无线通信技术的演进与挑战

需积分: 9 185 浏览量更新于2024-07-18 收藏 3.26MB PDF 举报

"Wireless Communications - Andrea Goldsmith" 是一本由斯坦福大学的Andrea Goldsmith教授撰写的关于无线通信的书籍。这本书深入探讨了无线通信的历史、技术挑战、现有系统以及未来愿景，并涵盖了各种无线网络标准和技术。本书首先概述了无线通信的重要性，指出其在通信行业中增长速度最快，全球约有20亿用户使用无线通信，特别是在商业和个人生活中，无线通信已经成为不可或缺的一部分。无线局域网（WLAN）逐渐取代有线网络，同时新兴的应用如无线传感器网络、自动化高速公路、智能家庭和远程医疗等正从研究阶段转向实际应用。作者回顾了无线通信的历史，从古代的烟雾信号到现代的蜂窝网络、卫星通信系统。然后，讨论了无线通信的愿景，包括需要克服的技术难题，以实现这一愿景。这些挑战包括设计出能够支持新兴应用并具有足够性能的稳健无线网络。书中详细介绍了当前的无线系统，例如： 1. 蜂窝电话系统：作为最广泛使用的无线通信系统，它们经历了指数级的增长。 2. 无绳电话：在家庭和小型办公室中提供便捷的通信。 3. 无线局域网（WLAN）：在住宅、企业和校园中广泛应用。 4. 广域无线数据服务：提供了移动设备的数据连接。 5. 宽带无线接入：用于高速互联网访问。 6. 页码系统：提供个人通知服务。 7. 卫星网络：覆盖全球范围的通信。 8. 低成本低功耗无线电：如蓝牙和Zigbee，用于短距离设备互联。 9. 超宽带无线电：提供高数据速率和低延迟的通信。此外，书中还讨论了无线频谱管理，包括分配方法和现有系统的频谱分配，以及标准的制定，这些都是确保无线通信系统有效运行的关键要素。第2章开始深入技术细节，如路径损耗和阴影衰落，这是无线通信中的基础物理现象，影响信号传播和网络设计。这本书是无线通信领域的综合指南，对于理解该领域的技术发展、挑战和未来趋势有着重要的价值。

Thus, technological breakthroughs to enable higher frequency systems with the same cost and performance would

greatly reduce the spectrum shortage. However, path loss at these higher frequencies is larger, thereby limiting

range, unless directional antennas are used.

As a signal propagates through a wireless channel, it experiences random ﬂuctuations in time if the transmitter,

receiver, or surrounding objects are moving, due to changing reﬂections and attenuation. Thus, the characteristics

of the channel appear to change randomly with time, which makes it difﬁcult to design reliable systems with

guaranteed performance. Security is also more difﬁcult to implement in wireless systems, since the airwaves are

susceptible to snooping from anyone with an RF antenna. The analog cellular systems have no security, and one

can easily listen in on conversations by scanning the analog cellular frequency band. All digital cellular systems

implement some level of encryption. However, with enough knowledge, time and determination most of these

encryption methods can be cracked and, indeed, several have been compromised. To support applications like

electronic commerce and credit card transactions, the wireless network must be secure against such listeners.

Wireless networking is also a signiﬁcant challenge. The network must be able to locate a given user wherever

it is among billions of globally-distributed mobile terminals. It must then route a call to that user as it moves at

speeds of up to 100 Km/hr. The ﬁnite resources of the network must be allocated in a fair and efﬁcient manner

relative to changing user demands and locations. Moreover, there currently exists a tremendous infrastructure of

wired networks: the telephone system, the Internet, and ﬁber optic cable, which should be used to connect wireless

systems together into a global network. However, wireless systems with mobile users will never be able to compete

with wired systems in terms of data rates and reliability. Interfacing between wireless and wired networks with

vastly different performance capabilities is a difﬁcult problem.

Perhaps the most signiﬁcant technical challenge in wireless network design is an overhaul of the design

process itself. Wired networks are mostly designed according to a layered approach, whereby protocols associated

with different layers of the system operation are designed in isolation, with baseline mechanisms to interface

between layers. The layers in a wireless systems include the link or physical layer, which handles bit transmissions

over the communications medium, the access layer, which handles shared access to the communications medium,

the network and transport layers, which routes data across the network and insure end-to-end connectivity and data

delivery, and the application layer, which dictates the end-to-end data rates and delay constraints associated with

the application. While a layering methodology reduces complexity and facilitates modularity and standardization,

it also leads to inefﬁciency and performance loss due to the lack of a global design optimization. The large

capacity and good reliability of wired networks make these inefﬁciencies relatively benign for many wired network

applications, although it does preclude good performance of delay-constrained applications such as voice and

video. The situation is very different in a wireless network. Wireless links can exhibit very poor performance,

and this performance along with user connectivity and network topology changes over time. In fact, the very

notion of a wireless link is somewhat fuzzy due to the nature of radio propagation and broadcasting. The dynamic

nature and poor performance of the underlying wireless communication channel indicates that high-performance

networks must be optimized for this channel and must be robust and adaptive to its variations, as well as to network

dynamics. Thus, these networks require integrated and adaptive protocols at all layers, from the link layer to the

application layer. This cross-layer protocol design requires interdiciplinary expertise in communications, signal

processing, and network theory and design.

In the next section we give an overview of the wireless systems in operation today. It will be clear from

this overview that the wireless vision remains a distant goal, with many technical challenges to overcome. These

challenges will be examined in detail throughout the book.

1.4 Current Wireless Systems

This section provides a brief overview of current wireless systems in operation today. The design details of these

system are constantly evolving, with new systems emerging and old ones going by the wayside. Thus, we will

focus mainly on the high-level design aspects of the most common systems. More details on wireless system

standards can be found in [1, 2, 3] A summary of the main wireless system standards is given in Appendix D.

1.4.1 Cellular Telephone Systems

Cellular telephone systems are extremely popular and lucrative worldwide: these are the systems that ignited the

wireless revolution. Cellular systems provide two-way voice and data communication with regional, national, or

international coverage. Cellular systems were initially designed for mobile terminals inside vehicles with antennas

mounted on the vehicle roof. Today these systems have evolved to support lightweight handheld mobile terminals

operating inside and outside buildings at both pedestrian and vehicle speeds.

The basic premise behind cellular system design is frequency reuse, which exploits the fact that signal power

falls off with distance to reuse the same frequency spectrum at spatially-separated locations. Speciﬁcally, the

coverage area of a cellular system is divided into nonoverlapping cells where some set of channels is assigned

to each cell. This same channel set is used in another cell some distance away, as shown in Figure 1.1, where

denotes the channel set used in a particular cell. Operation within a cell is controlled by a centralized base

station, as described in more detail below. The interference caused by users in different cells operating on the same

channel set is called intercell interference. The spatial separation of cells that reuse the same channel set, the reuse

distance, should be as small as possible so that frequencies are reused as often as possible, thereby maximizing

spectral efﬁciency. However, as the reuse distance decreases, intercell interference increases, due to the smaller

propagation distance between interfering cells. Since intercell interference must remain below a given threshold

for acceptable system performance, reuse distance cannot be reduced below some minimum value. In practice it

is quite difﬁcult to determine this minimum value since both the transmitting and interfering signals experience

random power variations due to the characteristics of wireless signal propagation. In order to determine the best

reuse distance and base station placement, an accurate characterization of signal propagation within the cells is

needed.

Initial cellular system designs were mainly driven by the high cost of base stations, approximately one million

dollars apiece. For this reason early cellular systems used a relatively small number of cells to cover an entire city

or region. The cell base stations were placed on tall buildings or mountains and transmitted at very high power with

cell coverage areas of several square miles. These large cells are called macrocells. Signal power was radiated

uniformly in all directions, so a mobile moving in a circle around the base station would have approximately

constant received power if the signal was not blocked by an attenuating object. This circular contour of constant

power yields a hexagonal cell shape for the system, since a hexagon is the closest shape to a circle that can cover a

given area with multiple nonoverlapping cells.

Cellular systems in urban areas now mostly use smaller cells with base stations close to street level transmit-

ting at much lower power. These smaller cells are called microcells or picocells, depending on their size. This

evolution to smaller cells occured for two reasons: the need for higher capacity in areas with high user density and

the reduced size and cost of base station electronics. A cell of any size can support roughly the same number of

users if the system is scaled accordingly. Thus, for a given coverage area a system with many microcells has a

higher number of users per unit area than a system with just a few macrocells. In addition, less power is required

at the mobile terminals in microcellular systems, since the terminals are closer to the base stations. However, the

evolution to smaller cells has complicated network design. Mobiles traverse a small cell more quickly than a large

cell, and therefore handoffs must be processed more quickly. In addition, location management becomes more

complicated, since there are more cells within a given area where a mobile may be located. It is also harder to

Base

Station

Figure 1.1: Cellular Systems.

develop general propagation models for small cells, since signal propagation in these cells is highly dependent

on base station placement and the geometry of the surrounding reﬂectors. In particular, a hexagonal cell shape is

generally not a good approximation to signal propagation in microcells. Microcellular systems are often designed

using square or triangular cell shapes, but these shapes have a large margin of error in their approximation to

microcell signal propagation [9].

All base stations in a given geographical area are connected via a high-speed communications link to a mobile

telephone switching ofﬁce (MTSO), as shown in Figure 1.2. The MTSO acts as a central controller for the network,

allocating channels within each cell, coordinating handoffs between cells when a mobile traverses a cell boundary,

and routing calls to and from mobile users. The MTSO can route voice calls through the public switched telephone

network (PSTN) or provide Internet access. A new user located in a given cell requests a channel by sending a call

request to the cell’s base station over a separate control channel. The request is relayed to the MTSO, which accepts

the call request if a channel is available in that cell. If no channels are available then the call request is rejected. A

call handoff is initiated when the base station or the mobile in a given cell detects that the received signal power for

that call is approaching a given minimum threshold. In this case the base station informs the MTSO that the mobile

requires a handoff, and the MTSO then queries surrounding base stations to determine if one of these stations can

detect that mobile’s signal. If so then the MTSO coordinates a handoff between the original base station and the

new base station. If no channels are available in the cell with the new base station then the handoff fails and the

call is terminated. A call will also be dropped if the signal strength between a mobile and its base station drops

below the minimum threshold needed for communication due to random signal variations.

The ﬁrst generation of cellular systems used analog communications, since they were primarily designed in

the 1960’s, before digital communications became prevalent. Second generation systems moved from analog to

digital due to its many advantages. The components are cheaper, faster, smaller, and require less power. Voice

quality is improved due to error correction coding. Digital systems also have higher capacity than analog systems

since they can use more spectrally-efﬁcient digital modulation and more efﬁcient techniques to share the cellular

spectrum. They can also take advantage of advanced compression techniques and voice activity factors. In addition,

MOBILE

TELEPHONE

SWITCHING

OFFICE

LOCAL

EXCHANGE

LONG−DISTANCE

NETWORK

PHONE

CELLULAR

BASE

STATION

INTERNET

Figure 1.2: Current Cellular Network Architecture

encryption techniques can be used to secure digital signals against eavesdropping. Digital systems can also offer

data services in addition to voice, including short messaging, email, Internet access, and imaging capabilities

(camera phones). Due to their lower cost and higher efﬁciency, service providers used aggressive pricing tactics to

encourage user migration from analog to digital systems, and today analog systems are primarily used in areas with

no digital service. However, digital systems do not always work as well as the analog ones. Users can experience

poor voice quality, frequent call dropping, and spotty coverage in certain areas. System performance has certainly

improved as the technology and networks mature. In some areas cellular phones provide almost the same quality as

landline service. Indeed, some people have replaced their wireline telephone service inside the home with cellular

service.

Spectral sharing in communication systems, also called multiple access, is done by dividing the signaling

dimensions along the time, frequency, and/or code space axes. In frequency-division multiple access (FDMA) the

total system bandwidth is divided into orthogonal frequency channels. In time-division multiple access (TDMA)

time is divided orthogonally and each channel occupies the entire frequency band over its assigned timeslot. TDMA

is more difﬁcult to implement than FDMA since the users must be time-synchronized. However, it is easier to ac-

commodate multiple data rates with TDMA since multiple timeslots can be assigned to a given user. Code-division

multiple access (CDMA) is typically implemented using direct-sequence or frequency-hopping spread spectrum

with either orthogonal or non-orthogonal codes. In direct-sequence each user modulates its data sequence by a

different chip sequence which is much faster than the data sequence. In the frequency domain, the narrowband

data signal is convolved with the wideband chip signal, resulting in a signal with a much wider bandwidth than

the original data signal. In frequency-hopping the carrier frequency used to modulate the narrowband data signal

is varied by a chip sequence which may be faster or slower than the data sequence. This results in a modulated

signal that hops over different carrier frequencies. Typically spread spectrum signals are superimposed onto each

other within the same signal bandwidth. A spread spectrum receiver separates out each of the distinct signals by

separately decoding each spreading sequence. However, for non-orthogonal codes users within a cell interfere

with each other (intracell interference) and codes that are reused in other cells cause intercell interference. Both

the intracell and intercell interference power is reduced by the spreading gain of the code. Moreover, interference

in spread spectrum systems can be further reduced through multiuser detection and interference cancellation. More

details on these different techniques for spectrum sharing and their performance analysis will be given in Chap-

ters 13-14. The design tradeoffs associated with spectrum sharing are very complex, and the decision of which

technique is best for a given system and operating environment is never straightforward.

Efﬁcient cellular system designs are interference-limited, i.e. the interference dominates the noise ﬂoor since

otherwise more users could be added to the system. As a result, any technique to reduce interference in cellular

systems leads directly to an increase in system capacity and performance. Some methods for interference reduction

in use today or proposed for future systems include cell sectorization, directional and smart antennas, multiuser

detection, and dynamic resource allocation. Details of these techniques will be given in Chapter 15.

The ﬁrst generation (1G) cellular systems in the U.S., called the Advance Mobile Phone Service (AMPS),

used FDMA with 30 KHz FM-modulated voice channels. The FCC initially allocated 40 MHz of spectrum to

this system, which was increased to 50 MHz shortly after service introduction to support more users. This total

bandwidth was divided into two 25 MHz bands, one for mobile-to-base station channels and the other for base

station-to-mobile channels. The FCC divided these channels into two sets that were assigned to two different ser-

vice providers in each city to encourage competition. A similar system, the European Total Access Communication

System (ETACS), emerged in Europe. AMPS was deployed worldwide in the 1980’s and remains the only cellular

service in some of these areas, including some rural parts of the U.S.

Many of the ﬁrst generation cellular systems in Europe were incompatible, and the Europeans quickly con-

verged on a uniform standard for second generation (2G) digital systems called GSM

. The GSM standard uses

a combination of TDMA and slow frequency hopping with frequency-shift keying for the voice modulation. In

contrast, the standards activities in the U.S. surrounding the second generation of digital cellular provoked a rag-

ing debate on spectrum sharing techniques, resulting in several incompatible standards [10, 11, 12]. In particular,

there are two standards in the 900 MHz cellular frequency band: IS-54, which uses a combination of TDMA and

FDMA and phase-shift keyed modulation, and IS-95, which uses direct-sequence CDMA with binary modulation

and coding [13, 14]. The spectrum for digital cellular in the 2 GHz PCS frequency band was auctioned off, so

service providers could use an existing standard or develop proprietary systems for their purchased spectrum. The

end result has been three different digital cellular standards for this frequency band: IS-136 (which is basically

the same as IS-54 at a higher frequency), IS-95, and the European GSM standard. The digital cellular standard

in Japan is similar to IS-54 and IS-136 but in a different frequency band, and the GSM system in Europe is at a

different frequency than the GSM systems in the U.S. This proliferation of incompatible standards in the U.S. and

internationally makes it impossible to roam between systems nationwide or globally without a multi-mode phone

and/or multiple phones (and phone numbers).

All of the second generation digital cellular standards have been enhanced to support high rate packet data

services [15]. GSM systems provide data rates of up to 100 Kbps by aggregating all timeslots together for a single

user. This enhancement is called GPRS. A more fundamental enhancement, Enhanced Data Services for GSM

Evolution (EDGE), further increases data rates using a high-level modulation format combined with FEC coding.

This modulation is more sensitive to fading effects, and EDGE uses adaptive techniques to mitigate this problem.

Speciﬁcally, EDGE deﬁnes six different modulation and coding combinations, each optimized to a different value

of received SNR. The received SNR is measured at the receiver and fed back to the transmitter, and the best

modulation and coding combination for this SNR value is used. The IS-54 and IS-136 systems currently provide

data rates of 40-60 Kbps by aggregating time slots and using high-level modulation. This evolution of the IS-136

standard is called IS-136HS (high-speed). The IS-95 systems support higher data using a time-division technique

called high data rate (HDR)[16].

The third generation (3G) cellular systems are based on a wideband CDMA standard developed within the

auspices of the International Telecommunications Union (ITU) [15]. The standard, initially called International

Mobile Telecommunications 2000 (IMT-2000), provides different data rates depending on mobility and location,

from 384 Kbps for pedestrian use to 144 Kbps for vehicular use to 2 Mbps for indoor ofﬁce use. The 3G standard

is incompatible with 2G systems, so service providers must invest in a new infrastructure before they can provide

3G service. The ﬁrst 3G systems were deployed in Japan. One reason that 3G services came out ﬁrst in Japan

is the process of 3G spectrum allocation, which in Japan was awarded without much up-front cost. The 3G

spectrum in both Europe and the U.S. is allocated based on auctioning, thereby requiring a huge initial investment

for any company wishing to provide 3G service. European companies collectively paid over 100 billion dollars

The acronym GSM originally stood for Groupe Sp

eciale Mobile, the name of the European charter establishing the GSM standard. As

GSM systems proliferated around the world, the underlying acronym meaning was changed to Global Systems for Mobile Communications.

剩余570页未读，继续阅读

weixin_42146564

粉丝: 0
资源: 1

无线通信技术的演进与挑战

无线通信英文版 歌德史密斯Wireless Communication By Goldsmith

Wireless Communications-Andrea Goldsmith 书籍电子档

Wireless Communications-Andrea Goldsmith

Wireless+communications-andrea-goldsmith

Wireless-Communications-Andrea-Goldsmith--Stanfor_PDF_

Wireless Communications-by Andrea Goldsmith习题解答

Wireless Communications by Andrea Goldsmith

Wireless communications(Andrea Goldsmith)

Wireless Communications (Andrea Goldsmith) 无线通信课本

wireless communications - GoldSmith.pdf

最新资源

无线通信英文版歌德史密斯Wireless Communication By Goldsmith