2. Multiple Radio Access
MULTIPLE RADIO ACCESS
•Wireless devices have become both popular and cost-effective and
have attracted considerable interest from the industry and the
academia.
•Users of wireless networks, either walking on the street, driving a
car, or operating a portable computer on an aircraft, enjoy the
exchange of information without worrying about how technology
makes such exchange possible.
•To make this communication feasible, a user needs access to a
control channel, which can be exclusively assigned for this purpose
or can be shared among numerous subscribers.
•Such an expensive commodity is shared among users, as needed,
using predefined rules or algorithms if there is no central authority
to handle such allocation.
•Even if there is a controller like a BS, MSs need to use a control
channel to inform the BS before using the traffic channel or the
information channel.
3. MULTIPLE RADIO ACCESS
Multiple Radio Access
•A typical scenario in a wireless network is shown in fig MSs
have to compete for a shared channel. Each MS has a
transmitter/receiver that communicates with other MSs using a
single channel.
•In a general scheme, transmission from any MS can be received
by all other MSs in the neighborhood.
•Therefore, if more than one MS attempts to transmit at one
time, using the shared channel, collision occurs, wherein signals
in the channel frequency range (air in the case of wireless
devices) are garbled, and MSs receiving the information cannot
interpret or differentiate what is being transmitted.
•These situations are called collisions in the channel or multiple
access issues.
4. MULTIPLE RADIO ACCESS
Multiple Radio Access
•Collisions must be avoided, and we need to follow some protocols
to determine which MS has exclusive access to the shared medium
at a given time and for a given duration so that the MS can transmit
and other MSs can receive, understand, and interpret the received
control information in a wireless system.
•For handling multiple access issues, there are two different types
of protocols: the contention-based protocol and the conflict-free (or
collision-free) protocol. Contention-based protocols resolve a
collision after it occurs.
•These protocols execute a collision resolution protocol after
detection of each collision. Collision-free protocols (e.g., a bit-map
protocol and binary countdown) ensure that a collision never
occurs.
5. MULTIPLE RADIO ACCESS
Multiple Radio Access
Shared multiple
access channel
MS 4
MS 3
MS 2
MS 1 MS n
Figure Multiple access of shared channel in a wireless network.
•Channel-sharing techniques can be classified into two methods:
static channelization and dynamic medium access control.
•In static allocation, the channel assignment is done in a
prespecified way and does not change with time. In the dynamic
technique, the channel is allocated as needed and changes with
time.
6. MULTIPLE RADIO ACCESS
FDMA
•The orthogonality condition of the two signals in FDMA is
given by
•that there is no overlapping frequency in frequency domain F
for the signals si( f, t) and sj( f, t) and the two signals do not
interfere with each other.
•FDMA is a multiple-access system that has been widely
adopted in existing analog systems for portable and automobile
wireless telephones.
•The BS dynamically assigns a different carrier frequency to
each active user (MS).
•A frequency synthesizer is used to adjust and maintain the
transmission and reception frequencies. The concept of FDMA
is shown in Figure below
7. MULTIPLE RADIO ACCESS
FDMA
User 1
User 2
User n
Time
Frequency
Fig. The concept of FDMA.
•There is a pair of channels for the communication between the
BS and the MS. The paired channels are called forward channel
(downlink) and reverse channel (uplink).
•Different frequency bandwidths are assigned to different users.
This implies that there is no frequency overlapping between the
forward and reverse channels.
8. •The radio antenna is at a much higher elevation and the MSs
are shown at the same level in fig.although these are not
necessarily at the same relative height.
•Also, if the physical separation between the BS and MSs is
drawn to scale, the MSs will become too small to be
represented by a point, and all other details will be
MULTIPLE RADIO ACCESS
FDMA
