Multiple Access Techniques

Multiple Access Techniques
Looking at the transmission of multiple data streams sharing a common medium, their
separation is managed by multiplexing techniques in single-user scenarios or multiple
access techniques in multiuser communications. In order to ensure reliable communication,
many systems try to avoid interference by choosing orthogonal access schemes so
that no multiple access interference (MAI) disturbs the transmission. However, in most
cases, orthogonality cannot be maintained due to the influence of the mobile radio channel.
The next subsections introduce the most important multiplexing and multiple access
strategies.
Time Division Multiplexing (TDM) and Multiple Access (TDMA)
This relatively common multiple access technique divides the time axis into different time
slots, each of length Tslot according to Figure 1.2. Each data packet or burst is assigned
to a certain slot, whereby a user can also occupy several slots. A defined number Nslot
of slots build a frame that is periodically repeated. Hence, each user has periodical access
to the shared medium. Due to the influence of the transmission channel (cf. Section 1.2)
and the restrictions of practical filter design, guard intervals of length T have to be
inserted between successive slots in order to avoid interference between them. Within these
intervals, no information is transmitted so that they represent redundancy and reduce the
spectral efficiency of the communication system.
Frequency Division Multiplexing (FDM) and Multiple Access (FDMA)
Alternatively, the frequency axis can be divided into Nf subbands each of width B as illustrated
in Figure 1.3. The data streams are now distributed on different frequency bands, rather than on different time slots. However, in mobile environments, the signals’ bandwidths
are spread by the Doppler effect, so that neighboring subbands interfere. Thus, gaps
of an appropriate width f combating this effect at the expense of a reduced spectral
efficiency are required for Frequency division multiple access (FDMA).
Code Division Multiplexing (CDM) and Multiple Access (CDMA)
In contrast to both the preceding schemes, CDMA allows simultaneous access on the channel
in the same frequency range. The basic principle is to spectrally spread the data streams
with specific sequences called spreading codes (Spread Spectrum technique). The signals
can be distinguished by assigning them individual spreading codes. This opens a third
dimension, as can be seen in Figure 1.4. One intuitive choice would lead to orthogonal
codes, ensuring a parallel transmission of different user signals. However, the transmission
channel generally destroys the orthogonality and multiuser interference (MUI) becomes a
limiting factor concerning spectral efficiency (cf. Chapters 4 and 5).
Space Division Multiplexing (SDM) and Multiple Access (SDMA)
The fourth access scheme exploits the resource space (Figure 1.4 and Figure 1.5). Spatially
separated data streams can simultaneously access the channel in the same frequency band, provided that the locations of transmit and receive antennas are appropriately chosen. In
mobile environments, this requirement is sometimes difficult to fulfill, because users are
changing their position during the connection. Therefore, quasi-static scenarios or combinations
with the aforementioned access techniques are often considered. Mutual interference
is also likely to occur in Space division multiple access (SDMA) systems because the transmitter
and the receiver have no perfect channel knowledge of what would be necessary to
totally avoid interference.
As expected, all the mentioned access schemes can be combined. The well-known
Global System for Mobile (GSM) Communications and Digital Cellular System (DCS)-
1800 standards both combine Time division multiple access (TDMA) and FDMA. In UMTS
(Universal Mobile Telecommunications System) or IMT-2000 (International Mobile Communications)
systems, CDMA is used in connection with TDMA and FDMA (Dahlman
et al. 1998; Ojanper¨a and Prasad 1998b; Toskala et al. 1998). While TDMA, FDMA, and
CDMA have already been used for a fairly long time, SDMA is rather recent in comparison.
This development is a result of the demand to use licenses that are assigned to certain
frequency bands as efficiently as possible. Hence, all the resources have to be exploited for
reaching this goal.