Multiple Access Techniques

1. Multiple Access Techniques

2. Duplexing • Duplexing facilitates communications in both directions simultaneously: base station to mobile and mobile to base station • Duplexing is done either using frequency or time domain techniques: • Frequency division duplexing (FDD) • Time division duplexing (TDD) • FDD is suitable for radio communication systems, whereas TDD is more suitable for fixed wireless systems

3. Multiple Access Techniques • Frequency Division Multiple Access • Time Division Multiple Access • Spread Spectrum Multiple Access • Space Division Multiple Access • Packet Radio

4. Frequency Division Multiple Access (FDMA) • The frequency spectrum is divided into unique frequency bands or channels • These channels are assigned to users on demand • Multiple users cannot share a channel • Users are assigned a channel as a pair of frequencies (forward and reverse channels) • FDMA requires tight RF filtering to reduce adjacent channel interference

5. Channel-1 Channel-5 Channel-6 Channel-7 Channel-8 Channel-9 FDMA TIME Channel-2 Channel-3 Channel-4 Channel-5 Channel-6 Channel-7 Channel-8 Channel-9 FREQUENCY

6. Time Division Multiple Access (TDMA) • TDMA systems divides the radio spectrum into time slots, and in each time slot only one use is allowed to either transmit or receive • Transmission for any user is non-continuous • In each TDMA frame, the preamble contains the address and synchronization information • TDMA shares a single carrier frequency with several users • TDMA could allocate varied number of time slots per frame to different users

7. TDMA FREQUENCY Channel-10 Channel-3 Channel-4 Channel-5 Channel-6 Channel-8 Channel-9 Channel-1 Channel-7 Channel-2 TIME

8. Spread Spectrum Multiple Access (SSMA) • SSMA uses signals that have a transmission bandwidth several orders of magnitude greater than the minimum required RF bandwidth • SSMA provides immunity to multipath interference and robust multiple access capability • SSMA is bandwidth efficient in multi-user environment • SSMA techniques: • Frequency hopped (FH) multiple access • Direct sequence (DS) multiple access • Also known as code division multiple access (CDMA)

9. FHMA • Carrier frequencies of individual users are varied in a pseudorandom fashion within a wideband channel • Data of each user is broken into uniform size bursts that are transmitted on different channels at different time instants based on their pseudo-noise (PN) code sequence • In the FH receiver, a locally generated PN code is used to synchronize the receiver’s instantaneous frequency with that of the transmitter • FHMA provides inherent security, and guard against erasures through error control coding and interleaving • Use: Bluetooth and HomeRF

10. CDMA • The narrowband message signal is multiplied by a very large bandwidth signal called spreading signal • The spreading signal has a pseudo-noise code sequence that has a chip rate which is orders of magnitudes greater than the data rates of the message • All users use the same carrier frequency and can transmit simultaneously • Each use has its own pseudorandom codeword that is orthogonal to the others • The receivers need to know the codeword of the corresponding sender • Power control is used to combat the near-far problem • Near-far problem: • When many mobile users share the same channel, the strongest received mobile signal will capture the demodulator at a base station. Thus a nearby subscriber could overpower the base-station receiver by drowning out the signals of far away subscribers.

11. CDMA Features • Many users of CDMA share the same frequency – either TDD or FDD may be used • CDMA has soft capacity limit – system performance is inversely proportional to the number of users • Multipath fading is reduced because of the signal spread • Channel data rates are very high • Prone to self-jamming and near-far problem • Self-jamming: when the spreading sequences of different users are not exactly orthogonal • Near-far problem occurs at a CDMA receiver if an undesired user has a high detected power compared to the desired user

12. Space Division Multiple Access (SDMA) • SDMA controls the radiated energy for each user and serves different users by using spot beam antennas • Different areas covered by antenna beam may be served by same or different frequencies • Reverse links present difficulty: • Transmitted power from each subscriber must be controlled to prevent any single user from driving up the interference level • Transmit power is limited by battery consumption at the subscriber unit

13. Packet Radio • In Packet Radio (PR) access techniques, many subscribers attempt to access a single channel in an uncoordinated manner • Collisions from simultaneous transmissions from multiple transmitters are detected at the base station receiver, in which case an ACK or NACK signal is broadcasted by the base station to alert the user • PR subscribers use a contention technique to transmit on a common channel

14. Packet Radio Protocols • If a packet transmission duration is t, the time duration during which it is susceptible to a collision (vulnerable period) is t1+ 2t, where t1 is the transmission time instant • Throughput = ltPr[no collisions] • Contention protocols: • Random access • Scheduled access • Hybrid access • ALOHA protocols: pure and slotted

15. Carrier Sense Multiple Access Protocols (CSMA) • In CSMA protocols, each terminal on the network is able to monitor the status of the channel before transmitting information • Variations: • 1-persistent CSMA • non-persistent CSMA • p-persistent CSMA • CSMA/CD