broadband radio access to homes and businesses: mmds and lmds

Ž .Computer Networks 31 1999 379–393

Broadband radio access to homes and businesses: MMDS andLMDS

Hikmet Sari )

Alcatel Access Systems DiÕision, 5, rue Noel-Pons, 92734 Nanterre Cedex, France

Abstract

Ž .This paper gives an overview of digital microwave multipoint distribution systems MMDS and local multipointŽ .distribution systems LMDS originally developed for digital TV broadcasting and later extended to offer interactive services

Ž .for homes and businesses. We first describe the specification work carried out by the Digital Video Broadcasting DVBŽ .project and the Digital Audio Visual Council DAVIC which forms the technical basis of these systems. Next, after

presenting initial frequency allocations particularly in Europe and North America, we discuss the potential of MMDS andLMDS to offer broadband services to homes as well as to small- and medium-size businesses. We also discuss the majortechnical issues related to the design of these systems, frequency planning and reuse patterns, as well as future technicalevolutions to make them more efficient in terms of performance, capacity, and the service offered. This discussion includes

Ž .the use of higher-level modulations, multiple access techniques such as code-division multiple access CDMA andŽ .orthogonal frequency-division multiple access OFDMA , and of adaptive arrays for electronic beamforming. q 1999

Elsevier Science B.V. All rights reserved.

Keywords: Broadband; Internet access; DAVIC; DVB; MAC protocol; Local access network; Frequency planning; DAVIC; CDMA;OFDMA

1. Introduction

Originally driven by digital TV applications, therehas been a tremendous effort over the past few yearsto develop standards for digital broadcasting over avariety of media including satellite, cable networks,and radio transmission at microwave and milli-meter-wave frequencies. Standardization was firstinitiated in Europe with the establishment of the

Ž .Digital Video Broadcasting DVB project conductedunder the auspices of the European BroadcastingUnion. The DVB project was in charge of elaborat-ing the commercial requirements and technical speci-

) E-mail: [email protected].

fications of different broadcast technologies. Thetechnical specifications elaborated by the DVB pro-ject were next passed to the European Telecommuni-

Ž .cations Standards Institute ETSI for further proce-dures toward publication of the standards. This Euro-pean initiative has had a large impact worldwide, andthe DVB specifications have been adopted in severalother regions of the world.

From the different transmission media, the firstDVB specifications were released for satellite ser-

w xvices 1 , but the technical specifications for digitalTV broadcasting over cable networks followed im-

w xmediately 2 . Then, in addition to digital TV broad-casting over the terrestrial VHF and UHF channelsw x3 , attention was turned toward microwave radio

1389-1286r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-7552 98 00270-0

( )H. SarirComputer Networks 31 1999 379–393380

transmission which was divided into two categories:The first, referred to as microwave multipoint distri-

Ž .bution systems MMDS , operates at frequenciesbelow 10 GHz, and the second, referred to as local

Ž .multipoint distribution systems LMDS , operates atfrequencies above 10 GHz. Although the DVB pro-

Žject has preferred the terminology of MVDS multi-.point video distribution systems to LMDS, we will

exclusively use here the terminology of LMDS whichis undoubtedly more appropriate to designate short-range millimeter-wave radio systems that will offerdata and telephony services in addition to TV pro-grams.

Another international body which was set up toelaborate technical specifications for broadcast aswell as for interactive services over cable networks,satellites, and radio is the Digital AudioVisual Coun-

Ž .cil DAVIC which groups major network operators,service providers, and consumer electronics,telecommunications and computer industries. Al-though DAVIC is not sponsored by any officialstandard-making body, the tremendous number ofmember organizations has made it extremely power-ful on the international scene.

The DVB project originally started with the speci-fication of one-way broadcast services only, leaving

Ž .the definition of the upstream return channel forinteractive services to a later stage. In contrast,DAVIC focused on the definition of the upstreamchannel, and particularly of the medium-access con-

Ž .trol MAC protocol. Unfortunately, this partitioningof roles was not strictly followed by the two organi-zations, and a number of divergences appeared be-tween their respective specifications for both thedownstream and upstream channels. DAVIC did notredefine a modulation and coding scheme for satel-

w xlite broadcasting, but it elaborated specifications 4for cable networks, MMDS, and LMDS, which dif-fered to some extent from those specified by theDVB.

As mentioned earlier, the original driver of thestandardization carried out by both organizations wasdigital TV broadcasting as well as interactive ser-

Žvices video-on-demand, pay-per-view, home shop-.ping, internet access, . . . to residential customers,

and digital satellite and cable TV broadcasting ser-vices have been deployed for several years. Theintroduction of digital broadcasting services by

MMDS and LMDS have been much slower, particu-larly due to the competition from direct broadcastsatellites. But in addition to the originally intendedresidential market, MMDS and LMDS have beenrecognized to be attractive to supply broadband dataand telephony services to small- and medium-sizebusiness customers. This is more particularly true forLMDS to which a substantial amount of frequencyhas been allocated in Europe, North America, andother regions of the world.

The purpose of the present paper is to give ageneral overview of MMDS and LMDS systems,discuss their potential to offer broadband services tohomes and businesses, and present the major techni-cal issues related to their implementations. We alsodiscuss frequency allocation as well as frequencyplanning and reuse, and indicate some potential tech-nologies for possible future evolutions. First, in thenext section, we briefly describe the DVB satelliteand cable specifications, which were later extendedto LMDS and MMDS, respectively. Next, in Section3, we describe the DAVIC specifications for MMDSand LMDS, and point out their communalities withand differences from DVB specifications. In addition

Ž .to the forward downstream channel, we also de-scribe in this section the specifications of the up-stream channel and of the MAC protocol. Section 4discusses the frequency allocation in Europe and inthe US. Section 5 analyzes the current situation ofMMDS and LMDS, and points out that for the nextfew years to come, the driving application is likely tobe telephony and data services for small- andmedium-size businesses rather than TV and enter-tainment services for residential customers as it wasoriginally anticipated. In Section 6, we discuss thebasic technical issues in receiver design for sub-

Ž .scriber premises equipment CPE , frequency plan-ning and reuse patterns, as well as some potentialfuture evolutions. Finally, in Section 7, we give asummary of the discussions and our conclusions.

2. The DVB specifications

2.1. Satellite and LMDS

The standard of modulation and channel codingfor satellite TV services in the 11r12 GHz band was

( )H. SarirComputer Networks 31 1999 379–393 381

w xpublished by the ETSI in December 1994 1 . Thesystem was first intended to provide direct-to-homesatellite services and feed cable TV head-end sta-tions. The same specifications were later extendedwithout any changes to the downlink in LMDS

w x Ž .systems 5 so that a satellite set-top box STB canbe used and LMDS can benefit from the mass mar-ket of direct broadcast satellites.

The modulation, channel coding, and related func-tions for satellite broadcasting and the LMDS down-

Ž .stream channel from a central station to subscribersare summarized in Table 1. The system uses a

Ž .quaternary phase-shift keying QPSK modulationŽ .and a concatenated forward error correction FEC

coding scheme with a convolutional inner code and aŽ .Reed–Solomon RS outer code. The transmission

frame is based on the MPEG2 transport data streamw x6 , and prior to channel coding, a scrambler is usedto randomize the input signal.

The outer code has a block length of 204 bytes,carries 188 information bytes, and can correct up to8 byte errors per block. This code is obtained by

Ž .shortening the RS 255, 239 Reed–Solomon code toa block length of 204. The associated bandwidthexpansion is approx. 8.5%. A convolutional inter-

w xleaver 7 with interleaving depth of Is12 is in-serted between the inner and outer encoders in orderto uniformly distribute the errors which occur bybursts at the Viterbi decoder output in the receiver.The interleaver and deinterleaver block diagram issketched in Fig. 1. In the interleaver, the input databytes are cyclically fed to the 12 parallel brancheswhich consist of simple first-in-first-out shift regis-ters. The first branch has a zero delay, the secondhas a 17-byte delay, the third has a 2=17-bytedelay, and so forth, until the 12th branch which

Žincludes an 11=17-byte delay. More generally, aconvolutional interleaver of length N and depth I

Fig. 1. Block diagram of the convolutional interleaver and deinter-leaver used in the DVB specifications for satellite and LMDSsystems.

comprises I branches, and the ith branch includes aŽ . .delay element of iy1 NrI units. The output

switch moves cyclically in phase synchronism withthe input switch. The deinterleaver has an identicalstructure except that the order of delay elements isreversed with respect to the interleaver.

With this interleaving scheme, a 12-byte errorburst at the Viterbi decoder output appears as 12isolated byte errors with a spacing of 204 bytes atthe RS decoder input, and the RS decoder correctsall of these errors. In fact, since the RS code em-ployed can correct 8 byte errors per block, thisinterleaving scheme can handle error bursts of up to8=12s96 bytes or 384 QPSK symbols. The innercode is a rate-1r2 convolutional code with constraint

Žlength Ks7 the NASA code which has become aw x.de facto industry standard 8 , but the DVB specifi-

Žcations also include higher code rates 2r3, 3r4,.5r6, and 7r8 by puncturing this basic code. This

allows to trade off coding gain against useful datarate on a given satellite transponder or an LMDSlink.

The DVB specifications define all transmit andreceive functions and system parameters except thesymbol rate at which the modems must operate. Thereason for this is that there are a variety of satellites

Table 1DVB specifications for satellite and LMDS systems

Transmission frame MPEG2 transport stream

Modulation QPSKTransmit filtering Square-root raised-cosine Nyquist with roll-off factor as0.35Coding Concatenated codingInner code Convolutional, Ks7, with code rates 1r2, 2r3, 3r4, 5r6, and 7r8

Ž .Outer code Reed–Solomon 204, 188, ts8Interleaver Convolutional, Is12


with different transponder bandwidths, and no fre-quency planning is readily available for LMDS links.Furthermore, even for a given satellite transponder,the operators were reluctant to fix a symbol rate inorder to have the freedom of selecting it later inaccordance with their needs. As an example, supposethat a 26 Mbaud symbol rate is used on a 36 MHzsatellite transponder. With QPSK signaling, this givesa gross bit rate of 52 Mbitrs, and after RS decoding,we have a bit rate of 47.92 Mbitrs. The net bit rateon the transponder is a function of the convolutionalcode rate used, and is given in Table 2 for all coderates included in the DVB standard.

2.2. Cable and MMDS

The basic modulation scheme for digital cable TVservices is the 64-state quadrature amplitude modula-

Ž .tion 64-QAM , but the initial standard also includedtwo lower-level modulations, namely 16-QAM and32-QAM, which increase system margin and robust-ness at the expense of reduced data throughput. The128- and 256-QAM signal constellations were ini-tially mentioned as future extensions, but they havebeen approved in the meantime, and they are now afull part of the standard.

The cable TV standard shares all coding, inter-leaving, framing, and scrambling functions of thesatellite TV standard, except that channel coding is

Ž .reduced to the RS 204, 188, ts8 code employed asan outer code in the satellite TV standard. In otherwords, the cable TV standard does not employ aconcatenated error correction coding scheme, achoice that can be justified by the fact that typical

Ž .signal-to-noise ratio SNR values encountered incable networks are substantially higher than thosecorresponding to satellite links. Conversely, the ca-

Table 2Useful bit rates on the satellite transponder corresponding to asymbol rate of 26 Mbaud

Ž .Code rate Useful bit rate Mbitrs

1r2 23.962r3 31.953r4 35.945r6 39.937r8 41.93

ble channel suffers from echoes due to imperfectcable terminations and impedance mismatches, andthis requires the use of an adaptive equalizer at thereceiver. Further, since no training symbols are in-cluded in the transmitted data stream, the equalizermust be able to operate in a ‘blind’ mode. Of coursethe standard does not specify what kind of equalizerneeds to be implemented, and this issue is left to themanufacturers. Next, although there is no concate-nated coding scheme in the DVB cable standard, thesatellite TV interleaver is used after the RS encoder.Its role here is different from the satellite, and it isused in this case to combat impulse noise typicallyencountered in CATV networks.

The roll-off factor of the overall Nyquist filteringused is as0.15, and is evenly divided betweentransmitter and receiver. For an 8-MHz channelŽ .European CATV frequency plans , the maximumsymbol rate without spectral overlapping and mutualinterference of adjacent channels is 6.96 Mbaud.With this maximum symbol rate, the useful bit rate islimited to 38.46 Mbitrs with 64-QAM, but this canbe increased to 44.87 Mbitrs with 128-QAM and to51.28 Mbitrs with 256-QAM.

The DVB cable specifications were later extendedto MMDS systems without any changes so that acable STB can be used by MMDS subscribers. Thisis analogous to the extension of the satellite specifi-cations to LMDS, and the idea is to make MMDSbenefit from the cable mass market. This assumes, ofcourse, that the equalizer in the cable STB is capableof compensating for echoes encountered in MMDSnetworks.

3. DAVIC specifications

3.1. Downstream channel

As mentioned earlier, DAVIC did not addresssatellite broadcasting, and even in the US, a numberof satellite operators adopted the DVB specificationsexcept for the roll-off factor which characterizes thesquare-root raised-cosine Nyquist filters used attransmitter and receiver. Specifically, these operatorsfavored the use of as0.2 with respect to as0.35specified by the DVB in order to increase the datarate on satellite transponders.


The DAVIC specifications for LMDS follow thoseof the DVB except that DAVIC allows the use ofeither as0.35 or as0.2 for channel filtering andeither QPSK or 16-QAM for modulation. In terms offiltering, this is not a true divergence, because state-of-the-art receivers employ digital filtering, and it isnot difficult to make these filters programmable sothat they can accommodate both roll-off factors andthe receiver can be used in both modes. Regardingmodulation, DAVIC in fact specifies two grades:Grade A is based on QPSK only, whereas a Grade Breceiver must be able to accommodate both QPSKand 16-QAM. From this discussion, it is clear thatthe DVB LMDS specifications are a subset of theDAVIC LMDS specifications, and that there are noserious compatibility problems between the two stan-dards. Another way to view this is that DAVICfollows DVB, but also defines future extensionswhich employ more bandwidth-efficient modulationand filtering.

The situation is a little more complicated for cableand MMDS systems. For cable networks, DAVICadopted the framing and channel coding functions ofthe DVB specifications, but focused exclusively onsquare QAM signal constellations, namely 16-, 64-,and 256-QAM. Grade A DAVIC specifications in-clude the 16- and 64-QAM modulations, and coin-cide with those of the DVB except for the missing32-QAM modulation. Grade B also includes the256-QAM modulation, and in the 256-QAM modewhich is the major focus of US cable operators,DAVIC adopted a different interleaver and also themapping function of the input bits to the 256-QAMconstellation points differ from that adopted by theDVB. The DAVIC convolutional interleaver for 256-QAM has a depth of Is204, which is 17 times thatof the interleaver specified for 64-QAM, which mayseem excessive if we take into account the fact thatthe symbol period is only reduced by 25% in 256-QAM. DAVIC also specifies a roll-off factor of 0.15for European 8-MHz channels, and 0.13 for 6-MHzchannels used in the US, but the difference betweenthese two values is not even measurable in practice.The strong interest of US cable operators in the256-QAM modulation is motivated by the fact thatthe channel spacing is only 6 MHz, and they wish totransmit a data rate similar to that achieved by64-QAM on the European 8 MHz channels.

Finally, the DAVIC MMDS specifications werefollowing the cable standard in order to be able touse the same STB both in cable networks and inMMDS, but a strong pressure came from someoperators to include trellis coding with the aim ofincreasing the coverage of MMDS transmitters. Inthe end, DAVIC defined 3 different grades: Grade Aemploys the 16- and 64-QAM signal constellationswithout a trellis code. Grade Aq is restricted to thesame modulation schemes as Grade A, but alsosupports a trellis code. The trellis code in question isthe 16-state 4-dimensional trellis code which is an

Ž .ITU-T former CCITT standard for high-speedw xvoiceband modems 9 . Finally, Grade B supports the

16-, 64-, and 256-QAM signal constellations and thetrellis code of Grade Aq. It is also specified that thetrellis code in Grade Aq and in Grade B must bebypassable so that the same STBs can also be usedon cable networks.

It should also be pointed out that in addition tothe MPEG2 transport stream used for digital TVbroadcasting, DAVIC defines a mapping function totransport ATM data on the downstream channel.This mapping function is as follows: 7 consecutive53-byte ATM cells from the incoming data are ap-pended with 3 control bytes as shown in Fig. 2 toform two 187-byte packets. Next, one sync byte isappended to each of these packets to form a 188-byteframe. Finally, 16 redundancy bytes are added toeach frame for RS encoding, and this results in twoconsecutive MPEG2 transport stream frames.

To conclude this section, the differences betweenthe DVB and DAVIC specifications have motivatedchip vendors and equipment manufacturers to offer

Fig. 2. Mapping of 7 ATM cells onto 2 consecutive MPEG2transport frames.


solutions which accommodate both of them throughappropriate configuration. This obviously involvesan increased complexity, but the complexity increaseis fortunately modest and does not have a significantimpact on the cost of CPEs.

3.2. The return channel

The lead for the definition of a return channel forcable, LMDS, and MMDS networks was clearlytaken by DAVIC, because the DVB project wasexclusively concerned with broadcast services in itsfirst phase. Accordingly, we will focus here on the

Ž .DAVIC upstream return channel specifications,which were also adopted with little or no changes bythe DVB project. DAVIC defined a return channelfirst for cable networks and then for LMDS systems.At that time, MMDS was viewed as a one-wayŽ .broadcast system due to the lack of frequenciesallocated to the return channel. Later, some interestappeared in offering interactive services also onMMDS networks, and a debate started within DAVICon whether to adopt for MMDS the return channelspecifications of LMDS or those of cable networks.In the end, the council adopted the LMDS returnchannel specifications, and therefore, it is not rele-vant to discuss here the return channel specificationsfor cable networks.

The multiple access technique used on the LMDSreturn channel is time-division multiple accessŽ .TDMA . The MAC protocol allocates time slots todifferent users, and each CPE transmitter can trans-mit only when a time slot is allocated to it. The timeslots are composed of 68 bytes which include a4-byte preamble and a 1-byte guard interval at theend. The remaining 63 bytes include 53 informationbytes and 10 parity-check bytes for the RS codeemployed. That is, the return channel employs a

Ž .shortened Reed–Solomon code RS 63,53 with 8-bitcode symbols and a 5-byte error correction capabilityper block. Clearly, each time slot carries one ATMcell. Before Reed–Solomon encoding, the data pack-ets appended by the preamble and the guard intervalare randomized through a byte randomizer. Themodulation scheme is a differentially-encoded burst-type QPSK. Channel filtering is of raised-cosineNyquist type evenly split between transmitter andreceiver. The roll-off factor is 0.3.

Clearly, error protection on the upstream channelis not as efficient as the concatenated coding schemeused on the downstream channel. In addition tocoding itself, the bursty nature of traffic leads tofurther performance degradation. Indeed, the burstQPSK receiver at the central station will typicallyhave a degradation on the order of 2 dB with respectto the QPSK receiver of the user terminal whichoperates on a continuous data stream. These funda-mental differences can be compensated, however, bythe design of transmit and receive functions on theupstream and downstream channels.

3.3. MAC protocol

We will briefly discuss here the medium accessŽ .control MAC protocol used to allocate resources to

different user terminals by the central station. Recallthat the downstream channel is a time-division multi-

Ž .plex TDM and the upstream channel employsTDMA. Both downstream and upstream framed andthe frames are divided into time slots that encapsu-late exactly one ATM cell.

Each frame on the downstream channel includes aframe start slot followed by random access slotswhich carry MAC messages and higher layer data.The upstream frame is divided into polling responseslots, contention slots, and reserved time slots. Thepolling time slots are allocated to one subscriberterminal and may be utilized for a poll response afterreceiving a poll request from the central station. Thecontention slots are the time slots that are typicallyallocated to more than one terminal and utilization ofcontention time slots may cause a collision withanother terminal trying to use the same slot. When acollision occurs, the contention may be resolved by anumber of algorithms such as random retransmissiondelays which indicates to each terminal how manyframes it has to wait before retransmission. Reservedtime slots are reserved for use by only one terminal.The terminal transmits on these time slots wheneverit has data to transmit. If no data is available, ittransmits an idle cell. The contention and pollingtime slots are determined by the central station whichdecides which carrier frequency and time slots are tobe used by each terminal. The MAC protocol in-cludes a combination of circuit mode reservation forconstant bit rate services and dynamic reservation for


variable bit rate and unspecified bit rate data ser-vices.

The central station periodically polls each userterminal to establish, maintain, and terminate con-nections. The polls are periodically repeated at aninterval of less than or equal to 2 s. It declares thatthe terminal is not responding if it receives noresponse to a polling request for 10 s. When aterminal attempts to enter the network, it acquires adownstream channel and listens for the poll directedto it. If it receives no poll request for 2 s, it switchesto the next downstream channel and listens again.This process is repeated until the terminal finds thedownstream channel on which it is being polled. Thefirst task is to calibrate the user terminal in terms ofclock phase so that it can transmit on poll responsetime slots without interfering with adjacent time slotsand other terminals. In addition to the clock phase,the terminal also performs power control and carrierfrequency control. Power control compensates forunequal signal attenuations resulting from differentphysical distances of user terminals to the centralstation on one hand and different propagation condi-tions on the other hand. This control loop sets andperiodically updates the signal level transmitted bythe terminal such that the central station receives apredetermined nominal signal level. Similarly, theuser terminal performs carrier frequency control inorder to compensate for the large frequency uncer-tainty and drifts of the microwave oscillator used inthe transceiver which may be far beyond the capabil-ity of the demodulator.

4. Frequency allocation

As pointed out earlier, the DVB and DAVICspecifications were essentially developed to providebroadcast TV and interactive residential serviceswhich require a narrowband return channel only. Inthe US, there exists today analog MMDS networks

Ž .in the 2.5 GHz band 2.500–2.686 GHz , and similarsystems also exist in some other countries. Thechannel spacing in the US is 6 MHz as in cablenetworks, and it is therefore natural to try to adoptthe same standard for cable and MMDS so that thesame STB can be used for both systems. This fre-quency allocation covers 31 6-MHz channels which

allow to transmit 100 to 200 TV programs dependingon the targeted picture quality. Now, there is also a

Ž .12 MHz frequency band 2.150–2.162 GHz totransmit upstream information for interactive ser-vices.

In contrast, with very few exceptions, there are nofrequencies available in Europe below 10 GHz forthese kind of services, and therefore, European coun-tries turned toward the millimeter-wave frequencyband and allocated the 40.5–42.5 GHz band to LMDSservices. Since this frequency band is primarily in-

Ž .tended for broadcast, video-on-demand VoD , andsimilar applications which only require a narrowbandreturn channel, only a 100 MHz of this spectrum isdedicated to the return channel. There are currently anumber of trials in Europe for analog and digital TVbroadcasting in this frequency band, and interactiveservice trials are envisioned in a next step.

In addition to the 2.5 GHz band allocated toMMDS, the US also allocated the 27.5–28.35 GHzband for LMDS services. This band was initiallyintended for downstream transmission, and the31.075–31.225 GHz frequency band was allocatedfor the return channel. As the 40.5–42.5 GHz Euro-pean frequency band, this frequency allocation in theUS was primarily intended for TV broadcast, VoD,and internet services to residential costumers whichrequire a narrowband return channel only. No fre-

Ž .quency planning channel spacing is specified atthis stage, but this is expected to be in the 20–40MHz range for the downstream channel and below 5MHz for the return channel.

To summarize, the frequency plans mentionedabove are all intended for residential type serviceswhich are asymmetric in nature. That is, the up-stream channel capacity is much lower than that ofthe downstream channel. The systems support broad-cast as well as two-way services with limited interac-tivity. Comparing MMDS and LMDS, we can makethe following comments: Due to the lack of frequen-cies, MMDS is viable only in a limited number ofcountries, while a much larger frequency band isavailable for LMDS in most, if not in all, countries.This gives LMDS a much stronger market potentialthan MMDS. Another important point in favor ofLMDS is the cell coverage and the frequency reusefactor. The cell radius in MMDS typically exceeds50 km and may be as large as 80 km. In contrast, the


cell radius in LMDS systems does not exceed a fewkm. Depending on the power transmitted, it can be 2to 3 km at 28 GHz, and in the range of 1 km or lessat 40 GHz. Higher ranges may be achieved, but onlyat the expense of increased transmit power which isincompatible with cost objectives of user terminalsfor the several years to come. The implication of thisis that frequencies are reused many more times inLMDS than in MMDS in a given geographical area,which further increases the relative capacity of LMDSsystems. As an example, consider an MMDS systemwith a cell radius of 50 km and an LMDS systemwith a cell radius of 2 km. One MMDS cell in thisexample is equivalent to 252 s625 LMDS cells, andthe frequency reuse factor is 625 times larger inLMDS. If frequency allocation is such that bothsystems have the same capacity within a cell, theLMDS system will have 625 times the capacity ofthe LMDS system in the same geographical area.These considerations make LMDS systems muchmore suitable to supply high-speed data services indensely populated areas.

5. Broadband wireless access

Despite the development of standards and specifictechnologies, it is today questionable that massivedeployment of MMDS and LMDS networks willtake place in the near future for broadcast applica-tions. The reason is that direct-to-home satellites arealready in place, and it is quite unlikely that MMDS

Žand LMDS technologies will be able to economi-.cally compete with satellite systems in the several

years to come.In contrast, MMDS and LMDS technologies are

attractive to provide broadband wireless access forhomes as well as for small- and medium-size busi-nesses. This is particularly true for the LMDS tech-nology which is suitable for millimeter-wave radioaccess. Unlike consumer applications largely domi-nated by broadcast and interactive TV services,broadband radio access to business customers isessentially symmetric in nature, because it is primar-ily intended for telephony and data services such as

Ž .local area network LAN interconnection. Small-and medium-size businesses today make use of leasedlines for these applications, and it is expected that

point-to-multipoint broadband radio systems willsubstantially reduce the cost of these services withrespect to the point-to-point links used today.

Broadband wireless access systems are of particu-lar interest to new fixed service operators, becausethey are quick to install on one hand, and are eco-nomically cost-effective on the other hand. Anothermajor application of broadband wireless systems isthe interconnection of base stations to the fixed

Ž .public switched telephone network PSTN in cellu-Ž .lar and personal communication systems PCS . Cel-

lular and PCS operators too make use today ofpoint-to-point radios for this application, but broad-band wireless access systems will substantially re-duce the cost of base station interconnection in thenear future.

At this point, an important remark is in order: Inconsumer applications, standards are needed so thatSTBs from different manufacturers can operate withcentral station transmit equipments from other ven-dors. The situation in business applications is quitedifferent, because operators need complete turnkeysolutions, and there is no need in principle to followany standard. Clearly, interoperability is not an issuehere, but despite this, the existing DVB and DAVICspecifications offer a unique opportunity due to thelow-cost technology that has been developed for theconsumer mass market.

We will now discuss the current situation offrequency allocation for broadband wireless accessin Europe and North America. First, in Europe, the24.5–26.5 GHz and the 27.5–29.5 GHz frequencybands originally reserved for point-to-point links arenow available for point-to-multipoint broadband ra-dio systems. In addition, there is some bandwidth

Žavailable in the 10 GHz range 10.15–10.3 and.10.5–10.65 GHz in the United Kingdom and Scan-

dinavia. At this moment, the 38 GHz ETSI band isnot open for point-to-multipoint radios, and the40.5–42.5 GHz band is intended for consumer-typeapplications, but the situation may change in thefuture. The 27.5–29.5 GHz frequency band is alsoenvisioned for LMDS services in Latin America aswell as in the Asia Pacific region.

In the US, the first band which was allocated toLMDS services is the 24 GHz band which comes in

Žtwo blocks of 200 MHz 24.25–24.45 and 25.05–.25.25 GHz . The Federal Communications Commis-


Ž .sion FCC has now started the auctions for the 28GHz which comes in three blocks, namely 27.5–28.35, 29.1–29.25, and 31.075–31.225 GHz. Thesecond and the third of these blocks are allocated todownlinks and uplinks, respectively. As for the firstblock which is 850 MHz wide, its use is unspecified,and can be used in principle for either downlinks oruplinks. In other words, depending on the use madeof this block, the 28 GHz US band can be used eitherfor largely asymmetric residential services, or essen-tially symmetric business services.

One problem which has recently emerged con-cerning the 28 GHz ETSI band is that some forth-

Ž .coming low earth orbit LEO satellite systems willbe allowed to operate in the 28.6–29.1 GHz band.Interference considerations will preclude the use ofthis band in terrestrial point-to-multipoint systems,and this will leave only the 27.5–28.6 GHz and29.1–29.5 GHz bands from the original 2-GHz wideETSI band. The situation is much simpler with the26 GHz ETSI band. It can be separated into two1-GHz wide bands, and each of those can be allo-cated to one direction of the transmission, so that thesame capacity can be achieved on the downlink andthe uplink, and symmetric services can be supportedefficiently.

6. Some technical issues and perspectives

In this section, we will discuss some technicalissues related to receiver design in CPEs, as well asfrequency planning and reuse patterns suitable forthe deployment of LMDS and MMDS systems, andsome perspectives for future evolutions.

6.1. ReceiÕer design

In MMDS and LMDS systems, the low-cost con-straints are much more stringent for the CPE than forthe central station equipments, because the latter areshared by many subscribers. The CPE is comprisedof one outdoor unit which includes the transmit andreceive antenna and of an indoor unit which inter-faces with different subscriber equipments such astelephones and PCs. The indoor unit accepts thereceived signal after downconversion by the outdoor

Ž .unit to an intermediate frequency IF , demodulatesand demultiplexes it, and then interfaces with the

connected subscriber equipments. The downstreamIF is the UHF band in MMDS and the satellite IFŽ .950–2050 MHz in LMDS systems. On the uplinkside, the indoor unit accepts signals from differentsubscriber equipments, multiplexes them, and modu-lates the resulting signal on an IF before passing it tothe outdoor unit.

From available propagation data, MMDS systemsare prone to severe multipath propagation problems,and the CPE needs to incorporate an efficient adap-tive equalizer to cope with this phenomenon. Echodelays on this channel are much longer than thoseencountered on cable networks, while the symbolrates and the modulation formats are typically thesame. More specifically, echo delays in MMDS sys-tems may attain several microseconds, but fortu-nately, the echo amplitude quickly reduces with echodelay, and a linear equalizer of 20 to 30 taps is ingeneral sufficient to compensate for most echoesencountered in practical applications. The equalizermust be able to converge in a ‘blind’ mode, i.e.,without a priori known transmitted reference dataw x10 , because no preamble is available in the receiveddata stream.

Another major issue in the receiver design is thedifficulty to achieve a large frequency acquisitionrange in the carrier recovery loop together with asmall steady-state phase jitter compatible with thehigh-level signal formats used. Conventional fre-quency sweeping may be used here, but it leads toslow frequency acquisition, and phaserfrequency de-

w xtectors 11 which have become popular in point-to-point digital microwave radio systems are an appeal-ing solution which offers faster acquisition.

An example of a QAM demodulator was de-w xscribed in 12 . This all-digital variable symbol rate

demodulator, which was integrated in a single VLSIchip, can handle all signal formats from 16- to256-QAM and all symbol rates from 5 to 7 Mbaudusing the same oscillator. The equalizer can compen-sate for echoes with a delay in excess of 3 ms, andthe carrier recovery loop compensates for frequencyoffsets in excess of "100 kHz. Higher frequencyoffsets with this chip can be compensated through

Ž .the automatic frequency control AFC functionwhich feeds control information back to the tuner.

An LMDS receiver closely resembles MMDS re-ceivers except for the carrier frequencies, IFs, and


the modulation formats used on the downstreamchannel. In the outdoor unit, the receivedmillimeter-wave signal is first low-noise amplifiedand frequency downconverted to the satellite IF sothat a conventional satellite tuner can be used. Thetuner, located in the indoor unit, performs channelselection and frequency downconversion functions,and its output is passed to a QPSK demodulator andthen to a demultiplexer which selects the data blockintended to that user.

First generation LMDS systems will use QPSKdemodulation chips developed for satellite broadcast-ing which do not include an equalizer. It is quiteunclear at this moment whether an adaptive equalizeris truly required in LMDS systems, but as technol-ogy advances, one can expect that even satellitereceivers will include an equalizer to compensate for

Ž .the carrier-to-noise ratio CNR degradation causedby imperfections of the modem and of the radiofunctions. As far as propagation is concerned, it canbe anticipated that channel equalization is not amajor issue in LMDS, since millimeter-wave linksare very short and the receive antennas are fairlydirectional. Equalization may be necessary, however,to compensate for signal propagation on the cablethat connects the rooftop outdoor unit to the indoor

Ž .unit s which may seriously distort the received sig-nal. This is particularly true in large buildings wherethe cable length typically spans several hundreds ofmeters.

6.2. Frequency planning

In Europe, the ETSI specifies frequency planswith channel spacings obtained through successivedivisions by 2 of 112 MHz. In other words, either ofthe following channel spacings are usable by opera-tors: 112, 56, 28, 14, 7, and 3.5 MHz. Consider, forexample, the 26 GHz ETSI plan, and assume that thefirst half is allocated to the downlink, and the secondhalf is allocated to the uplink. Also suppose thatbandwidth occupancy is 28 MHz for downstreamchannels and 7 MHz for upstream channels. Obvi-ously, four upstream channels are needed for each

Ždownstream channel in order to have at least ap-.proximately the same capacity in both directions of

the transmission. Note that even in that case, capac-ity is not exactly identical on the downstream andupstream channels, because physical layer functions

Žare different between the two directions channel.coding, filtering, . . . .

We will now discuss the possible frequency reuseplans in a given geographical area. The first possibil-ity is to use hexagonal frequency patterns as in

w xmobile cellular systems 13 . This is illustrated inFig. 3 where each cell is labelled with a digit which

Ž .indicates the channel or group of channels allo-cated to it. Subscriber terminals in each hexagonal

Ž .cell are served by the central or base station locatedin the cell center. This frequency allocation schemerequires 3 times the bandwidth allocated to one cell.Central station antennas are assumed omnidirec-tional, but subscriber antennas are directional anten-nas with a narrow beamwidth pointing to the centralstation serving their cell. A close inspection of thisfigure reveals that the worst case interference occurswhen the desired central station and another centralstation employing the same frequency are located inthe same direction for the subscriber terminal.

Another possibility is to use rectangular cells with908 sector antennas as shown in Fig. 4. Each quad-rant of a cell in this figure is labelled with a digit

Žwhich indicates the frequency or group of frequen-.cies used in that sector. The frequency reuse factor

Ž .in Fig. 4 a is 4, which indicates that covering of aregion requires 4 times the frequency bandwidthused in each sector. The frequency reuse pattern of

Ž .Fig. 4 b reduces the bandwidth requirements by 2Žor equivalently, it increases network capacity by a

.factor of 2 in the same frequency band using twoorthogonal polarizations. In this pattern, each digitindicates a frequency, and a superscript ) indicates

Fig. 3. Hexagonal frequency reuse patterns with 3 frequencies.


Ž .Fig. 4. Rectangular freqeuncy reuse patterns. a 4 frequencies perŽ .sector. b 2 frequencies and two polarizations per sector.

orthogonal polarization. As in hexagonal frequencyreuse patterns, subscriber antennas are highly direc-tional and point toward the central station servingtheir cell. Antenna sectoring within a cell rather thansplitting each cell into further cells to increase capac-ity has the advantage of reducing maintenance costin addition to easing network upgrade. The reason isthat all equipments serving a cell are located in thesame place with antenna sectoring, whereas cellsplitting requires to install equipments in the centersof the newly defined smaller size cells and to makefurther connections to the fixed network.

Referring back to the hexagonal frequency reusepattern of Fig. 3, the worst case of co-frequency

interference occurs when a subscriber is located on aline which passes through the central station towardwhich its antenna is pointed and a central stationusing the same frequency at a distance of 3d , where0

d denotes the cell radius. The carrier-to-interference0Ž .ratio CrI in this case is given by

CrIs20 log 1q3d rd 1Ž . Ž .0

where d denotes the distance between the subscriberand the center of its cell. This function is minimizedfor dsd , i.e., when the subscriber is located at the0

cell boundary. Then, we have

CrIs20 log 4 s12 dB. 2Ž . Ž .

Interference from the next central station using thesame channels is another 5 dB down from this value.

Next, proceeding similarly with rectangular fre-quency reuse patterns, we find that the worst caseco-frequency interference gives

CrIs20 log 5 s14 dB, 3Ž . Ž .

which indicates that interference is 2 dB lower thanthat encountered in hexagonal patterns.

6.3. Future eÕolutions

It is obvious that future technical and technologi-cal evolutions will give the possibility to improveperformance and capacity of MMDS and LMDSsystems. We will not discuss here any technologicalissues such as transmit power amplifiers, low-noiseamplifiers, and antenna technologies for central sta-tions and CPEs, the progress in these technologiesobviously leading either to lower cost or to improvedperformance and capacity. Instead, we will focus onnew transmission, multiple access, and signal pro-cessing techniques.

6.3.1. Higher-leÕel modulationsFirst generation LMDS systems are based on

QPSK which does not make an efficient use of theavailable spectrum compared to QAM signal formatswith a higher number of states. Currently, this modu-lation is not only used on the downstream channelsof LMDS, but also on the upstream channels of bothMMDS and LMDS. This choice is perfectly justified


by the current state of technology and low-costobjectives, but there is no doubt that the trend ofever-increasing capacity in communication networkswill continue in the future, and this trend will natu-rally lead to the use of higher-level modulations.

The 64-QAM modulation adopted for the down-stream channels of MMDS is 3 times more band-width-efficient than the QPSK used on the upstreamchannels. This factor, combined with the frequencyallocation plan which consists of allocating 31 chan-nels to downstream transmission and only 2 channelsto the upstream leads to very asymmetric systems.The implication of this is that MMDS systems aretoday suited to offer asymmetric services such as TVbroadcasting, VoD, internet access, etc., which onlyrequire a narrowband return channel. If the returnchannel is upgraded in the future to employ ahigher-level modulation scheme, the return channelbottleneck can be relaxed, and more broadband ser-vices can be offered. Note that with TDMA asmultiple access technique on the return channel,upstream modulators and demodulators operate inburst mode. Coherent detection in burst-mode trans-mission is not desirable, because it requires a signifi-cant amount of overhead which reduces the systemcapacity. Therefore, rather than the conventionalQAM signal constellations whose states are on asquare grid, differentially-encoded amplitude and

Ž .phase shift keying DAPSK signal constellationsw x14 seem to be the right choice for this application.

Fig. 5. 16-DAPSK signal constellation.

The 16-state DAPSK signal constellation issketched in Fig. 5. It consists of two sets of 8 signalpoints located on two concentric circles. Each of

Ž .those sets forms an 8-state phase shift keying 8-PSKsignal constellation, and the two sets are perfectlyphase aligned. Since the signal constellation has 16states, it carries 4 bits per symbol. From those, 3 bits

Žare carried by phase transitions as in differential.8-PSK , and 1 bit is carried by amplitude transitions.

Ž .Provided that a)1q2 sin pr8 , the average powerof this constellation is related to the minimum dis-tance through the relation

1qa 22Ps d . 4Ž .min28 sin pr8Ž .

This is to be contrasted to Ps5d2 r2 for 16-QAM,min

and to

12Ps d 5Ž .min24 sin pr16Ž .

for 8-PSK. From these expressions, it is easily seenthat in terms of minimum distance, 16-DAPSK loses1.5 dB with respect to 16-QAM and gains 2.7 dBwith respect to 16-PSK. In terms of sensitivity tophase noise, it is similar to 8-PSK, i.e., it is morerobust than both 16-QAM and 16-PSK.

The potential of increasing network capacity iseven higher in LMDS systems, because both thedownstream and the upstream channels can be up-graded from QPSK to 16- or even 64-state modula-tions. This means 16- and 64-QAM signal constella-tions for the downstream channels, and 16- or 64-DAPSK for the upstream channels. When both direc-tions are upgraded to the same modulation levels, the

Žsystem remains symmetric assuming the redundancy.is the same in both directions . If only the down-

stream channels are upgraded, or if the upgrades arenot carried out symmetrically, the downstream willhave a higher capacity than the upstream. For exam-ple, if downstream channels are upgraded to 64-QAMand upstream channels are unchanged, the down-stream direction will have 3 times the capacity of theupstream direction. One way to make the systemsymmetric in this situation is to change the fre-quency allocation plans so as to assign a higherbandwidth to the upstream. More precisely, threequarters of the total available bandwidth needs to beallocated to the upstream direction to make the sys-


tem symmetric in the case at hand. The overallsystem capacity increase is then 50%. Next, supposethat downstream channels are upgraded to 64-QAMand upstream channels are upgraded to 16-DAPSK.In that case, 40% of the available bandwidth needs tobe allocated to downstream channels, and 60% of itto the upstream channels in order to keep the samecapacity in both directions. The network capacity inthis scenario is increased by 140%.

6.3.2. Multiple access techniquesAs mentioned previously, the multiple access

technique used in current MMDS and LMDS sys-tems is TDMA. More precisely, TDMA is used toshare resources of each upstream carrier, but eachuser has access to the resources of all carriers allo-cated to its sector. In other words, the multipleaccess scheme is combined TDMArFDMA. An ob-vious question is whether other multiple accessschemes such as code-division multiple accessŽ .CDMA or orthogonal frequency-division multiple

Ž . w xaccess OFDMA 15 offer new perspectives to thedevelopment of MMDS and LMDS systems in thefuture.

First, let us analyze the potential of CDMA. Thistechnique, which originates from spread-spectrumtechnology, is used in the North American cellular

w xradio standard IS-95 13 , and has recently beenadopted for other future cellular and satellite sys-tems. Comparison of TDMA and CDMA has been avery controversial subject often dominated by com-mercial interests, and therefore, it is difficult to finda truly objective comparison in the literature. An-

w xother difficulty is the fact that, as stated in 16 ,comparisons are often between systems with differ-ent maturity levels and in which TDMA or CDMA isonly one ingredient among many others. The oftenclaimed superiority of CDMA in terms of capacity is

w xstill to be demonstrated, and as it is shown in 17 ,CDMA is not superior either in terms of robustnessto narrowband interference, which may seem surpris-ing as it contradicts a common belief

There are essentially two classes of CDMA: Or-Ž .thogonal CDMA OCDMA which employs a set of

orthogonal sequences, e.g., Walsh–Hadamard se-quences, and nonorthogonal CDMA which employssome kind of pseudo-noise sequences. Practical sys-tems often use a combination of these two tech-

niques. For example, in the IS-95 mobile radio stan-dard, OCDMA is used to share resources in eachcell, and on top of these ortogonal spreading se-quences, pseudo-noise sequences are overlayed inorder to separate signals of different cells. First, it isnot difficult to demonstrate that OCDMA has identi-cal capacity to TDMA. If W designates the band-width required by one user, N users can be accom-modated in both TDMA and OCDMA when the totalavailable bandwidth is NPW. TDMA accommodatesthese users by allocating different time slots, andOCDMA accommodates them by allocating mutuallyorthogonal spreading sequences. Since the number oforthogonal sequences of length N is exactly N,OCDMA can not accommodate any more users thanTDMA. The other class of CDMA, i.e., pseudo-noiseCDMA is more difficult to evaluate in terms ofcapacity, because the capacity in that case is not afixed number. In this class of CDMA, all usersinterfere with each other, and capacity depends on

Žhow much interference and performance degrada-.tion one is prepared to tolerate. In fact, it can be

speculated that pseudo-noise CDMA supports a sig-nificantly smaller number of users than TDMA andOCDMA if the performance degradation is to bekept at an acceptable level.

Despite these negative arguments, CDMA has asignificant advantage over TDMA in terms of peak-to-average signal power, and this favors its use onthe upstream channels. For a given average transmitpower of the CPE, the peak power is N times largerin TDMA, because this multiple access techniqueconcentrates the transmitted signal energy on theallocated slots, whereas CDMA spreads it over theentire frame. This holds when TDMA employs onlyone slot per frame and CDMA allocates one se-quence only. If the user requires more resources,TDMA will allocate a higher number of time slotsper frame, and CDMA will allocate several spread-ing sequences. In that case, the advantage of CDMAover TDMA will be diminished, but still CDMA hasa significant advantage which can be exploited tomake low-cost user terminals. The situation is ex-actly the opposite for the central station, because ifthe transmitted signal is QPSK in time-division mul-

Ž .tiplexing TDM , it is a sum of QPSK signals with ahigher peak-to-average power in code-division multi-

Ž .plexing CDM .


As for OFDMA, this multiple access techniquewas proposed for use on narrowband interferencechannel such as the return channel of CATV net-works. OFDMA yields the same capacity as TDMAand OCDMA on Gaussian noise channels, but cansupport much higher levels of interference thanTDMA and CDMA. Further, the reduced peak-to-average power ratio of CDMA also applies toOFDMA, which makes it attractive for user termi-nals. Finally, channel equalization is much easier inthe case of OFDMA, which is reduced to multiplyingby a complex coefficient each demodulated carrier atthe receiver. No equalization is needed at all if themodulation is QPSK and detection is made differen-tially. The disadvantage of OFDMA is its highersensitivity to phase noise which makes it necessaryto use highly stable low-noise oscillators in themodulator.

6.4. Smart antennas

The antennas used at central station sites cover anentire sector of, say, 908. Instead of such fixed beam

w xantennas, it is appealing to use an adaptive array 18which points its beam in the direction of the user. Anadaptive array consists of a set of sensors located ona rectangular grid whose outputs are linearly com-bined using a set of complex coefficients. The beamcan be moved horizontally and vertically by appro-priately changing the coefficients of the array. Byforming a narrow beam in the direction of the user,adaptive arrays maximize the antenna gain, whilereducing interference from users located in otherdirections This opens up two new perspectives: Thefirst is that adaptive arrays give the possibility to

Ž .implement space-division multiple access SDMAin order to increase capacity in each sector of a cell.The principle of SDMA is to form beams simultane-ously in more than one direction so as to accommo-date several users at the same carrier frequency andduring the same time slot in the same sector. Thisspatial reuse of resources is feasible as long assignals originate from two directions that are separa-ble by the adaptive array used. What is needed is tofeed the respective outputs of the array elements to aset of independent linear combiners. Also, the MACprotocol needs to use the location information ofeach subscriber which can be stored in the centralstation controller. The other perspective is to in-crease the frequency reuse factor in a given geo-

graphic area by synchronizing central stations andcoordinating resource allocation among them. Morespecifically, resource allocation in this case is per-formed in such a way as to minimize interferencefrom other cells which use the same set of frequen-cies.

Note that the concept of SDMA is particularlyattractive when the basic multiple access scheme isTDMA, and it is not as attractive in CDMA systems.This is due to the fact that users are active sequen-tially in TDMA, whereas they are active simultane-ously in CDMA. With two linear combiners, TDMA

Žcan allocate each time slot to two users and double.the sector capacity by switching beam directions at

the time slot rate. In contrast, two combiners areneeded in CDMA in order to assign one particularspreading sequence to two different users. The sameapplies to all spreading sequences, and therefore, 2 Ncombiners are needed to double the capacity of asector by reusing all of the N sequences in the samesector. This obviously leads to an excessive com-plexity with respect to that involved in the case ofTDMA.

7. Conclusions

In this paper, we have given an overview ofMMDS and LMDS systems on the basis of recentstandardization activities which have been carriedout by the DVBrETSI and DAVIC. First, we pre-sented the DVB specifications for digital satellite TVbroadcasting and cable TV networks, and indicatedthat these were extended without any modificationsto LMDS and MMDS, respectively. Next, we pre-sented the DAVIC specifications for the downstreamand upstream channels, as well as for the MACprotocol. The differences between the DVB andDAVIC specifications have been highlighted, and theimpact of this on product design has been indicated.Then, we described the current frequency allocationfor MMDS and LMDS, and pointed out that thestandards and technologies originally developed forbroadcast TV, VoD, and similar consumer servicesare now finding applications to offer broadband ra-dio access services for small- and medium-size busi-nesses. Frequency allocation in Europe and NorthAmerica has been discussed for both consumer andbusiness applications.


Finally, we presented the major design issues ofCPE receivers in MMDS and LMDS, and discussedfrequency planning and reuse, as well as potentialfuture evolutions of these systems. These include theuse of higher-level modulations to increase capacity,multiple access techniques such as CDMA andOFDMA which may lead to lower-cost CPEs byreducing the peak power of the transmitted signal,and adaptive arrays with dynamic beamformingwhich increase wireless network capacity either byspatial reuse of radio resources in each sectorŽ .SDMA or by increasing the frequency reuse factorin a given geographical area.

References

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Hikmet Sari was born in Antakya,Turkey, on February 1, 1954. He re-ceived the Dipl. Eng. and Dr. Eng. de-grees from the Ecole NationaleSuperieure des Telecommunications´ ´ ´Ž .ENST , Paris, France, in 1978 and1980, respectively. He also received theHabilitation degree from the Faculte des´Sciences d’Orsay, Universite de Paris´XI, in January 1992. From 1978 to 1989,he worked for the Laboratoires d’Elec-tronique et de Physique Appliquee´

Ž .LEP , Limeil-Brevannes, France, where he successively held the´positions of Research Scholar, Research Engineer, and GroupSupervisor. From September 1989 to August 1996, he was with

Ž .the Societe Anonyme de Telecommunications SAT , Paris, as a´ ´ ´ ´Department Head in the company’s Telecommunications Division.In September 1996, he joined Alcatel Telecom, where he currentlyholds the position of R&D Director for Broadband Radio Access.He also held the position of Adjunct Professor at the ENST andserved on its Scientific Committee from 1987 to 1991, andcurrently serves as a Scientific Advisor at the ENSTB, Brest. Hehas published over 90 journal and conference papers in the fieldof digital communications, and organized and chaired numeroustechnical sessions at major international conferences. Dr. Sari wasthe Editor for Channel Equalization of the IEEE Transactions onCommunications from 1987 to 1991. He served as a Guest Editor

Ž .of the European Transactions on Telecommunications ETT for aSpecial Issue devoted to the applications of coded-modulationtechniques and published in MayrJune 1993. From 1994 to 1997,he was on the Editorial Board of the Annals of Telecommunica-tions. In 1995, he was elected to the IEEE Fellow Grade for hiscontributions to advanced signal processing in digital microwaveradio systems, and received the Andre Blondel Medal from the´French Electrical and Electronics Engineering Society SEE. Cur-rently, he is a Guest Editor of the IEEE Journal on Selected Areas

Ž .in Communications JSAC preparing a Special Issue devoted toBroadband Wireless Techniques.

broadband radio access to homes and businesses: mmds and lmds

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