03-08-2012, 01:00 PM
CELLULAR MOBILE RADIO COMMUNICATION
[attachment=30020]
The subject of this book is code division multiple access (CDMA) communications.
A major application of CDMA is wireless communication including
mobile radio. In this chapter we introduce the basic concepts of mobile radio
systems, including cellular concepts, consider the general structure of a cellular
system, and study different principles of multiple-access (time, frequency, and
code division) and spread spectrum concepts.
This chapter begins with an overview of the principles of cellular radio
systems. Next, given the focus on simultaneous wideband transmission of all
users over a common frequency spectrum, we consider direct-sequence CDMA
systems, frequency-hopped CDMA systems, and pulse position-hopped CDMA
systems. The chapter concludes with a description of this book. The book is
devoted to the analysis of different aspects of CDMA communication. Given
the rapid and continuing growth of cellular radio systems throughout the world,
CDMA digital cellular radio systems will be the widest-deployed form of spread
spectrum systems for voice and data communication. It is a major technology of
the twenty-first century.
CELLULAR MOBILE RADIO SYSTEMS
A cellular radio system provides a wireless connection to the public telephone network
for any user location within the radio range of the system. The term mobile
has traditionally been used to classify a radio terminal that can be moved during communication. Cellular systems accommodate a large number of mobile units
over a large area within a limited frequency spectrum. There are several types
of radio transmission systems. We consider only full duplex systems. These are
communication systems that allow simultaneous two-way communication. Transmission
and reception for a full duplex system are typically on two different channels,
so the user may constantly transmit while receiving signals from another user.
FREQUENCY DIVISION AND TIME DIVISION
MULTIPLE ACCESS
Multiple access schemes are used to allow many mobile users to share simultaneously
a common bandwidth. As mentioned above, a full duplex communication
system typically provides two distinct bands of frequencies (channels) for every
user. The forward band provides traffic from the base station to the mobile, and
the reverse band provides traffic from the mobile to the base station. Therefore,
any duplex channel actually consists of two simplex channels.
Frequency division multiple access (FDMA) and time division multiple access
(TDMA) are the two major access techniques used to share the available bandwidth
in a conventional mobile radio communication systems.
Frequency division multiple access assigns individual channels (frequency
bands) to individual users. It can be seen from Figure 1.3 that each user is
allocated a unique frequency band. These bands are assigned on demand to users
who request service. During the period of the call, no other user can share the
same frequency band. The bandwidths of FDMA channels are relatively narrow
(25–30 kHz) as each channel supports only one call per carrier. That is, FDMA
is usually implemented in narrowband systems. If an FDMA channel is not in
use (for example, during pauses in telephone conversation) it sits idle and cannot
be used by other users to increase the system capacity.
DIRECT SEQUENCE CDMA
A completely different approach, realized in CDMA systems, does not attempt
to allocate disjoint frequency or time resources to each user. Instead the system
allocates all resources to all active users.
In direct sequence (DS) CDMA systems, the narrowband message signal is
multiplied by a very large-bandwidth signal called the spreading signal. All
users in a DS CDMA system use the same carrier frequency and may transmit
simultaneously. Each user has its own spreading signal, which is approximately
orthogonal to the spreading signals of all other users. The receiver performs a
correlation operation to detect the message addressed to a given user. The signals
from other users appear as noise due to decorrelation. For detecting the message
signal, the receiver requires the spreading signal used by the transmitter. Each
user operates independently with no knowledge of the other users (uncoordinated
transmission).
[attachment=30020]
The subject of this book is code division multiple access (CDMA) communications.
A major application of CDMA is wireless communication including
mobile radio. In this chapter we introduce the basic concepts of mobile radio
systems, including cellular concepts, consider the general structure of a cellular
system, and study different principles of multiple-access (time, frequency, and
code division) and spread spectrum concepts.
This chapter begins with an overview of the principles of cellular radio
systems. Next, given the focus on simultaneous wideband transmission of all
users over a common frequency spectrum, we consider direct-sequence CDMA
systems, frequency-hopped CDMA systems, and pulse position-hopped CDMA
systems. The chapter concludes with a description of this book. The book is
devoted to the analysis of different aspects of CDMA communication. Given
the rapid and continuing growth of cellular radio systems throughout the world,
CDMA digital cellular radio systems will be the widest-deployed form of spread
spectrum systems for voice and data communication. It is a major technology of
the twenty-first century.
CELLULAR MOBILE RADIO SYSTEMS
A cellular radio system provides a wireless connection to the public telephone network
for any user location within the radio range of the system. The term mobile
has traditionally been used to classify a radio terminal that can be moved during communication. Cellular systems accommodate a large number of mobile units
over a large area within a limited frequency spectrum. There are several types
of radio transmission systems. We consider only full duplex systems. These are
communication systems that allow simultaneous two-way communication. Transmission
and reception for a full duplex system are typically on two different channels,
so the user may constantly transmit while receiving signals from another user.
FREQUENCY DIVISION AND TIME DIVISION
MULTIPLE ACCESS
Multiple access schemes are used to allow many mobile users to share simultaneously
a common bandwidth. As mentioned above, a full duplex communication
system typically provides two distinct bands of frequencies (channels) for every
user. The forward band provides traffic from the base station to the mobile, and
the reverse band provides traffic from the mobile to the base station. Therefore,
any duplex channel actually consists of two simplex channels.
Frequency division multiple access (FDMA) and time division multiple access
(TDMA) are the two major access techniques used to share the available bandwidth
in a conventional mobile radio communication systems.
Frequency division multiple access assigns individual channels (frequency
bands) to individual users. It can be seen from Figure 1.3 that each user is
allocated a unique frequency band. These bands are assigned on demand to users
who request service. During the period of the call, no other user can share the
same frequency band. The bandwidths of FDMA channels are relatively narrow
(25–30 kHz) as each channel supports only one call per carrier. That is, FDMA
is usually implemented in narrowband systems. If an FDMA channel is not in
use (for example, during pauses in telephone conversation) it sits idle and cannot
be used by other users to increase the system capacity.
DIRECT SEQUENCE CDMA
A completely different approach, realized in CDMA systems, does not attempt
to allocate disjoint frequency or time resources to each user. Instead the system
allocates all resources to all active users.
In direct sequence (DS) CDMA systems, the narrowband message signal is
multiplied by a very large-bandwidth signal called the spreading signal. All
users in a DS CDMA system use the same carrier frequency and may transmit
simultaneously. Each user has its own spreading signal, which is approximately
orthogonal to the spreading signals of all other users. The receiver performs a
correlation operation to detect the message addressed to a given user. The signals
from other users appear as noise due to decorrelation. For detecting the message
signal, the receiver requires the spreading signal used by the transmitter. Each
user operates independently with no knowledge of the other users (uncoordinated
transmission).