Wireless networks - Lecture 6: Multiple access techniques

Wireless networks - Lecture 6: Multiple access techniques. The main topics covered in this chapter include: frequency division multiple access (FDMA); time division multiple access (TDMA); random access; random access methods in wireless networks; code division multiple access (CDMA);...

Wireless Networks

Lecture Multiple Access Techniques

Dr Ghalib A Shah

Last Lecture Review

Multiple Access Techniques

 Frequency Division Multiple Access

 FDMA was the initial multiple-access

technique for cellular systems

 Separates large band into smaller

channels

 Each channel has the ability to

support user

channel preventing co-channel

 Advantages

► Simple to implement in terms of hardware.

► Fairly efficient with a small base population and with

constant traffic.

► Network and spectrum planning are intensive and

time consuming.

► Channels are dedicated for a single user, idle

channels add spectrum inefficiency.

 Entire bandwidth is available to the

user for finite period of time

 Users are allotted time slots for a

channel allowing sharing of a single

channel

 Each of the user takes turn in

transmitting and receiving data in a

How it works?

 User presses Push-to-Talk (PTT) button

 A control channel registers the radio to the closest base

station.

 The BS assigns an available pair of channels.

 Unlike FDMA, TDMA system also assigns an available

time slot within the channel.

 Data transmission is not continuous rather sent and

received in bursts.

 The bursts are reassembled and appear like

continuous transmission.

 Advantages

► Extended battery life and talk time

► More efficient use of spectrum, compared to FDMA

► Will accommodate more users in the same spectrum space than an

FDMA system

► Network and spectrum planning are intensive

► Multipath interference affects call quality

► Dropped calls are possible when users switch in and out of different

cells.

► Too few users result in idle channels (rural versus urban environment)

► Higher costs due to greater equipment sophistication

 SS has been used in

military applications due to

anti-jamming and security.

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 Advantages  

► Greatest spectrum efficiency:

► CDMA improves call quality by filtering out background noise,

cross-talk, and interference

► Simplified frequency planning - all users on a CDMA system use the

same radio frequency spectrum

► Random Walsh codes enhance user privacy; a spread-spectrum

advantage

► Precise power control increases talk time and battery size for mobile

phones

► Backwards compatibility techniques are costly

► Currently, base station equipment is expensive

► Low traffic areas lead to inefficient use of spectrum and equipment

Random Acces s  

► more efficient way of managing medium acces s  

for communicating s hort burs ty mes s ages

• in contrast to fixed-access schemes, each user gains access to medium only when needed -has some data to send

• drawback: users must compete to access the medium (‘random access’)

• collision of contending transmissions

 Random Acces s  Methods  in Wireles s  

Networks

ALOHA­bas ed Random Acces s

 us er acces s es  medium as  s oon as  it has  a packet 

ready to trans mit

► after transmission, user waits a length of time > round-trip

delay in the network, for an ACK from the receiver

► if no ACK arrives, user waits a random interval of time (to avoid

repeated collision) and retransmits

Pure-ALOHA

Slotted ALOHA

 time is  divided into equal time s lots  –when a us er 

has  a packet to trans mit, the packet is  buffered and  trans mitted at the s tart of the next time s lot

► BS transmits a beacon signal for timing, all users must

synchronize their clocks

 advantages:

► partial packet collision avoided

 Disadvantages

► throughput still quite low!

► there is either no collision or a complete collision

Slotted ALOHA

► Two types  of channels  in GSM:

• Traffic channels  (TCH): used for transmission of user data –based on FDMA/TDMA

• Signalling channels, used for control and management of a cellular network

• Random Acces s  Channel (RACH): signalling channel for establishing access to the network (i.e BS)

• employs Slotted ALOHA

• only channel in GSM where contention can occur

Res ervation ALOHA

 Time s lots  are divided into res ervation and 

trans mis s ion s lots  / periods

► during reservation period, stations can reserve future slots in

transmission period

► reservation slot size << transmission slot size

► collisions occur only in reservation slots

 advantages:

► higher throughput under heavy loads

► max throughput up to 80% of channel capacity

 disadvantages:

► more demanding on users as they have to obtain / keep

‘reservation list’ up-to-date

R-ALOHA