无线通信基础与前沿技术：Goldsmith教授解读

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"《无线通信》(Wireless Communication) 是由Andrea Goldsmith教授编著的一本关于无线通信领域的经典著作，涵盖了众多前沿技术。这本书是斯坦福大学EE359无线通信课程的教材，旨在深入讲解支撑各种无线应用的基本通信技术，特别是通信系统设计。" 在无线通信这个快速发展的行业中，本书对这一领域的广泛应用进行了全面探讨。随着全球手机用户数量预计在未来几年内超过10亿，以及无线局域网的指数级增长，无线技术不仅支持全球任意地点的语音、数据和视频通信，还为传感器网络、智能家居、远程医疗、远程学习、自动化工厂和车辆等提供了核心技术。书中首先对当前的各种无线系统和标准进行了简要概述，包括2G和3G蜂窝系统、无线局域网、广域无线数据系统、卫星网络以及蓝牙技术。这些系统和标准构成了现代无线通信的基础，并为读者构建了理解无线通信技术的背景框架。接下来，作者详细分析了无线信道的特性，如路径损耗、多径传播、衰落效应以及干扰等，这些都是无线通信中至关重要的因素。通过理解这些信道特性，读者能够更好地掌握如何设计能在实际环境中有效工作的无线通信系统。此外，书中的内容还涉及调制与编码技术，包括频分复用、时分复用、码分多址(CDMA)以及正交频分复用(OFDM)等，这些技术对于提高无线通信系统的频谱效率和鲁棒性至关重要。同时，无线网络的信道接入协议，如CSMA/CA（载波监听多路访问/冲突避免）和多址接入协议也在书中有所阐述。书中还深入讨论了无线通信系统的错误控制编码，如卷积码、Turbo码和LDPC码，这些编码技术能显著提高数据传输的可靠性。另外，无线通信中的信号检测和估计理论，包括最大似然检测、匹配滤波器以及最优接收机设计也是核心内容之一。最后，无线通信系统的优化与性能分析也是本书的重点，这包括功率控制、多天线技术（MIMO）、空间多工和波束成形等，这些技术极大地提高了无线通信的容量和覆盖范围。通过阅读《无线通信》，读者将获得对无线通信系统设计的深刻理解，能够应对不断变化的无线通信环境，为未来的技术创新打下坚实基础。这本书是无线通信领域研究人员、工程师和学生不可多得的参考资料。

1.3 Technical Issues

The technical problems that must be solved to make the wireless vision a reality extend across all levels of

the system design. At the hardware level the terminal must have multiple modes of operation to support

the diﬀerent applications and media. Desktop computers currently have the capability to process voice,

image, text, and video data, but breakthroughs in circuit design are required to implement multimode

operation in a small, lightweight, handheld device. Since most people don’t want to carry around a

twenty pound battery, the signal processing and communications hardware of the portable terminal must

consume very little power, which will impact higher levels of the system design. Many of the signal

processing techniques required for eﬃcient spectral utilization and networking demand much processing

power, precluding the use of low power devices. Hardware advances for low power circuits with high

processing ability will relieve some of these limitations. However, placing the processing burden on ﬁxed

sites with large power resources has and will continue to dominate wireless system designs. The associated

bottlenecks and single points-of-failure are clearly undesirable for the overall system. Moreover, in some

applications (e.g. sensors) network nodes will not be able to recharge their batteries. In this case the ﬁnite

battery energy must be allocated eﬃciently across all layers of the network protocol stack [5]. The ﬁnite

bandwidth and random variations of the communication channel will also require robust compression

schemes which degrade gracefully as the channel degrades.

The wireless communication channel is an unpredictable and diﬃcult communications medium. First

of all, the radio spectrum is a scarce resource that must be allocated to many diﬀerent applications and

systems. For this reason spectrum is controlled by regulatory bodies both regionally and globally. In

the U.S. spectrum is allocated by the FCC, in Europe the equivalent body is the European Telecommu-

nications Standards Institute (ETSI), and globally spectrum is controlled by the International Telecom-

munications Union (ITU). A regional or global system operating in a given frequency band must obey

the restrictions for that band set forth by the corresponding regulatory body as well as any standards

adopted for that spectrum. Spectrum can also be very expensive since in most countries, including the

U.S., spectral licenses are now auctioned to the highest bidder. In the 2 GHz spectral auctions of the

early 90s, companies spent over nine billion dollars for licenses, and the recent auctions in Europe for 3G

spectrum garnered over 100 billion dollars. The spectrum obtained through these auctions must be used

extremely eﬃciently to get a reasonable return on its investment, and it must also be reused over and

over in the same geographical area, thus requiring cellular system designs with high capacity and good

performance. At frequencies around several Gigahertz wireless radio components with reasonable size,

power consumption, and cost are available. However, the spectrum in this frequency range is extremely

crowded. Thus, technological breakthroughs to enable higher frequency systems with the same cost and

performance would greatly reduce the spectrum shortage, although path loss at these higher frequencies

increases, thereby limiting range.

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

transmitter or receiver is 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 you 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 [23, 24, 25, 26]. The network must be able to

locate a given user wherever it is amongst millions of globally-distributed mobile terminals. It must then

route a call to that user as it moves at speeds of up to 100 mph. 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 rate and reliability. The design of protocols to interface between wireless and wired networks with

vastly diﬀerent performance capabilities remains a challenging topic of research.

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 the layers of the OSI model:

each layer is designed independent from the other layers with baseline mechanisms to interface between

layers. This methodology greatly simpliﬁes network design, although it leads to some ineﬃciency and

performance loss due to the lack of a global design optimization. However, the large capacity and good

reliability of wired network links make it easier to buﬀer high-level network protocols from the lower level

protocols for link transmission and access, and the performance loss resulting from this isolated protocol

design is fairly low. However, 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. The dynamic nature and poor performance of the underlying wireless communication

channel indicates that high-performance wireless networks must be optimized for this channel and must

adapt to its variations as well as to user mobility. Thus, these networks will require an integrated and

adaptive protocol stack across all layers of the OSI model, from the link layer to the application layer.

In summary, technological advances in the following areas are needed to implement the wireless

vision outlined above:

• Measurements and models for wireless indoor and outdoor channels.

• Hardware for low-power handheld computer and communication terminals.

• Techniques to mitigate wireless channel impairments and to improve the quality and spectral eﬃ-

ciency of communication over wireless channels.

• Better means of sharing the limited spectrum to accommodate the diﬀerent wireless applications.

• Protocols for routing and mobility management which support users on the move.

• An architecture to connect the various wireless subnetworks together and to the backbone wireline

network.

• An integrated and adaptive protocol stack for wireless networks that extends across all layers of

the OSI model.

Given these requirements, the ﬁeld of wireless communications draws from many areas of expertise,

including physics, communications, signal processing, network theory and design, software design, and

hardware design. Moreover, given the fundamental limitations of the wireless channels and the explo-

sive demand for its utilization, communication between these interdisciplinary groups is necessary to

implement the most rudimentary shell of the wireless vision depicted above.

We now 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 challenges remaining before it will

be realized. Many of these challenges will be examined in detail in later chapters.

1.4 Current Wireless Systems

1.4.1 Cellular Telephone Systems

Cellular telephone systems, also referred to as Personal Communication Systems (PCS), are extremely

popular and lucrative worldwide: these systems have sparked much of the optimism about the future

of wireless networks. Cellular telephone systems are designed to provide two-way voice communication

at vehicle speeds with regional or national 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 path loss 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 1.4, where f

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 to maximize the spectral eﬃciency

obtained by frequency reuse. 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 path loss, shadowing,

and multipath. In order to determine the best reuse distance and base station placement, an accurate

characterization of signal propagation within the cells is needed. This characterization is usually obtained

using detailed analytical models, sophisticated computer-aided modeling, or empirical measurements.

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. Signals propagated out from base stations uniformly in all directions, so a mobile moving in a

circle around the base station would have approximately constant received power. 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 telephone systems are now evolving to smaller cells with base stations close to street level or

inside buildings transmitting at much lower power. These smaller cells are called microcells or picocells,

depending on their size. This evolution is driven by two factors: 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. Small cells also have better propagation conditions since the lower base stations have reduced

shadowing and multipath. 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 city where a mobile may be located. It is also harder to develop

f f

Base

Station

Figure 1.4: Cellular Systems.

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 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 [7].

All base stations in a city are connected via a high-speed communications link to a mobile telephone

switching oﬃce (MTSO), as shown in Figure 1.5. 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 in conjunction with the public switched telephone

network (PSTN). 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. False

handoﬀs may also be initiated if a mobile is in a deep fade, causing its received signal power to drop

below the minimum threshold even though it may be nowhere near a cell boundary.

Cellular telephone systems have recently moved from analog to digital technology. Digital technology

has many advantages over analog. 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 are not limited to frequency division for multiple access, and they can

take advantage of advanced compression techniques and voice activity factors. In addition, encryption

MOBILE

TELEPHONE

SWITCHING

OFFICE

LOCAL

EXCHANGE

LONG−DISTANCE

NETWORK

PHONE

CELLULAR

BASE

STATION

WIDE−AREA

Figure 1.5: Current Cellular Network Architecture

techniques can be used to secure digital signals against eavesdropping. All cellular systems being deployed

today are digital, and these systems provide voice mail, paging, and email services in addition to voice.

Due to their lower cost and higher eﬃciency, service providers have used aggressive pricing tactics to

encourage user migration from analog to digital systems. Since they are relatively new, digital systems

do not always work as well as the old analog ones. Users experience poor voice quality, frequent call

dropping, short battery life, and spotty coverage in certain areas. System performance will certainly

improve as the technology and networks mature. However, it is unlikely that cellular phones will provide

the same quality as wireline service any time soon. The great popularity of cellular systems indicates

that users are willing to tolerate inferior voice communications in exchange for mobility.

Spectral sharing in digital cellular can be done using frequency-division, time-division, code-division

(spread spectrum), or hybrid combinations of these techniques (see Chapter 14). In time-division the

signal occupies the entire frequency band, and is divided into time slots t

which are reused in distant

cells [8]. Time division is depicted by Figure 1.4 if the f

s are replaced by t

s. Time-division is more

diﬃcult to implement than frequency-division since the users must be time-synchronized. However, it is

easier to accommodate multiple data rates with time-division since multiple timeslots can be assigned

to a given user. Spectral sharing can also be done using code division, which is commonly implemented

using either direct-sequence or frequency-hopping spread spectrum [9]. In direct-sequence each user

modulates its data sequence by a diﬀerent pseudorandom 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 - hence the name

spread spectrum. In frequency hopping the carrier frequency used to modulate the narrowband data

signal is varied by a pseudorandom chip sequence which may be faster or slower than the data sequence.

Since the carrier frequency is hopped over a large signal bandwidth, frequency-hopping also spreads the

data signal to a much wider bandwidth. Typically spread spectrum signals are superimposed onto each

other within the same signal bandwidth. A spread spectrum receiver can separate each of the distinct

signals by separately decoding each spreading sequence. However, since the codes are semi-orthogonal,

the users within a cell interfere with each other (intracell interference), and codes that are reused in other

cells also cause interference (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.

In the U.S. the standards activities surrounding the second generation of digital cellular systems

provoked a raging debate on multiple access for these systems, 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 IS-95, which uses semi-orthogonal CDMA [13, 14]. The

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