Introduction
The common feature of the next generation wireless technologies will be the convergence
of multimedia services such as speech, audio, video, image, and data. This implies that
a future wireless terminal, by guaranteeing high-speed data, will be able to connect to
different networks in order to support various services: switched traffic, IP data packets
and broadband streaming services such as video. The development of wireless terminals
with generic protocols and multiple-physical layers or software-defined radio interfaces is
expected to allow users to seamlessly switch access between existing and future standards.
The rapid increase in the number of wireless mobile terminal subscribers, which cur-
rently exceeds 1 billion users, highlights the importance of wireless communications in
this new millennium. This revolution in the information society has been happening, espe-
cially in Europe, through a continuous evolution of emerging standards and products by
keeping a seamless strategy for the choice of solutions and parameters. The adaptation of
wireless technologies to the user’s rapidly changing demands has been one of the main
drivers of this revolution. Therefore, the worldwide wireless access system is and will
continue to be characterized by a heterogeneous multitude of standards and systems. This
plethora of wireless communication systems is not limited to cellular mobile telecom-
munication systems such as GSM, IS-95, D-AMPS, PDC, UMTS or cdma2000, but also
includes wireless local area networks (WLANs), e.g., HIPERLAN/2, IEEE 802.11a/b and
Bluetooth, and wireless local loops (WLL), e.g., HIPERMAN, HIPERACCESS, and IEEE
802.16 as well as broadcast systems such as digital audio broadcasting (DAB) and digital
video broadcasting (DVB).
These trends have accelerated since the beginning of the 1990s with the replacement of
the first generation analog mobile networks by the current 2nd generation (2G) systems
(GSM, IS-95, D-AMPS and PDC), which opened the door for a fully digitized network.
This evolution is still continuing today with the introduction of the deployment of the
3rd generation (3G) systems (UMTS, IMT-2000 and cdma2000). In the meantime, the
research community is focusing its activity towards the next generation beyond 3G, i.e.
fourth generation (4G) systems, with more ambitious technological challenges.
The primary goal of next-generation wireless systems (4G) will not only be the intro-
duction of new technologies to cover the need for higher data rates and new services, but
also the integration of existing technologies in a common platform. Hence, the selection
of a generic air-interface for future generation wireless systems will be of great impor-
tance. Although the exact requirements for 4G have not yet been commonly defined, its
new air interface shall fulfill at least the following requirements:
Multi-Carrier and Spread Spectrum Systems K. Fazel and S. Kaiser
2003 John Wiley & Sons, Ltd ISBN: 0-470-84899-5
2 Introduction
— generic architecture, enabling the integration of existing technologies,
— high spectral efficiency, offering higher data rates in a given scarce spectrum,
— high scalability, designing different cell c onfigurations (hot spot, ad hoc), hence bet-
ter coverage,
— high adaptability and reconfigurability, supporting different standards and technolo-
gies,
— low cost, enabling a rapid market introduction, and
— future proof, opening the door for new technologies.
From Second- to Third-Generation Multiple Access Schemes
2G wireless systems are mainly characterized by the transition of analog towards a fully
digitized technology and comprise the GSM, IS-95, PDC a nd D-AMPS standards.
Work on the pan-European digital cellular standard Global System for Mobile commu-
nications (GSM) started in 1982 [14][37], where now it accounts for about two-thirds of
the world mobile market. In 1989, the technical specifications of GSM were approved by
the European Telecommunication Standard Institute (ETSI), where its commercial suc-
cess began in 1993. Although GSM is optimized for circuit-switched services such as
voice, it offers low-rate data services up to 14.4 kbit/s. High speed data services up to
115.2 kbit/s are possible with the enhancement of the GSM standard towards the General
Packet Radio Service (GPRS) by using a higher number of time slots. GPRS uses the
same modulation, frequency band and frame structure as GSM. However, the Enhanced
Data rate for Global Evolution (EDGE) [3] system which further improves the data rate
up to 384 kbit/s introduces a new modulation scheme. The final evolution from GSM is
the transition from EDGE to 3G.
Parallel to GSM, the American IS-95 standard [43] (recently renamed cdmaOne)was
approved by the Telecommunication Industry Association (TIA) in 1993, where its first
commercial application started in 1995. Like GSM, the first version of this standard (IS-
95A) offers data services up to 14.4 kbit/s. In its second version, IS-95B, up to 64 kbit/s
data services are possible.
Meanwhile, two other 2G mobile radio systems have been introduced: Digital Advanced
Mobile Phone Services (D-AMPS/IS-136), called TDMA in the USA and the Personal
Digital Cellular (PDC) in Japan [28]. Currently PDC hosts the most convincing example
of high-speed internet services to mobile, called i-mode. The high amount of congestion
in the PDC system will urge the Japanese towards 3G and even 4G systems.
Trends towards more capacity for mobile receivers, new multimedia services, new
frequencies and new technologies have motivated the idea of 3G systems. A unique
international standard was targeted: Universal/International Mobile Telecommunication
System (UMTS/IMT-2000) with realization of a new generation of mobile communica-
tions technology for a world in which personal communication services will dominate.
The objectives of the third generation standards, namely UMTS [17] and cdma2000 [44]
went far beyond the second-generation systems, especially with respect to:
— the wide range of multimedia services (speech, audio, image, video, data) and bit rates
(up to 2 Mbit/s for indoor and hot spot applications),
From Second- to Third-Generation Multiple Access Schemes 3
— the high quality of service requirements (better speech/image quality, lower bit error
rate (BER), higher number of active users),
— operation in mixed cell scenarios (macro, micro, pico),
— operation in different environments (indoor/outdoor, business/domestic, cellular/cord-
less),
— and finally flexibility in frequency (variable bandwidth), data rate (variable) and radio
resource management (variable power/channel allocation).
The commonly used multiple access schemes for second and third generation wire-
less mobile communication systems are based on either Time Division Multiple Access
(TDMA), Code Division Multiple Access (CDMA) or the combined access schemes in
conjunction with an additional Frequency Division Multiple Access (FDMA) component:
— The GSM standard, employed in the 900 MHz and 1800 MHz bands, first divides the
allocated bandwidth into 200 kHz FDMA sub-channels. Then, in each sub-channel,
up to 8 users share the 8 time slots in a TDMA manner [37].
— In the IS-95 standard up to 64 users share the 1.25 MHz channel by CDMA [ 43]. The
system is used in the 850 MHz and 1900 MHz bands.
— The aim of D-AMPS (TDMA IS-136) is to coexist with the analog AMPS, where the
30 kHz channel of AMPS is divided into three channels, allowing three users to share
a single radio channel by allocating unique time slots to each user [27].
— The recent ITU adopted standards for 3G (UMTS and cdma2000) are both based on
CDMA [17][44]. For UMTS, the CDMA-FDD mode, which is known as wideband
CDMA, employs separate 5 MHz channels for both the uplink and downlink directions.
Within the 5 MHz bandwidth, each user is separated by a specific code, resulting in
an end-user data rate of up to 2 Mbit/s per carrier.
Table 1 summarizes the key characteristics of 2G and 3G mobile communication sys-
tems.
Beside tremendous developments in mobile communication systems, in public and
private environments, operators are offering wireless services using WLANs in selected
Table 1 Main parameters of 2G and 3G mobile radio systems
Parameter 2G systems 3G systems
GSM IS-95 IMT-2000/UMTS
(WARC’92 [39])
Carrier frequencies 900 MHz
1800 MHz
850 MHz
1900 MHz
1900–1980 MHz
2010–2025 MHz
2110–2170 MHz
Peak data rate 64 kbit/s 64 kbit/s 2 Mbit/s
Multiple access TDMA CDMA CDMA
Services Voice, low rate data Voice, low rate data Voice, data, video
4 Introduction
Table 2 Main parameters of WLAN communication systems
Parameter Bluetooth IEEE 802.11b IEEE 802.11a HIPERLAN/2
Carrier frequency 2.4 GHz (ISM) 2.4 GHz (ISM) 5 GHz 5 GHz
Peak data rate 1 Mbit/s 5.5 Mbit/s 54 Mbit/s 54 Mbit/s
Multiple access FH-CDMA DS-CDMA
with carrier
sensing
TDMA TDMA
Services Ethernet Ethernet Ethernet Ethernet, ATM
spots such as hotels, train stations, airports and conference rooms. As Table 2 shows,
there is a similar objective to go higher in data rates with WLANs, where multiple access
schemes TDMA or CDMA are employed [15][30].
FDMA, TDMA and CDMA are obtained if the transmission bandwidth, the transmission
time or the spreading code are related to the different users, respectively [2].
FDMA is a multiple access technology widely used in satellite, c able and terrestrial
radio networks. FDMA subdivides the total bandwidth into N
c
narrowband sub-channels
which are available during the whole transmission time (see Figure 1). This requires band-
pass filters with sufficient stop band attenuation. Furthermore, a sufficient guard band is
left between two adjacent spectra in order to cope with frequency deviations of local
oscillators and to minimize interference from adjacent channels. The main advantages
of FDMA are in its low required transmit power and in c hannel equalization that is
either not needed or much simpler than with other multiple access techniques. However,
its drawback in a cellular system might be the implementation of N
c
modulators and
demodulators at the base station (BS).
TDMA is a popular multiple access technique, which is used in several international
standards. In a TDMA system all users employ the same band and are separated by
allocating short and distinct time slots, one or several assigned to a user (see Figure 2).
In TDMA, neglecting the overhead due to framing and burst formatting, the multiplexed
signal bandwidth will be approximately N
c
times higher than in an FDMA system, hence,
Frequency
Time
Power density
Figure 1 Principle of FDMA (with N
c
= 5 sub-channels)
From Second- to Third-Generation Multiple Access Schemes 5
Frequency
Time
Power density
Figure 2 Principle of TDMA (with 5 time slots)
Frequency
Time
Power density
Figure 3 Principle of CDMA (with 5 spreading codes)
Table 3 Advantages and drawbacks of different multiple access schemes
Multiple access
scheme
Advantages Drawbacks
FDMA
– Low transmit power
– Robust to multipath
– Easy frequency planning
– Low delay
– Low peak data rate
– Loss due to guard bands
– Sensitive to narrow band
interference
TDMA
– High peak data rate
– High multiplexing gain in
case of bursty traffic
– High transmit power
– Sensitive to multipath
– Difficult frequency planning
CDMA
– Low transmit power
– Robust to multipath
– Easy frequency planning
– High scalability
– Low delay
– Low peak data rate
– Limited capacity per sector
due to multiple access
interference
leading to quite complex equalization, especially for high-data rate applications. The chan-
nel separation of TDMA and FDMA is based on the orthogonality of signals. Therefore, in
a cellular system, the co-channel interference is only present from the reuse of frequency.
On the contrary, in CDMA systems all users transmit at the same time on the same
carrier using a wider bandwidth than in a TDMA system (see Figure 3). The signals of
6 Introduction
users are distinguished by assigning different spreading codes with low cross-correlation
properties. Advantages of the spread spectrum technique are immunity against multi-
path distortion, simple frequency planning, high flexibility, variable rate transmission and
resistance to interference.
In Table 3, the main advantages and drawbacks of FDMA, TDMA and CDMA
are summarized.
From Third- to Fourth-Generation Multiple Access Schemes
Besides offering new services and applications, the success of the next generation of wire-
less systems (4G) will strongly depend on the choice of the concept and technology inno-
vations in architecture, spectrum allocation, spectrum utilization and exploitation [38][39].
Therefore, new high-performance physical layer and multiple access technologies are
needed to provide high speed data rates with flexible bandwidth allocation. A low-cost
generic radio interface, being operational in mixed-cell and in different environments
with scalable bandwidth and data r ates, is expected to have better acceptance.
The technique of spread spectrum may allow the above requirements to be at least par-
tially fulfilled. As explained earlier, a multiple access scheme based on direct sequence
code division multiple access (DS-CDMA) relies on spreading the data stream using an
assigned spreading code for each user in the time domain [40][45][47][48]. The capability
of minimizing multiple access interference (MAI) is given by the cross-correlation prop-
erties of the spreading codes. In the case of severe multipath propagation in mobile com-
munications, the capability of distinguishing one component from others in the composite
received signal is offered by the autocorrelation properties of the spreading codes [45].
The so-called rake receiver should contain multiple correlators, each matched to a dif-
ferent resolvable path in the received composite signal [40]. Therefore, the performance
of a D S-CDMA system will strongly depend on the number of active users, the channel
characteristics, and the number of arms employed in the rake. Hence, the system capacity
is limited by self-interference and MAI, which results from the imperfect auto- and cross-
correlation properties of spreading codes. Therefore, it will be difficult for a DS-CDMA
receiver to make full use of the received signal energy scattered in the time domain and
hence to handle full load conditions [40].
The technique of multi-carrier transmission has recently been receiving wide interest,
especially for high data-rate broadcast applications. The history of orthogonal multi-
carrier transmission dates back to the mid-1960s, when Chang published his paper on
the synthesis of band-limited signals for multichannel transmission [5][6]. He introduced
the basic principle of transmitting data simultaneously through a band-limited channel
without interference between sub-channels (without inter-channel interference,ICI)and
without interference between consecutive transmitted symbols (without inter-symbol inter-
ference, ISI) in time domain. Later, Saltzberg performed further analyses [41]. However,
a major contribution to multi-carrier transmission was presented in 1971 by Weinstein
and Ebert [49] who used Fourier transform for base-band processing instead of a bank
of sub-carrier oscillators. To c ombat ICI and ISI, they introduced the well-known guard
time between the transmitted symbols with raised cosine windowing.
The main advantages of multi-carrier transmission are its robustness in frequency
selective fading channels and, in particular, the reduced signal processing complexity
by equalization in the frequency domain.
From Third- to Fourth-Generation Multiple Access Schemes 7
The basic principle of multi-carrier modulation relies on the transmission of data by
dividing a high-rate data stream into several low-rate sub-streams. These sub-streams are
modulated on different sub-carriers [1][4][9]. By using a large number of sub-carriers, a
high immunity against multipath dispersion can be provided since the useful symbol dura-
tion T
s
on each sub-stream will be much larger than the c hannel time dispersion. Hence,
the effects of ISI will be minimized. Since the amount of filters and oscillators necessary
is considerable for a large number of sub-carriers, an efficient digital implementation of a
special form of multi-carrier modulation, called orthogonal frequency division multiplex-
ing (OFDM), with rectangular pulse-shaping and guard time was proposed in [1]. OFDM
can be easily realized by using the discrete Fourier transform (DFT). OFDM, having
densely spaced sub-carriers with overlapping spectra of the modulated signals, abandons
the use of steep band-pass filters to detect each sub-carrier as it is used in FDMA schemes.
Therefore, it offers a high spectral efficiency.
Today, progress in digital technology has enabled the realization of a DFT also for large
numbers of sub-carriers (up to several thousand), through which OFDM has gained much
importance. The breakthrough of OFDM came in the 1990s as it was the modulation cho-
sen for ADSL in the USA [8], and it was selected for the European DAB standard [11].
This success continued with the choice of OFDM for the European DVB-T standard [13]
in 1995 and later for the WLAN standards HIPERLAN/2 and IEEE802.11a [15][30]
and recently in the interactive terrestrial return channel (DVB-RCT) [12]. It is also
a potential candidate for the future fixed wireless access standards HIPERMAN and
IEEE802.16a [16][31]. Table 4 summarizes the main characteristics of several standards
employing OFDM.
The advantages of multi-carrier modulation on one hand and the flexibility offered
by the spread spectrum technique on the other hand have motivated many researchers
to investigate the combination of both techniques, known as Multi-Carrier Spread Spec-
trum (MC-SS). This combination, published in 1993 by several authors independently [7]
[10][18][25][35][46][50], has introduced new multiple access schemes called MC-CDMA
and MC-DS-CDMA. It allows one to benefit from several advantages of both multi-carrier
modulation and spread spectrum systems by offering, for instance, high flexibility, high
Table 4 Examples of wireless transmission systems employing OFDM
Parameter DAB DVB-T IEEE 802.11a HIPERLAN/2
Carrier
frequency
VHF VHF and UHF 5 GHz 5 GHz
Bandwidth 1.54 MHz 8MHz
(7 MHz)
20 MHz 20 MHz
Max. data rate 1.7 Mbit/s 31.7 Mbit/s 54 Mbit/s 54 Mbit/s
Number of
sub-carriers
(FFT size)
192 up to 1536
(256 up to
2048)
1705 and 6817
(2048 and
8196)
52
(64)
52
(64)
8 Introduction
spectral efficiency, simple and robust detection techniques and narrow band interference
rejection capability.
Multi-carrier modulation and multi-carrier spread spectrum are today considered poten-
tial candidates to fulfill the requirements of next generation (4G) high-speed wireless
multimedia communications systems, where spectral efficiency and flexibility will be
considered the most important criteria for the choice of the air interface.
Multi-Carrier Spread Spectrum
Since 1993, various combinations of multi-carrier modulation with the spread spectrum
technique as multiple access schemes have been introduced. It has been shown that
multi-carrier spread spectrum (MC-SS) offers high spectral efficiency, robustness and
flexibility [29].
Two different philosophies exist, namely MC-CDMA (or OFDM-CDMA) and MC-DS-
CDMA (see Figure 4 and Table 5).
MC-CDMA is based on a serial c oncatenation of direct sequence (DS) spreading with
multi-carrier modulation [7][18][25][50]. The high-rate DS spread data stream of pro-
cessing gain P
G
is multi-carrier modulated in the way that the chips of a spread data
symbol are transmitted in parallel and the assigned data symbol is simultaneously trans-
mitted on each sub-carrier (see Figure 4). As for DS-CDMA, a user may occupy the total
bandwidth for the transmission of a single data symbol. Separation of the user’s signal
is performed in the code domain. Each data symbol is copied on the sub-streams before
multiplying it with a chip of the spreading code assigned to the specific user. This r eflects
that an MC-CDMA system performs the spreading in frequency direction and, thus, has
an additional degree of freedom compared to a DS-CDMA system. Mapping of the chips
spreading code
spread data symbols
data symbols
0
1
•
•
L-1
0
1
•
L-1
sub-carrier f
0
sub-carrier f
1
sub-carrier f
N
c
− 1
MC-CDMA
(Frequency diversity)
spreading code
T
d
serial-
to-
parallel
converter
spread data symbols
01•
•
L − 1
01•
L −
1
sub-carrier f
0
sub-carrier f
1
sub-carrier f
N
c
− 1
MC-DS-CDMA
(Time diversity)
Figure 4 General principle of MC-CDMA and MC-DS-CDMA systems
Multi-Carrier Spread Spectrum 9
Table 5 Main characteristics of different MC-SS concepts
Parameter MC-CDMA MC-DS-CDMA
Spreading Frequency direction Time direction
Sub-carrier spacing F
S
=
P
G
N
c
T
d
F
S
P
G
N
c
T
d
Detection algorithm MRC, EGC, ZF, MMSE
equalization, IC, MLD
Correlation detector
(coherent rake)
Specific characteristics Very efficient for the
synchronous downlink by
using orthogonal codes
Designed especially for an
asynchronous uplink
Applications Synchronous uplink and
downlink
Asynchronous uplink and
downlink
in the frequency direction allows for simple methods of signal detection. This concept
was proposed with OFDM for optimum use of the available bandwidth. The realization
of this concept implies a guard time between adjacent OFDM symbols to prevent ISI or
to assume that the symbol duration is significantly larger than the time dispersion of the
channel. The number of sub-carriers N
c
has to be chosen sufficiently large to guarantee
frequency nonselective fading on each sub-channel. The application of orthogonal codes,
such as Walsh–Hadamard c odes for a synchronous system, e.g., the downlink of a cellu-
lar system, guarantees the absence of MAI in an ideal channel and a minimum MAI in
a real channel. For signal detection, single-user detection techniques such as maximum
ratio combining ( MRC), equal gain combining (EGC), zero forcing (ZF) or minimum
mean square error (MMSE) equalization, as well as multiuser detection techniques like
interference cancellation (IC) or maximum likelihood detection (MLD), can be applied.
As depicted in Figure 4, MC-DS-CDMA modulates sub-streams on sub-carriers with a
carrier spacing proportional to the inverse of the chip rate. This will guarantee orthogo-
nality between the spectra of the sub-streams [42]. If the spreading code length is smaller
or equal to the number of sub-carriers N
c
, a single data symbol is not spread in the fre-
quency direction, instead it is spread in the time direction. Spread spectrum is obtained by
modulating N
c
time spread data symbols on parallel sub-carriers. By using high numbers
of sub-carriers, this concept benefits from tim e diversity. However, due to the frequency
nonselective fading per sub-channel, frequency diversity can only be exploited if channel
coding with interleaving or sub-carrier hopping is employed or if the same information
is transmitted on several sub-carriers in parallel. Furthermore, higher frequency diversity
could be achieved if the sub-carrier spacing is chosen larger than the chip rate. This
concept was investigated for an asynchronous uplink scenario. For data detection, N
c
coherent receivers can be used.
It can be noted that both schemes have a generic architecture. In the case where the
number of sub-carriers N
c
= 1, the classical DS-CDMA transmission scheme is obtained,
whereas without spreading (P
G
= 1) it results in a pure OFDM system.
10 Introduction
By using a variable spreading factor in frequency and/or time and a variable sub-carrier
allocation, the system can easily be adapted to different environments such as multicell
and single cell topologies, each with different coverage areas.
Today, the field of multi-carrier spread spectrum communications is considered to be
an independent and important research topic; see [19] to [23], [26], [36]. Several deep
system analysis and c omparisons of MC-CDMA and MC-DS-CDMA with DS-CDMA
have been performed that show the superiority of MC-SS [24][29][32][33][34]. In addi-
tion, new application fields have been proposed such as high-rate cellular mobile (4G),
high-rate wireless indoor and fixed wireless access (FWA). In a ddition to system-level
analysis, a multitude of research activities have been addressed to develop appropriate
strategies for detection, interference cancellation, channel coding, modulation, synchro-
nization (especially uplink) and low-cost implementation design.
The Aim of this Book
The interest in multi-carrier transmission, especially in multi-carrier spread spectrum, is
still growing. Many researchers and system designers are involved in system aspects and
the implementation of these new techniques. However, a comprehensive collection of
their work is still missing.
The aim of this book is first to describe and analyze the basic concepts of the combina-
tion of multi-carrier transmission with spread spectrum, where the different architectures
and the different detection strategies are detailed. Concrete examples of its applications for
future cellular mobile communications systems are given. Then, we examine other deriva-
tives of MC-SS (e.g., OFDMA, SS-MC-MA and interleaved FDMA) and other variants
of the combination of OFDM with TDMA, which are today part of WLAN, WLL and
DVB-RCT standards. Basic OFDM implementation issues, valid for most of these com-
binations, such as channel coding, modulation, digital I/Q-generation, synchronization,
channel estimation, and effects of phase noise and nonlinearity are further analyzed.
Chapter 1 covers the fundamentals of today’s wireless communications. First a detailed
analysis of the radio channel (outdoor and indoor) and its modeling are presented. Then
the principle of OFDM multi-carrier transmission is introduced. In addition, a general
overview of the spread spectrum technique, especially of DS-CDMA, is given. Examples
of applications of OFDM and DS-CDMA for broadcast, WLAN, and cellular systems
(IS-95, UMTS) are briefly presented.
Chapter 2 describes the combinations of multi-carrier transmission with the spread spec-
trum technique, namely MC-CDMA and MC-DS-CDMA. It includes a detailed description
of the different detection strategies (single-user and multiuser) and presents their perfor-
mance in terms of bit error rate (BER), spectral efficiency and complexity. Here a cellular
system with a point- to multi-point topology is considered. Both downlink and uplink
architectures are examined.
Hybrid multiple access schemes based on MC-SS, OFDM or spread spectrum are
analyzed in Chapter 3. This chapter covers OFDMA, being a derivative of MC-CDMA,
OFDM-TDMA, SS-MC-MA, interleaved FDMA and ultra wide band (UWB) schemes.
All these multiple access schemes have recently received wide interest. Their concrete
application fields are detailed in Chapter 5.
The issues of digital implementation of multi-carrier transmission systems, essential
especially for system- and hardware designers, are addressed in Chapter 4. Here, the
References 11
different functions such as digital I/Q generation, analog/digital conversion, digital multi-
carrier modulation/demodulation, synchronization (time, frequency), channel estimation,
coding/decoding and other related RF issues such as nonlinearities, phase noise and narrow
band interference rejection are analyzed.
In Chapter 5, concrete application fields of MC-SS, OFDMA and OFDM-TDMA
for cellular mobile (4G), wireless indoor (WLAN), fixed wireless access (FWA/WLL)
and interactive multimedia communication (DVB-T return channel) are outlined, where
for each of these systems, the multi-carrier architecture and their main parameters are
described. The c apacity advantages of using adaptive channel coding and modulation,
adaptive spreading and scalable bandwidth allocation are discussed.
Finally, Chapter 6 covers further techniques that can be used to enhance system capac-
ity or offer more flexibility for the implementation and deployment of the transmission
systems examined in Chapter 5. Here, diversity techniques such as space time/frequency
coding and Tx/Rx antenna diversity in MIMO concepts and software-defined radio (SDR)
are introduced.
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12 Introduction
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