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`14 CURRENT WIRELESS SYSTEMS
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`The cellular digital packet data(CDPD)system is a wide-area wireless data service
`layed on the analog cellular telephone network. CDPD shares the FDMA voice chi
`of the analog systems, since many of these channels are idle owing to thé growth of
`tal cellular, The CDPD service provides packet data transmission at rates of 19.2 kby
`ig available throughoutthe United States. However, since newer generationsof cellule
`tems also provide data services and at higher data rates, CDPD is mostly beingreplac
`these newerservices. Thus, wide area wireless data services have not been yery succe
`although emerging systems that offer broadband access may haye more appeal.
`
`1.4.5 Broadband Wireless Access
`Broadband wireless access provides high-rate wireless communications betweenafix:
`cess point and multiple terminals. These systems were initially proposed to support in
`tive video service to the home, but the application emphasis then shifted to providing
`high-speed data access (tens of Mbps) to theIntemet and the World Wide Web as
`high-speed data networks for homes arid businesses. In the United States, two freq
`bands were set aside for these systems: part of the 28-GHz spectrum for local distril
`systems (local multipoint distribution service, LMDS) and a band in the 2-GHz spe
`for metropolitan distribution service (multichannel multipointdistribution services, MN
`LMDS represents a quick means for new service providers to-enter the already stiff cc
`tition among wireless and wireline broadband service providers [5, Chap. 2.3]. MM
`a television and telecommunication delivery system with transmission ranges of 30-{
`[5, Chap. 11.11], MMDS has the capability of delivering more than a hundred digital
`TV channels along with telephony and access to the Internet. MMDSwill compete n
`with existing cable and satellite systems. Europe is developing 4standard similar to.M
`called Hiperaccess.
`WiMax is an emerging broadbandwireless technology based on the IBEE 802.16 sta
`[16; £7]. The core 802.16 specification is a standard for broadband wireless access sy
`operating atradio frequencies between 2 GHz and 11 GHz for non-line-of-sight oper
`and between 10 GHz and 66GHz forline-of-sight operation. Data rates of around 40
`will be available for fixed users and 15 Mbps for mobile users, with a range of several
`meters. Many manufacturers of laptops and PDAs(personal digital assistants) are pla
`to incorporate WiMax once it becomes available to satisfy demand for constant Intern
`cess and email exchange from any location. WiMax will compete with wireless LAN
`cellular services, and-possibly wireline services like cable and DSL (digital subscriber
`Theability ofWiMax'to challenge or supplant these systems will depend.onits relativ
`formance and cost, which remain to. bé seen.
`
`1.4.6 Paging Systems
`Paging systems broadcast a short paging message simultaneously from many tail bas
`tionsor satellites transmitting at very high power (hundreds of watts to kilowatts). Sy
`with terrestrial transmitters are typically localized to a particular geographic area, suc
`city or metropolitan region, while geosynchronoussatellite transmitters. provide natio:
`international coverage. In both types of systems, no location managementor routing
`tions are needed because thepaging message is broadcast over the entire coveragearea
`high complexity and power of the paging transmitters allows low-complexity, low-p
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`18
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`OVERVIEW OF WIRELESS COMMUNICA
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`pocket paging receivers capable oflong usage times from small and lightweight bat
`Inaddition, the high transmit power allows paging signals to easily penetrate building
`Paging service also costs less than cellular service, both for the initial device and {
`monthly usage charge, although this price advantage has declined considerably in
`years as cellular prices dropped. The low cost, small and lightweighthandsets, long t
`life, and ability ofpaging devices to work almost anywhere indoors or outdoors are the
`reasons ‘for their appeal.
`Early radio paging systems were analog I-bit messages signaling a user that sor
`was trying to reach himor her. These systems required callback over a landline tele
`to obtain the phone number of the paging party. The system evolved to allow a shor
`tal message,including a phone number andbrief text, tobe sent to the pagee as well.
`paging systems were initially extremely successful, with a peak of 50. million sutise
`in the United States alone. However, their popularity began to wane with the wides
`penetration and competitive cost of cellulartelephone systems. Eventually the compi
`from cellular phones forced paging systems to provide new capabilities. Some implen
`“answer-back” capability (i.c., two-way communication). This required a major chai
`designof the pager because now it needed to transmitsignals in addition to receiving
`ahd the transmission distance to a satellite or base station can be very large. Paging
`panies also teamed up with palmtop computer makers to incorporate paging function
`these devices [18]. Despite these developments, the market for paging devices has s
`considerably, although there is still a niche market among doctors and other profess
`who must be reachable anywhere.
`
`14.7 Satellite Networks
`Commercial satellite systems are another major component of the witeless communic
`infrastructure [2; 3]. Geosynchronous systems include Inmarsat and OmniTRACS,Tt
`mer is geared mainly for analog voice transmission from remote locations. For examp!
`commonly used byjournalists to provide live reporting from war zones, Thefirst-gene
`Inmarsat-A system was designed for large (1-m parabolic dish antenna) and rather é
`sive terminals. Newer generations of Inmarsats use digital techniques to enable sn
`less expensive terminals, about the size of a briefcase. Qualcomm’s OmniTRACSpx
`two-way communications as well as location positioning. The system is used primari
`alphanumeric messaging and location tracking of trucking:fleets. There are several
`difficulties in providing voice and data services over geosynchronous satellites. It ti
`great deal ofpowerto reach thesesatellites, so handsets aré typically large and bulky.
`dition, there is a large round-trip propagation delay; this delay is quite noticeable in tw:
`voice communication. Geosynchronoussatellites also have fairly low data rates of les
`10 kbps. For these reasons, lower-orbit LEO satellites were thoughtto be a better mat
`voice and data communications.
`LEOsystems require approximately 30-80 satellites to provide global coverage
`plansfor deploying such constellations were widespread in the late 1990s. One ofthe
`ambitious ofthese systems, the Iridium constellation, was launched at that time. Hov
`the cost to build, launch, andmaintain thesesatellites is much higher than costs for tern
`base stations, Although these LEO systems can certainly complementterrestrial syste
`low-population areas and are also. appealing to travelers desiringjust’one handset and |
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`16.6 ENERGY-CONSTRAINED NETWORKS
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`separating the cooperating nodes, Whenthere is less of a difference between the sepa
`of the cooperating nodes and the transmission distance between these clusters, the e
`cost required for the local exchange of information exceeds the energy benefits of cx
`ating. Cooperative MIMOis one form of cooperative diversity. Others were discus:
`Section 16.3.3, and these other techniques may provide energy savings comparable to
`ceeding those ofcooperative MIMO,depending on the network topology.
`
`16.6.3 Access, Routing, and Sleeping
`Random access schemes can bemade more energy efficient by (i) minimizing collisior
`the resulting retransmissions and (ii) optimizing transmit power to the minimum rec
`for successful transmission. One way to reducecollisions is to increase error protect
`collisions become more frequent [111]. Alternatively, adaptively minimizing power th
`probingas part ofthe random accessprotocol has been shown to significantly increase e
`efficiency (111; 38]. Another methodfor energy-efficient access is to formulate the d
`uted access problem using a game-theoretic approach, where energy and: delay are
`associated with the game [112]. Several different approaches to energy-efficient access
`evaluated in [113]. However, noclear winneremerged because the performanceofeac!
`tocol is highly dependent on channel chiracteristics. Delay arid faimess constraints ox
`be incorporated into'an energy-efficient access framework,as investigated in [114]. Mi
`these techniques avoidcollisions through a version of TDMA,although setting up ch:
`ized acces&under distributed control can lead to largé delays.
`If users have longstrings of packets ora continuous stream of data, then randoms
`works poorly since most transmissions result in collisions. Hence channels must be ass
`to users in a more systematic fashion by transmission scheduling. Energy constraints
`new wrinkle to scheduling optimization. In [100] it was shown that the energy requi!
`send a bit is minimized by transmitting it over all available bandwidth and time dimen
`However, when multiple users: wish to access the channel, the systeni time and band
`resources must be shared amongallusers. More recent work has investigated optimal s
`uling. algorithms to minimize transmit energy for multiple users sharing a channel [11
`this work, scheduling was optimized to minimize the transmission energy required by
`user subject to a deadline or delay constraint. The-energy minimization was based on
`ciously varying packet transmission time (and corresponding energy consumption) to
`the delay constraints of the data. This scheme was shown to be significantly more @
`efficient than a deterministic schedule with the same deadline constraint.
`Energy-constrained networks also require routing protocols that optimize routes te
`to energy consumption. Ifthe rate of energy consumptionis not evenly distributed acr¢
`nodes then some nodes may expire sooner than others, leading to. apartitioning of th
`work, Routing can be optimized to minimize end-to-end energy Consumption by apr
`the standard optimization procedure described in Section 16.3.3, withenergy perhop (in
`ofcongestion or delay) as the hop cost[116]. Alternatively, the routes can be computed '
`on costs associated with the batteries in cach node —for example, maximizing the mini
`battery lifetime acrosg all nodes in the network [116; [17]. Different cost functions to
`mize energy-constrained routing were evaluated via simulation in [116] and were all ro
`equivalent. The cost function can also be extended to include the,traditional metric of
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`562
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`AD HOC WIRELESS NETA
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`along with energy [118]. This method allowsthe route optimization to trade off betwe:
`lay and energy consumption through different weighting of their respective contribut
`the overall cost function. Note that computation and disseminationofrouting tables ci
`tail significant cost; this can be avoided by routingtraffic geographically (i.e., in the gi
`direction ofits destination), which requires little advance computation [119].
`Energy-constrained nodes consume significant power even in standby mode, whei
`atejust passive patticipants. in the network with minimal exchange of data: to maintait
`network status, The paging industry developed asolution to this.problem several decad:
`by scheduling “sleep” periods for pagers. The basic idea is that each pager need only
`fortransmissions during certain short periods oftime. This is a simple solution to.impli
`when a central controller is available,butit is less obvious how to implement suchstra
`within the framework ofdistributed network control. Sleep decisions musttake into ac
`network connectivity, so it followsthat these decisions are local but not autonomous.}
`anisms that support such decisions can be based on neighbor discovery coupled with
`means for ordering decisions within the neighborhood. In a given area, the opportu
`sleep should be circulated among the nodes, ensuring that connectivity is notlost th
`the coincidence of several simultaneous decisions to sleep.
`
`16.6.4 Cross-Layer Design under Energy Constraints
`The unique attributes ofenergy-constrained networks make them prime candidates for:
`layer design. If node batteries cannot be recharged, then each node can transmit only a
`number ofbits before it dies, after which time it is no longer available to perform its int
`function (¢.g, sensing) or to participate in network activities such as routing. Thus, ¢
`must be used judiciously across all layers of the protocol stack in order to prolong né
`lifetime and meet application requirements.
`Energy efficiency at all layers oftheprotocol stack typically imposes trade-offs be!
`energy consumption, delay, and throughput [120]. However, at any given layer, the of
`operating pointonthis trade-offcurve must be driven by considerationsat higherlayer
`example, if a node transmits slowly then it conserves transmit energy, but this compl
`access for other nodes and increases end-to-end delay. A routing protocol may use :
`trally located node for energy-efficient routing, but this will increase congestion and
`on that route and also bum up that node’s battery energy quickly, thereby removing it
`the network. Ultimately the trade-offs between energy, delay, throughput, and node/ne
`lifetime must be optimized relative to the application requirements. An emergency r
`operation needs on-the-scene information quickly, but typically the network supportin
`local information exchangenéed only last a few hours or days. In contrast, a sensor ne
`embedded into the concrete of a bridge to measure stress and strain must last decades,tl
`the information need only be collected every day ar week,
`
`16.6.5 Capacity per Unit Energy
`When transmit energy is constrained, it is not possible to transmit any finite number «
`with asymptotically small error probability. This is easy to see intuitively by consic
`the transmission of a single bit. The only way to ensure that two different values in ;
`space (representingthe two possible bit values) can be decoded with arbitrarily small
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