Wireless Communications: Current Outlook and Future Trends

Saud A. Al-SemariElectrical Engineering Department, KFUPM

E-mail: semari@kfupm.edu.sa

Abstract: Wireless communications have seen a tremendous growth in the past two decades. Thispaper gives an overview of the major wireless systems used today and provides insights on theproposed third generation wireless systems as well as the future market of wireless technology.

I. Introduction: Wireless communicationservices have been adopted by many peoplearound the world. The associated industry hasgrown by orders of magnitude. Generallyspeaking, wireless services can be classifiedinto the following systems:

Paging systems Cordless telephone systems Cellular telephone systems Wireless Local Area Networks Satellite Communication Systems Wireless Local Loops (WLL)

Paging systems are typically used to notify asubscriber of a brief message or the need tocall a particular telephone number. Simplepaging systems may be limited in range, whilewide area paging systems can provideworldwide coverage.

On the other hand, cordless telephones providea wireless connection between a portablehandset and a specific base station that isconnected to a dedicated telephone line. Amore advanced system is the cellular phone. Itprovides a wireless connection to the publicswitched telephone network (PSTN) for anymoving user within the coverage area of thesystem. WLL systems use radio to provide atelephone connection to the home. Table 1provides some comparisons between cellularand cordless systems.

Subscribers are allowed to simultaneouslysend and receive information to and from thebase station. This is done using duplexing.Frequency division duplexing (FDD) providestwo distinct bands of frequencies for each user(for the uplink and downlink). On the otherhand, Time Division Duplexing (TDD) usestime to provide both an uplink and downlink.

Multiple access techniques are used to allowmobile users to simultaneously share a finiteamount of the radio spectrum. They can begenerally classified into the followingschemes:

Frequency Division Multiple Access Time Division Multiple Access Frequency Hopped Multiple Access Code Division Multiple Access

In FDMA, each user is given a frequency slot.Analog or digital systems can be used. InTDMA, each user is allocated a time slot.TDMA can only be used in digital systems. InFHMA, the user will hop from one frequencyslot to another in a specific manner. This givesimmunity against multi-path effects.

In CDMA based systems, each user data ismultiplied by a Pseudo-Noise (PN) sequence.Multiplication by the PN sequence will resultin a spread of the signal over a wide spectrum.If the users PN sequences are orthogonal, thenmultiple users can share the same channel(which will have a wide bandwidth).Therefore, the number of users on eachchannel is not a hard limit but depends on thesystem's acceptable noise level.

In order for CDMA systems to functionproperly, the user signals received at the basestation must be of the same or similar powerlevel. With different channel environmentsand distances between users, an accurate andfast power control mechanism has to beimplemented. In fact, the capacity of CDMAsystems is heavily dependent on the success ofthe power control scheme used.

In this paper, an overview of current wirelessservices is presented. Our focus will be mainly

on cellular, cordless and mobile satellitesystems. Section II will give a brief descriptionof the major cordless and cellular systems thathave been deployed. Mobile satellite systemswill be discussed in Section III. Then, sectionIV will give insights on the future trends ofwireless technology.

II. Cordless and Cellular Systems:The first generation cordless systems wereintroduced in the early 1980's. Systems likethe US cordless, CT1 and NTT were verycommon around the world. Cordlesstelephones were mainly used for indoorlocations. The cordless terminal output powerwas much less than that of the mobile terminal(

Geostationary Systems are at 35786 km orbits.The advantage of GEO systems is that eachsatellite has a large footprint. However, theysuffer from long transmission delays (on theorder of 250 ms). Such delays make errordetection/retransmission strategies difficult touse. Moreover, high transmit power is requiredby both the user unit and the satellite. GEOsystems, such as INMARSAT are primarilyused for ship-to-shore communications.

On the other hand, LEO satellites provide shortvisibility from any point on earth. Thisdemands potentially large constellations. Also,the satellite lifetime is dramatically reducedwhen low orbiting (5-8 years for LEOscompared to 12-15 in GEO systems). Thesetwo attributes affect the economics of LEOsatellite systems. Examples of LEO systemsinclude Iridium, Globalstar and Teledesic.

The Iridium system, by Motorola, startedoperation in Novermber 1998. It is designedto carry voice, data (2.4 kbps), Fax, andlocation Services. It consists of 66 satellites in6 polar orbits (780 km). It has satellite-to-satellite links as well as to ground. Accesstechnologies are FDMA (uplink) and TDM(downlink). It also supports satellite handoffsduring calls. The system is designed to becomplementary to terrestrial cellulartelephony. Using dual mode operation,satellites reach remote rural areas, whileterrestrial cellular reaches densely populatedareas.

On the other hand, the Globalstar system, to bedeployed by Loral/Qualcomm, consists of 48satellites at an altitude of 1400 km. It providesvoice, data (9.6 kbps), Fax, and locationServices. It uses CDMA access techniques.

The Teledesic system is promoted byMicrosoft. The system was originally plannedto operate on 21 orbital planes (at 695-705 kmaltitude), with 40 satellites per plane, thusgiving a total of 840 satellites. Later, Teledesichas changed its design to one consisting of 12planes of 24 satellites per plane at an altitudeof 1350km (a total of 288 satellites). The totalestimated system cost is $9 billion. It isdesigned to carry voice and data upto 2 Mbps.

MEO systems represent a compromisebetween LEO systems and GEO systems. ICOis an examples of a MEO system. It consists of10 satellites and is scheduled to start operationin year 2000.

IV. Third-Generation WirelessSystems: Two efforts are underway to comeup with third generation wireless standards. In1998, the European TelecommunicationsStandards Institute (ETSI) decided on a singleair interface solution for Universal MobileTelecommunication System (UMTS). Themain objectives in making this decision was tohelp making an open-ended and cost-effectivetechnology.

The International Mobile Telecommunicationsby the year 2000 (IMT-2000) is an initiative ofITU-Radio, Task Group 8/1. The WorldAdministrative Radio Conference (WARC) 92assigned 1885-2025 and 2110-2200 MHz foruse by IMT-2000. The main objective of IMT-2000 is to unify diverse systems into oneuniversal system that can be used worldwide.Other objectives of the IMT-2000 are asfollows

Customers should see services, nottechnology

Provide a seamless radio infrastructure Maximize commonality of radio interfaces Enable cost-effective dual mode operation Universal personal mobility Evolution from 2nd generation

technologies

The IMT-2000 is being designed for a widerange of services (Voice and Non-voice/Multi-media), a wide range of propagationenvironments (indoor/Outdoor) and a widerange of user densities, with a performancecomparable to that of fixed networks.Spectrum efficiency is a major factor in thesystem design. The system is required to havean open architecture. Moreover, the IMT-2000must provide a seamless path of migrationfrom second generation wireless networks [3].More details of the system can be found in[3], [4] and [5].

V. Conclusions: The demand on wirelesscommunications will increase. No singletechnology will drive the industry. However,services will tend to merge. New thirdgeneration wireless systems will deliver bitrates up to 384 kbps for wide-area coverageand 2 Mbps for indoor or fixed applications. Inthe near future, there will be an increasingimportance of wireless e-mail. By year 2000,the number of internet users is expected to be400 million users. However, at that time thenumber of mobile phone users is expected tobe around 600 million.

1. Wireless Now, An Ericsson Newsletter, Issue 3, August 1998.2. Wireless Now, An Ericsson Newsletter, Issue 4, August 1998.3. T. Nilsson, "Toward Third-Generation Wireless Communication", Ericsson Review, 1998, No. 24. R. Prasad and T. Ojanpera, "An Overview of CDMA Evolution toward Wideband CDMA", IEEE

Communications Surveys, 1998, Quarter 4.5. T. Ojanpera and R. Prasad, An Overview of Third-Generation Wireless Personal Communications: A

European Perspective, IEEE Personal Communications, December 1998, PP. 59-65.6. T. Rappaport, Wireless Communications: Principles and Applications, Prentice Hall PTR, 1996.

System Cellular Cordless

Cell size Large (0.5-30km) Small (50-500 m)Mobility speed High (up to 150 km/h) Low (less than 6 km/h)Coverage Wide area ZonalHandset complexity Moderate LowBase complexity High LowHandset TX power High (100mW-600mW) Low (5-10 mW)

Table1: Basic differences between cellular and cordless systems

Standard GSM D-AMPS IS95 PDCWhere Global US Many JapanAccess TDMA TDMA CDMA TDMAFrequencyBands (MHz)

890-915935-9601710-17851805-1880

824-849869-894

824-849869-894

810-826940-9561429-14531477-1501

RF Spacing 200 kHz 30 kHz 1250 kHz 25 kHzChannelBit rate kbit/s

270.833 48.6 1288/9.6 42 kbit/s

Voicechannels/ RF

8 3 variable 3

Modulation GMSK p /4 DQPSK BPSK/ QPSK p /4 DQPSK

Table 2: Basic Comparison between different second generation cellular systems.

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