The Evolution of Untethered Communications (1997) / Chapter Skim
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2 TECHNOLOGY LIMITS, TRADE-OFFS, AND CHALLENGES
2 TECHNOLOGY LIMITS, TRADE-OFFS, AND CHALLENGES
Pages 56-107
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From page 56... ...
The characteristics of the propagation medium change randomly as users move, and the mobile radio channel introduces random variation in the received signal power 56
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From page 57... ...
Battery life is limited and is influenced by many aspects of terminal design as well as the technology of the network infrastructure. Scarce power constrains the signal processing capabilities and transmit power of the mobile terminal, motivating efforts to keep these units as simple as possible.
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From page 58... ...
2.1 COMMUNICATION LINK DESIGN The ideal wireless communications system would provide high data rates with high reliability and yet use minimum bandwidth and power. It would perform well in wireless propagation environments despite multiple channel impairments such as signal fading and interference.
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From page 59... ...
For example, the transmit power requirements of the mobile unit can be reduced if error-correction coding is used, but then additional power is needed to drive the encoding and decoding hardware. In cellular systems it is preferable to place much of the computational burden at the base station, which has fewer power restrictions than do the mobile units.
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From page 60... ...
The simplest model for path loss, which captures the key characteristics for most channels, is an exponential relationship: The received signal power is proportional to the transmit power and inversely proportional to the square of the trans
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From page 61... ...
Thus, systems designed for typical suburban or low-density urban outdoor environments require much higher transmit power to achieve the same desired performance in a dense jungle or downtown area packed with tall buildings. The BER of a wireless link is determined by the received signal power, noise introduced by the receiver hardware, interference, and channel characteristics.
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From page 62... ...
establishes an "error floor" in the received bits that cannot be reduced by an increase in signal power because doing so also increases the self-interference. Without compensation, the ISI forces a reduction in the data rate such that the product of the RMS delay spread and signal bandwidth is less than 0.1.
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From page 63... ...
The deep fades in signal power caused by flat fading also need to be counterbalanced by an increase in transmit power or some other approach (see Section 2.1.5.1~. Otherwise the transmitted signal typically exhibits bursts of errors that are difficult to correct.
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From page 64... ...
2.1.2 Capacity Limits of Wireless Channels The pioneering work of Claude Shannon determined the total capacity limits for simple wired and wireless channel models: These limits established an upper bound on the maximum spectral link efficiency, measured as the data rate per unit of bandwidth as a function of the received SNR. For a channel without fading, ISI, or Doppler shift, this maximum bandwidth efficiency was identified by Shannon to be the logarithm of the term [SNR + 1]
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From page 65... ...
As measured by spectral link efficiency, these adaptive techniques in both Rayleigh fading and lognormal-shadowed channels can support much higher data rates than are typical in today's wireless systems. For example, typical digital voice systems deliver 8 kbps in a 30-kHz channel, which corresponds to a spectral link efficiency of 8/30, far less than 1.
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66 T o y \ ~ `` ~_ ~ .' I ~b ~ O ED a) ~ N O~ iouolo1~3~ bulk AIRS ~ .
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From page 67... ...
2.1.5.1 Flat-Fading Countermeasures The random variation in received signal power caused by multipath flat fading results in a very large increase in BER. For example, to main
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From page 68... ...
A1most all of the multipath variation is removed by first creating and then later combining four independent paths, with each path weighted by its received signal power. Because the wavelength is inversely proportional to frequency, antenna arrays can be mounted on handheld units when using superhigh frequencies (above 10 GHz)
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From page 69... ...
The goal of equalization is to invert the effects of the channel or cancel the ISI. Channel inversion, or linear equalization, can be achieved by passing the received signal through a filter with a frequency response that is the inverse of the channel frequency response (the channel being the original "filter" for the transmitted signals)
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From page 70... ...
In general, equalizers (especially the DFE) are most beneficial at high data rates, when the product of RMS delay spread and the data rate is much greater than 1.
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From page 71... ...
During the demodulation process, multipath signal components and interference are reduced in two stages: First the spectrum-spreading modulation is removed, and then the remaining signal is demodulated using conventional frequency- or phase-shift techniques to obtain the original data signal. In direct-sequence systems, the received signal is multiplied with an exact copy of the code sequence, perfectly synchronized in time.
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From page 72... ...
2.1. 6.1 Fixed-Allocation Multiple Access Fixed-allocation multiple-access techniques assign dedicated channels to multiple users through some type of channel resource division.
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Another consideration with respect to CDMA is the need for stringent power control to prevent the "near-far problem," which arises when signals from mobile units close to the base station overwhelm those of units farther away. Such control is difficult to maintain in a fading environment and is one of the major challenges of spread-spectrum multiple access.
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From page 74... ...
In the case of power-limited satellite channels or battery-operated transmitters, access efficiency is a more appropriate measure. The access efficiency of an ALOHA random-access channel is the ratio of spectral link efficiency using the ALOHA protocol to the spectral link efficiency of a continuously transmitting channel with the same average power and total bandwidth.
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From page 75... ...
It is difficult to size channels that are assigned on demand for a wide and unpredictable range of user data rates. New, highly flexible randomaccess structures will probably be needed to enable the seamless integration of data services within a voice network as promised in some new personal communications networks.
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From page 76... ...
Because co-channel interference is subject to shadowing and multipath fading, the design of a static cellular system needs to assume worst-case propagation conditions in determining this separation distance. System performance can be improved through dynamic resource allocation, which involves allocating power and bandwidth based on propagation conditions, user demands, and system traffic; however, the increases in spectral and power efficiency are achieved at the price of increased system complexity.
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From page 77... ...
In analog cellular systems, the switching office also coordinates handoffs to neighboring cells when a mobile terminal traverses a cell boundary. In digital cellular systems and low-tier systems, base stations and terminals play a more active role in coordinating handoffs.
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Many of the challenges in packet radio system design are the same as those for any wide-area wireless communications system. These issues include how best to deal with the fading characteristics of RF propagation and whether to use a random or reservation access strategy.
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From page 79... ...
A more efficient strategy is dynamic resource allocation, in which channels, data rates, and power levels are assigned depending on the current interference, propagation, and traffic conditions. For cellular systems, dynamic resource allocation includes assignment of channels to base stations.
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A cable-ready television set is an example of a gateway, and a set-top box is an example of an adapter. With respect to military wireless communications systems, there will be no convergence to a single technology in the foreseeable future, for many reasons.
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From page 81... ...
Therefore, the network management component needs to disseminate connectivity information more rapidly than is necessary in wired networks. The network also needs to be able to handle gracefully any network partitions caused by link outages, which are more likely to occur in mobile packet radio networks than in a conventional wired network.
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From page 82... ...
For very large multihop packet radio networks, such schemes impose a hierarchy on the network topology, hiding changes in the distant parts of the network from local nodes (the next-hop routes to distant network nodes are not likely to change as rapidly as are routes within each cluster)
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From page 83... ...
The MH can also request a locally assigned care-of IP address in its roaming domain by invoking the dynamic host configuration protocol; this address could be used by the home agent directly, eliminating the foreign agent. When an MH enters a new mobile subnetwork it needs to obtain a care-of address.
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, in metropolitan areas (packet radio) , and regional areas (satellite)
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From page 85... ...
They need to model the nature of errors on the wireless link precisely because errors are not uniformly distributed but rather tend to cluster (Nguyen et al., 1996~. They also need to model node mobility, especially in the case of packet radio networks.
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From page 86... ...
Finally, no analysis tool is sophisticated enough to examine the performance of software radios or radio networks in the presence of interference sources common to wideband mobile communications. The evaluation and optimization of mobile wireless networks would be enhanced by the development of sophisticated, flexible models of communications traffic and node mobility.
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From page 87... ...
Proxies provide a convenient place to change data representations en route to the client (thereby mitigating the lossy, constrained bandwidths of wireless links) , perform type-specific compressions, cache data for rapid re-access, and fetch data in anticipation of access.
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From page 88... ...
The IETF is working to provide guaranteed services on the Internet (Peterson and Davie, 1996; Tanenbaum, 1996~. The Internet carries two broad classes of applications: delay tolerant and delay intolerant.
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From page 89... ...
The ATM approach involves breaking up data into short packets of fixed size called cells, which are interspersed by time division with data from other sources and delivered over trunk networks. An ATM network can scale up to high data rates because it uses fast switching and data multiplexing based on these fixed-format cells, which contain 48 bytes of traffic combined with a 5-byte header defining the virtual circuits and paths over which the data are to be transported.
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From page 90... ...
Wireless links, on the other hand, have high bit-error rates, high latencies due to link layer retransmissions, and unpredictable link bandwidths.~9 Furthermore, the quality of a wireless link varies over time, and connections can be lost completely. Two wireless end nodes sharing the same link can experience vastly different link bandwidths depending on their relative proximity to the base station, location in a radio fade, or loss of receiver synchronization in a multipath environment.
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From page 91... ...
When a threshold number of duplicate acknowledgments is received, the sender retransmits the lost segment and halves the congestion window; this part of the algorithm is known as fast retransmission and recovery. A more serious congestion event can cause the loss of so many packets that no duplicate acknowledgments are generated by the receiver.
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From page 92... ...
, in which the base station triggers local retransmissions of lost segments. By intercepting the duplicate acknowledgments, the base station shields the sender from the effects of local losses that would have the effect of shrinking the congestion window and reducing throughput.
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Link security was primarily a military concern before commercial wireless communications became prevalent. Military systems are designed to avert the detection of radio signals, jamming of communication links, and interception and decoding of messages.
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From page 94... ...
Similarly, although the growing use of wireless systems and growing dependence on networked communications have heightened concerns about the possible denial of service in commercial contexts, there is probably greater tolerance for private-service outages than for jamming in a military situation at this time. Hardware security also has different implications for commercial and military applications, although encryption keys typically need to be protected in both contexts.23 Commercial systems require sufficient security to prevent the fraudulent use of information in the event of theft or loss, and user databases need to be secured against unauthorized access.
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From page 95... ...
Directional antennas in cellular-system base stations focus power in a particular direction, thereby minimizing the required transmitter power and significantly reducing the amount of interference. Directional antennas need to be positioned carefully.
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From page 96... ...
Commercial applications for adaptive antennas are limited to relatively low-cost, singleband units with limited flexibility in beam pattern. Moreover, virtually all existing adaptive antennas for mobile radio applications are designed for use at base stations rather than mobile units.
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From page 97... ...
. In new radio architectures, referred to variously as software-defined radio, programmable radio, or simply software radio, analog functions such as tun
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From page 98... ...
software radios are to become a practical reality: advanced A/D converters, DSP chips, filters, and RF amplifier components. 2.4.2.1 Analog-to-Digital Converters The key enabling component, and the most complex and misunderstood element of wideband software radios, is the A/D converter.
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From page 99... ...
software radios may implement some functions, such as analog-to-digital conversion and signal processing, in single integrated circuits.
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From page 100... ...
The commercial sector continues to rely on older technology (e.g., mechanical filters are used in cellular telephone systems) whereas the military has unique needs to reduce cosite interference, both within software radios and across multisystem platforms, and cover wide frequency ranges.
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The following subsections review the limitations and the new technologies designed to overcome them. The commercial sector is making rapid advances in all these areas that the DOD can exploit to good advantage.
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From page 102... ...
Because consumers are demanding highly integrated yet portable computing devices, the commercial sector is performing R&D with the aim of increasing processor capabilities while also reducing power requirements. 2.4.3.3 Batteries The commercial sector has made tremendous strides in battery technology in recent years because it plays a role in many technologies, ranging from surgical implants to electric cars.
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From page 103... ...
However, current tools are inadequate to the task of modeling an untethered communications system that uses wideband signals and advanced components such as software radios. The DOD also has unique needs for interoperability and security of communications systems, although commercial concerns about system integrity and service availability are growing.
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From page 104... ...
The emergence of wideband, programmable radios for military applications will also depend on advances in hardware components such as antennas, which need to be designed for mobile units, and filters, which need to be miniaturized and designed for wideband applications. These issues are examined further in Chapter 3, which explores the opportunities for synergy between the commercial and military sectors in the development of advanced wireless communications systems.
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From page 105... ...
12. Assuming that a collision results in the loss of two packets, the maximum throughput in an ALOHA channel is about 18 percent of the peak data rate if the probability of a collision is to be reduced to a level acceptable to the user.
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From page 106... ...
However, joint compression and channel coding creates some problems. First, this approach requires that the compression algorithms, which typically sit at the application layer, have access to information about the link layer, which means that the layer separation of the open-systems interconnection model breaks down.
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From page 107... ...
20. Because of the error characteristics of wireless links, some of the QoS issues need to be addressed locally at the link layer rather than from an end-toend perspective.
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