1. Lecture Multiple Access Techniques Dr. Ghalib A. Shah
Wireless Networks
Lecture
Multiple Access Techniques
Dr. Ghalib A. Shah

2. Outlines Review of previous lecture #5 FDMA TDMA CDMA Random Access
ALOHA
Slotted ALOHA
Reservation-based ALOHA
Summary of today’s lecture

3. Last Lecture Review Block Codes ARQ Hamming BCH Reed Solmon
Sliding window
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4. Multiple Access Techniques
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Random Access
Code Division Multiple Access (CDMA)
The success of wireless communication has increased the demand of radio frequency spectrum, which is a limited natural resource.
In an effort to make the most efficient use of this resource, various technologies have been developed so that multiple or simultaneous users can be supported in a finite amount of spectrum. This concept is called Multiple Access.
The three most commonly used methods are:
FDMA
TDMA
CDMA
Consider a large room with a group of people divided into pairs. Each pair hold their own conversation with no interest whats being said by the other pair. For these conversations to take place without interruption from others, it is necessary to define an isolated environment for each conversation. In this example, a room can be considered a slice of frequency spectrum allocated to be used by this group.
Imagine each pair talking through cellular phones or radios.
Applying FDMA to this scenario, the single large room would be partitioned with many dividing walls and creating a large number of small rooms.
A single pair of people would enter into each small room and hold their conversation. Each room is like a single channel and no one could use this room until the current conversation is over.

5. FDMA
FDMA was the initial multiple-access technique for cellular systems
Separates large band into smaller channels.
Each channel has the ability to support user.
Guard bands are used to separate channel preventing co-channel interference
Narrow bandwidth (30 khz).
User 1
User 2
User 3
User 4
Time
Frequency
Code f1

6. Advantages Disadvantages Simple to implement in terms of hardware.
Fairly efficient with a small base population and with constant traffic.
Disadvantages
Network and spectrum planning are intensive and time consuming.
Channels are dedicated for a single user, idle channels add spectrum inefficiency.

7. TDMA
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.
Requires time synchronization.
Each of the user takes turn in transmitting and receiving data in a round robin fashion.
User 1
User 2
User 3
User 4
Time
Frequency
Code
As frequency spectrum experiences more traffic, spectrum efficiency becomes more important.
In digital systems, continuous transmission is not required because users do not use the allotted bandwidth all the time. In such systems,
TDMA is a complimentary access technique to FDMA. Global Systems for Mobile communications (GSM) uses the TDMA technique.

8. 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.

9. Advantages Disadvantages 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
Disadvantages
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

10. CDMA
CDMA is a spread spectrum technique used to increase spectrum efficiency.
SS has been used in military applications due to anti-jamming and security.
User 4
Time
Frequency
Code
User 1
User 2
User 3

11. Code-Division Multiple Access (CDMA)
Basic Principles of CDMA
D = rate of data signal
Break each bit into k chips
Chips are a user-specific fixed pattern
Chip data rate of new channel = kD

12. CDMA Example If k=6 and code is a sequence of 1s and -1s
For a ‘1’ bit, A sends code as chip pattern
<c1, c2, c3, c4, c5, c6>
For a ‘0’ bit, A sends complement of code
<-c1, -c2, -c3, -c4, -c5, -c6>
Receiver knows sender’s code and performs electronic decode function
<d1, d2, d3, d4, d5, d6> = received chip pattern
<c1, c2, c3, c4, c5, c6> = sender’s code

13. CDMA Example User A code = <1, –1, –1, 1, –1, 1>
To send a 1 bit = <1, –1, –1, 1, –1, 1>
To send a 0 bit = <–1, 1, 1, –1, 1, –1>
User B code = <1, 1, –1, – 1, 1, 1>
To send a 1 bit = <1, 1, –1, –1, 1, 1>
Receiver receiving with A’s code
(A’s code) x (received chip pattern)
User A ‘1’ bit: 6 -> 1
User A ‘0’ bit: -6 -> 0
User B ‘1’ bit: 0 -> unwanted signal ignored

14. Advantages Disadvantages 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
Disadvantages
Backwards compatibility techniques are costly
Currently, base station equipment is expensive
Low traffic areas lead to inefficient use of spectrum and equipment resources
capacity increases of 8 to 10 times that of an analog system and 4 to 5 times that of other digital systems which makes it most useful in high traffic areas with a large number of users and limited spectrum

15. Random Access Methods in Wireless Networks
more efficient way of managing medium access for communicating short bursty messages
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 Access Methods in Wireless Networks
can be divided into two groups:
ALOHA based-no coordination between users
carrier-sense based-indirect coordination -users sense availability of medium before transmitting

16. Random Access Collision Period User 4 User 3 rescheduled User 2 User 1
Time

17. ALOHA-based Random Access
user accesses medium as soon as it has a packet ready to transmit
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
advantages:
simple, no synchronization among users required
disadvantages:
low throughput under heavy load conditions
probability of collision increases as number of users increases
max throughput = 18% of channel capacity

18. Pure-ALOHA

19. Slotted ALOHA
time is divided into equal time slots –when a user has a packet to transmit, the packet is buffered and transmitted at the start of the next time slot
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
max throughput = 36% of channel capacity

20. Slotted ALOHA

21. Example slotted ALOHA in GSM 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 Access Channel (RACH): signalling channel for establishing access to the network (i.e. BS)
employs Slotted ALOHA
only channel in GSM where contention can occur

22. Reservation ALOHA
Time slots are divided into reservation and transmission slots / 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 is most commonly used in satellite systems
satellite collects requests, complies ‘reservation list’ and finally sends the list back to users

23. R-ALOHA

24. Summary FDMA TDMA CDMA Random Access Next Lecture ALOHA Slotted ALOHA
Reservation-based ALOHA
Next Lecture
Carrier-sense based random access
Spread Spectrum