Multiple-Access Techniques
In Section 1.2.1 we
discussed ways in which a symbol stream associated with a single user can be
transmitted. Many wireless channels, particularly in emerging systems, operate
as multiple-access systems, in which multiple users share the same radio
resources.
There are several ways in which radio resources can be shared
among multiple users. These can be viewed as ways of allocating regions in
frequency, space, and time to different users, as shown in Fig. 1.1. For example, a classic multiple-access technique
is frequency-division multiple access (FDMA), in
which the frequency band available for a given service is divided into subbands
that are allocated to individual users who wish to use the service. Users are
given exclusive use of their subband during their communication session, but
they are not allowed to transmit signals within other subbands. FDMA is the
principal multiplexing method used in radio and television broadcast and in
first-generation (analog voice) cellular telephony systems, such as the Advanced
Mobile Phone System (AMPS) and Nordic Mobile Telephone (NMT), developed
primarily in the 1970s and 1980s (cf. [458]). FDMA is also used in some form
in all other current cellular systems, in tandem with other multiple-access
techniques that are used to further allocate the subbands to multiple users.
Similarly, users can share the channel on the basis of time-division multiple access (TDMA), in which time is
divided into equal-length intervals, which are further divided into equal-length
subintervals, or time slots. Each user is allowed to transmit throughout the
entire allocated frequency band during a given slot in each interval but is not
allowed to transmit during other time slots when other users are transmitting.
So, whereas FDMA allows each user to use part of the spectrum all of the time,
TDMA allows each user to use all of the spectrum part of the time. This method
of channel sharing is widely used in wireless applications, notably in a number
of second-generation cellular (i.e., digital voice) sytems, including the widely
used Global System for Mobile (GSM) system [178, 407, 408] and in the IEEE 802.16 wireless
MAN standards. A form of TDMA is also used in Bluetooth networks, in which one
of the Bluetooth devices in the network acts as a network controller to poll the
other devices in time sequence.
FDMA and TDMA systems are intended to assign orthogonal
channels to all active users by giving each, for their exclusive use, a slice of
the available frequency band or transmission time. These channels are said to be
orthogonal because interference between users
does not, in principle, arise in such assignments (although, in practice, there
is often such interference, as discussed further below). Code-division multiple access (CDMA) assigns channels
in a way that allows all users to use all of the available time and frequency
resources simultaneously, through the assignment of a pattern or code to each
user that specifies the way in which these resources will be used by that user.
Typically, CDMA is implemented via spread-spectrum modulation, in which the
pattern is the pseudorandom code that determines the spreading sequence in the
case of direct sequence, or the hopping pattern in the case of frequency
hopping. In such systems, a channel is defined by a particular pseudorandom
code, so each user is assigned a channel by being assigned a pseudorandom code.
CDMA is used, notably, in the second-generation cellular standard IS-95 (Interim
Standard 95), which makes use of direct-sequence CDMA to allocate subchannels of
larger-bandwidth (1.25 MHz) subchannels of the entire cellular band. It is also
used, in the form of frequency hopping, in GSM to provide isolation among users
in adjacent cells. The spectrum spreading used in wireless LAN systems is also a
form of CDMA in that it allows a number of such systems to operate in the same
lightly regulated part of the radio spectrum. CDMA is also the basis for the
principal standards being developed and deployed for 3G cellular telephony
(e.g., [130, 361, 362, 407]).
Any of the multiple-access techniques discussed here can be
modeled analytically by considering multiple transmitted signals of the form (1.1). In particular, for a system of K users, we can write a transmitted signal for each
user as
Equation 1.8
where xk(·), {bk[0], bk[1], ..., bk[M-1]}, and
wi,k(·) represent the transmitted
signal, symbol stream, and ith modulation
waveform, respectively, of user k. That is, each
user in a multiple-access system can be modeled in the same way as in a
single-user system, but with (usually) differing modulation waveforms (and
symbol streams, of course). If the waveforms {wi,k(·)} are of the form (1.2) but with different carrier frequencies {wk}, say, this is
FDMA. If they are of the form (1.2) but
with time-slotted amplitude pulses {pk(·)}, say, this is TDMA. Finally, if they
are spread-spectrum signals of this form but with different pseudorandom
spreading codes or hopping patterns, this is CDMA. Details of these
multiple-access models will be discussed in the sequel as needed.
Sections
