codes is used to define powerful error correctionmechanisms. The performance improves as one repeatedly iterates the decoder. In return,one can reduce the SNR by 4-6 dB (even greater in many cases). Turbo coding is good fordata, but the interleaver/ deinterleaver and decoder delay is currently considered to be toohigh for voice applications.

Multi-User Detection: This is theoretically superior to co-channel suppression or cancellationtechniques, and can increase capacity/re-use considerably. Multi-user detection techniqueshave been well studied for DS-CDMA signals, but relatively less known for TDMA signals.The pilot signals on the W-CDMA uplink enable fairly sophisticated multi-user detection at

the base-station.

For the mobile cellular application, interoperability will be key, and multimode and/ormultiband functionality with some of the above advanced techniques need to employed. Tomake these ideas a reality, a combination of powerful DSPs and ASIC accelerators areneeded in the base band section. The algorithms have to be crafted carefully, with anoptimised hardware/software partition to minimise cost and power consumption.

On the other hand, it seems that for fixed wireless access, like in the wireless in local loopapplication, not much re-engineering of the 2G fixed wireless systems may be required inorder to support IMT-2000 bit rates for pedestrian and fixed applications. As an example, weconsider the evolution of DECT to 3G in the next section.

6.2.2 DECT evolution towards 3G

The 3G requirements are 384 kbps data for pedestrian applications, and 2Mbps speed forfixed applications. The DECT standard has been expanded to provide for these - only the3G mobile requirements (of 144 kbps) are not met.

In particular, the extended DECT standard is very attractive for fixed wireless applications. Inextended DECT, the gross bit-rate is tripled from 1.152Mbps to 3.456Mbps by replacingGMSK with 8-PSK modulations. Exploiting the asymmetry allowed by TDD systems is quitestraightforward to obtain 2Mbps shared downlink services on such a standard.

6.3 Internet Access using Wireless Networks

Fixed and mobile wireless network standards have been evolving to support increasinglyhigher data rates. Whereas the 2G systems provided for low bit-rate (9.6 kbps in the case ofGSM) circuit-switched data connection, the interim 2.5G mobile system using GPRS andGSM enhanced the bit-rate to a reasonable high value of 115.2 kbps, by transmitting inmultiple time slots and using circuit switched connection. Similarly the DECT-DPRS supportsa much higher data rates than that of 24 kbps rate supported in DECT. Both GPRS andDPRS systems are inherently circuit switched connections and hence can only support asmall number of data subscribers.



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The 3G standards aim to support high data rates and a large number of active datasubscribers. The EDGE, WCDMA and EP-DECT are the examples of 3G systems. Themain features of these systems are:

1. Large signal constellations: Instead of binary or QPSK modulation in 2/2.5Gsystems, these systems use 8-PSK modulation and thereby affect a three-foldincrease in the raw bit rate.

2. Decoupling of uplink and downlink streams: This enables one to communicate atmuch higher rates on the downlink compared to the uplink rate.

Asymmetry between uplink and downlink rates is an important feature of internet traffic.

3. Packet Switched Connections: This exploits the inherent "burstiness" of the datatraffic to support a large number of active users.

4. Contention based access: The active users contend using random access protocolssimilar to ALOHA, to transmit on the uplink.

5. Point-to-multipoint downlink: This feature allows for the downlink channel to beshared among all or a group of active users.

The data rates the 3G-system support vary from a low of 8 to 32 kbps to a high of 1 to 2Mbps. With contention based uplink schemes (in the case of DECT with half-slotconnections also) it is possible to support large number of users to be connected.

7 Summary and Conclusion

In this paper, an attempt has been made to compare the subscriber densities that can besupported by major wireless access standards including GSM, IS-95 and DECT. The

emphasis was on their applicability for the local loop, which implies not only the need tosupport very higher user densities, but also provide toll quality voice and simultaneouslycater to high speed Internet requirements. In this context, it emerged from this study that forlocal loop applications, micro-cellular networks which can very efficiently re-use thefrequency spectrum are absolutely essential in order to meet bit-rate demands of 64 kbps orhigher and user densities greater than 1000 subscribers/sqkm. Micro-cellular standards likeDECT or PACS use dynamic channel selection to allocate frequencies across base stations,which results in very efficient frequency reuse. This also directly brings down costs, but alsoallows for a flexible and easy expansion of the base station coverage regions.

Finally, this article also discussed some of the key ideas in the emerging third generation(3G) wireless standards. A brief description of the new technologies that 3G will introduce

into the network, and what will be their impact on Internet and multimedia bit-rates andservices were also provided. Some important ideas that the air-interface would need toemploy in order to cater to Internet traffic have been described

It is expected that with the advent of 3G wireless standards, not only will greater bit-rates bedelivered to mobile users, but there will be a substantial improvement in the bit-rates andservices for fixed wireless applications like the local loop and broadband to home as well.

References

1. ITU-T Recommendation G.703, "Physical/Electrical characteristics of hierarchicaldigital Interface, 1996

2. ITU-T Recommendations V5.2



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3. Ashok Jhunjhunwala, Bhaskar Ramamurthi, Timothy A. Gonsalves, "The Role ofTechnology in Telecom Expansion in India", IEEE Communication Magazine,November, 1998

4. W.C.Y.Lee, "Mobile Communications Engineering", 2nd Edition Mc Graw Hill, NewYork 1998

5. Reference in section 4.95 on DCS

6. Cox D.C., "Wireless Network Access for Personal Communications", IEEECommunications Magazine, pp.96-114, December 1992.

7. Rappaport, T.S., "Wireless Communications Principles and Practice", PrenticeHall, New Jessey, 1996.

8. Mouly, M., and Pautet, M.B., The GSM System for Mobile Comunication, ISBN: 2-9507190-0-7, 1992.

9. V.K. Garg and J.E. Wilkes, "Principles and Applications of GSM", Prentice Hall, NewJessey,1999.

10. EIA/TIA Interim Standard, "Cellular System Dual Mode Mobile Station LandStation Compatibility Specifications", IS-54, Electronic Industries Association, May1990.

11. M. Gilhousen, et.al., "On the Capacity of Cellular CDMA System", IEEETransactions on Vehicular Technology, Vol.40, No.2, pp.303-311, May 1991.

12. ANSI J-STD-008 Personal Station-Base Compatibility Requirements for 1.8-2.0GHz Code Division Multiple Access (CDMA) Personal Communications Systems,March 1995.

13. H. Ochsner, "DECT Digital European Cordless Telecommunications", IEEEVehicular Technology 39th Conference, pp 718 721, 1989.

14. D.C.Cox, "Wireless Network Access for Personal Communications", IEEECommunications Magazine, pp. 96-114, December 1992.

15. K. Ogawa, et. al., "Toward the Personal Communication Era the Radio AccessConcept from Japan", International Journal on Wireless Information Networks, Vol.1,No.1, pp.17-27, January 1994.

16. D.C.Cox, W.Arnold, and P.T.Porter, "Universal Digital Portable Communications: A

System Perspective", IEEE Journal on Selected Areas of Communications,Vol.SAC-5, No.5, pp.764, 1987.

17. Paulraj and C.B. Papadias "Space-time processing for Wireless Communications",IEEE Signal Processing Magazine, vol.14, pp.49-83, Nov 1997.

18. G.J. Foschini and M.J. Gans "On Limits of Wireless Communications in a fadingEnvironment using MULTIPLE Antennas", Wireless Personal Communications,vol.6, No.3, March 1998.

19. V. Tarokh, N. Seshadri and A.R. Calderbank, "Space-Time Codes for High DataRate Wireless Communication: Performance Criterias and Code Construction",

IEEE Transaction an Information theory, vol.44, No.2, March 1998.



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20. Special Issue on "Third Generation Mobile Systems in Europe", IEEE PersonalCommunications, vol.5, No.2, April 1998.

21. Special issue on, "IMT-2000: Standards Efforts of the ITU", IEEE PersonalCommunications vol.4, No.4, August 1997.

22. Special Issue on "UMTS" IEEE Transaction on Vehicular Technology, vol.47, No.4,November 1998.

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