Wireless Networks

1. 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 • Hamming • BCH • Reed Solmon • ARQ • Sliding window • Go-back-N

4. Multiple Access Techniques • Frequency Division Multiple Access (FDMA) • Time Division Multiple Access (TDMA) • Random Access • Code Division Multiple Access (CDMA)

5. Code User 4 User 3 User 2 User 1 Time Frequency 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). f1

6. Advantages • 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. Code User 4 User 3 User 2 User 1 Time Frequency 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.

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 • 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. Code User 4 User 4 User 3 Time User 2 Frequency User 1 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.

11. Code-Division Multiple Access (CDMA) • Basic Principles of CDMA • D = rate of data signal • Break each bit into kchips • 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 • 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

15. Random Access • Random Access Methods • 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 • ALOHA • Slotted ALOHA • Reservation-based ALOHA • Next Lecture • Carrier-sense based random access • Spread Spectrum