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Multiple Access Techniques for Wireless Communication

Multiple Access Techniques for Wireless Communication. FDMA : Frequency division Multiple Access TDMA : Time division Multiple Access CDMA : Code division Multiple Access SDMA : Space division Multiple Access PDMA : Polarization division Multiple Access. Introduction.

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Multiple Access Techniques for Wireless Communication

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  1. Multiple Access Techniques for Wireless Communication FDMA : Frequency division Multiple Access TDMA : Time division Multiple Access CDMA : Code division Multiple Access SDMA : Space division Multiple Access PDMA : Polarization division Multiple Access

  2. Introduction • Multiple access schemes are used to allow many users to share simultaneously a finite amount of radio spectrum • Sharing of spectrum is required to increase capacity • For high quality communication this sharing of spectrum should not degrade performance of the system • high performance • duplexing generally required • frequency domain • time domain

  3. A B A B A B Duplexing • What is Duplexing? • to talk and listen simultaneously is called duplexing • Classification of communication systems according to their connectivity • Simplex • Half-duplex • Duplex

  4. Duplexing Contd… • Duplexing may be done using • frequency domain technique • time domain technique

  5. Frequency division duplexing (FDD) • two bands of frequencies for every user • forward band ( for traffic from Base station to mobile unit) • reverse band (for traffic from mobile unit to Base station) • duplexer needed • frequency separation between forward band and reverse band is constant throughout the system reverse channel forward channel frequency separation/split f

  6. Time division duplexing (TDD) • uses different time slots for forward and reverse link • forward time slot • reverse time slot • no duplexer is required (a simple switch can be used) • Communication is not full-duplex reverse channel forward channel t time separation/split

  7. Trade-offs b/w FDD and TDD • FDD • Provides individual radio frequencies to each user hence, transceiver should work on two frequency bands • Frequency allocation must be carefully coordinated with Out-of-band users • Duplexer needed • TDD • Single frequency hence simple transceiver • Duplexer not needed, a switch can do the job • There is time latency, communication is not full-duplex

  8. Multiple Access Techniques in Wireless Communication System • Frequency division multiple access (FDMA) • Time division multiple access (TDMA) • Code division multiple access (CDMA) • Space division multiple access (SDMA) • grouped as: • narrowband systems • wideband systems

  9. Narrowband systems • Bandwidth of the signal is narrow compared with the coherence bandwidth of the channel • In NB systems available radio spectrum is divided into large number of narrowband channels usually FDD (large frequency split) • Narrowband FDMA • Narrowband TDMA

  10. Narrowband systems • Narrowband FDMA • a user is assigned a particular channel which is not shared by other users • if FDD is used then each channel has a forward and reverse link (called FDMA/FDD) • Narrowband TDMA • Allows users to share the same channel but allocates a unique time slot to each user • FDMA/FDD • FDMA/TDD • TDMA/FDD • TDMA/TDD

  11. Narrowband systems • FDMA/FDD • FDMA/TDD • TDMA/FDD • TDMA/TDD

  12. Logical separation FDMA/FDD forward channel user 1 reverse channel ... f forward channel user n reverse channel t

  13. Logical separation FDMA/TDD user 1 forward channel reverse channel ... f user n forward channel reverse channel t

  14. Logical separation TDMA/FDD forward channel forward channel ... user 1 user n f reverse channel reverse channel t

  15. Logical separation TDMA/TDD user 1 user n ... forward channel reverse channel forward channel reverse channel f t

  16. Wideband systems • The transmission BW of a single channel is much larger than the coherence bandwidth of the channel • users are allowed to transmit in a large part of the spectrum • large number of transmitters on one channel • TDMA techniques allocates time slots to different transmitters • CMA techniques allows the transmitters to access the channel at the same time

  17. Wideband systems • FDD or TDD multiplexing techniques • TDMA/FDD • TDMA/TDD • CDMA/FDD • CDMA/TDD

  18. FDMA power TDMA frequency time power CDMA frequency time power frequency time Multiple Access Techniques

  19. Multiple Access Techniques in use Multiple Access Technique Advanced Mobile Phone System (AMPS) FDMA/FDD Global System for Mobile (GSM) TDMA/FDD US Digital Cellular (USDC) TDMA/FDD Digital European Cordless Telephone (DECT) FDMA/TDD US Narrowband Spread Spectrum (IS-95) CDMA/FDD Cellular System

  20. Frequency division multiple access FDMA • one phone circuit per channel • idle time causes wasting of resources • simultaneously and continuously transmitting • usually implemented in narrowband systems • Complexity of FDMA mobile systems is lower compared to TDMA • FDMA uses duplexers • for example: AMPS is a FDMA system with bandwidth of 30 kHz

  21. FDMA compared to TDMA • fewer bits for synchronization • fewer bits for framing • higher costs for duplexer used in base station and subscriber units • FDMA requires RF filtering to minimize adjacent channel interference

  22. Nonlinear Effects in FDMA • many channels - same antenna • for maximum power efficiency operate near saturation • near saturation power amplifiers are nonlinear • nonlinearities causes signal spreading • intermodulation frequencies

  23. Nonlinear Effects in FDMA • IM are undesired harmonics • interference with other channels in the FDMA system • interference outside the mobile radio band: adjacent-channel interference • RF filters needed - higher costs

  24. Number of channels in a FDMA system • N … number of channels • Bt … total spectrum allocation • Bguard … guard band • Bc … channel bandwidth Bt - 2Bguard N= Bc

  25. Example: Advanced Mobile Phone System • AMPS • FDMA/FDD • analog cellular system • 12.5 MHz per simplex band - Bt • Bguard = 10 kHz ; Bc = 30 kHz 12.5E6 - 2*(10E3) N= = 416 channels 30E3

  26. Time Division Multiple Access • Time slots • one user per slot • Buffer and burst method • Non-continuous transmission Advantage: Total bandwidth is utilized Disadvantage: Strict Burst Timing is required at the earth station

  27. Repeating Frame Structure One TDMA Frame Preamble Information Message Trail Bits Slot 1 Slot 2 Slot 3 … Slot N Trail Bits Sync. Bits Information Data Guard Bits The frame is cyclically repeated over time.

  28. Features of TDMA • a single carrier frequency for several users • transmission in bursts • handoff process much simpler (can listen when idle) • Low battery consumption • Bandwidth can be supplied on demand • FDD : switch instead of duplexer • high synchronization overhead

  29. Number of channels in a TDMA system • N … number of TDMA channel slots • m … number of TDMA users per radio channel • Btot … total spectrum allocation • Bguard … Guard Band • Bc … channel bandwidth m*(Btot - 2*Bguard) N= Bc

  30. Example: Global System for Mobile (GSM) • TDMA/FDD • forward link at Btot = 25 MHz • radio channels of Bc = 200 kHz • if m = 8 speech channels supported, and • if no guard band is assumed : 8*25E6 N= = 1000 simultaneous users 200E3

  31. Efficiency of TDMA • percentage of transmitted data that contain information • frame efficiency f • usually end user efficiency < f , • because of source and channel coding • How get f ?

  32. Repeating Frame Structure One TDMA Frame Preamble Information Message Trail Bits Slot 1 Slot 2 Slot 3 … Slot N Trail Bits Sync. Bits Information Data Guard Bits The frame is cyclically repeated over time.

  33. Efficiency of TDMA bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg • bOH … number of overhead bits • Nr … number of reference bursts per frame • br … reference bits per reference burst • Nt … number of traffic bursts per frame • bp … overhead bits per preamble in each slot • bg … equivalent bits in each guard time interval

  34. Efficiency of TDMA • bT … total number of bits per frame • Tf … frame duration • R … channel bit rate bT = Tf * R

  35. Efficiency of TDMA • f … frame efficiency • bOH … number of overhead bits per frame • bT … total number of bits per frame f = (1-bOH/bT)*100%

  36. Spread Spectrum Multiple Access (SSMA) • SSMA uses Signals have transmission BW that is several orders of magnitude greater than the minimum required BW • A Pseudo-noise sequence converts a narrow band signal to a wideband noise-like signal before transmission • SSMA not BW efficient when used by a single user • Many users can share the same BW without interfering with one another • Type of SSMA techniques: • frequency hoped multiple access (FHMA) • Direct sequence multiple access (DS) or Code division multiple access (CDMA)

  37. Shift Register Stage 1 Stage 2 Stage 3 Register Output Clock Modulo 2 adder Pseudo-noise (PN) sequence • PN code sets can be generated from linear feedback shift registers • Example:

  38. Correlation matrix of 31-bit PN sequence

  39. Frequency Hopped Multiple Access (FHMA) • It is digital multiple access system • Carrier frequencies of individual users varied in pseudorandom fashion with in a wideband channel • Digital data broken into uniform sized bursts which are transmitted on different carriers • Instantaneous BW of any one transmission burst is much smaller than the total spread BW • Locally generated PN code is used to synchronize the receiver frequency with that of transmitter. • Provides a High level of security • Fast frequency hopping system • Slow frequency hopping

  40. FHMA • Erasures can occur when two or more users transmit on the same channel at the same time • FH(Frequency hopped) signal is immune to fading so error control coding can be combined to guard against erasures .

  41. CDMA • Narrowband signals is multiplied by a very large bandwidth signal called the spreading signal. • The spreading signal is pseudo noise code sequence that has a chip rate which is orders of magnitudes greater than data rate of the message • All users use the same carrier frequency and transmit simultaneously • Each user has its own pseudo random code word which is approximately orthogonal to other codewords

  42. CDMA • Receiver performs time co-relation • All other codewords appear as noise • Receiver needs to know the code word used by transmitter • Many users same frequency, TDD or FDD • Soft capacity(capacity increases linearly) • Self-jamming • Near far problem

  43. Packet Radio • Many subscribers attempt to access a single channel in an uncoordinated or minimally coordinated manner • Collisions from simultaneous transmissions are detected at the BS • ACK and NACK is broadcasted by BS (its like perfect feedback) • In case of NACK the signal retransmitted • PR easy to implement • Induces delays

  44. Packet Radio Protocols • Vulnerable period Vp: time interval during which the packets are susceptible to collisions with transmission from other users • If packet length in time is t • then a packet will suffer collision if other terminals transmit packets during the period t1 to t1+2t Packet A Packet B t1+2t t1

  45. Packet Radio contd … • Subscribers use contention techniques • ALOHA protocol example of contention techniques • Advantage: ability to serve many users without any overhead

  46. Pure ALOHA • Uncoordinated stations are contesting for the use of a single shared channel • Algorithm • Stations transmit whenever they have something to send • Colliding frames will be destroyed • If destroyed, the sender waits a random amount of time and sends it again • Maximum channel utilization is 18.4% • 81.6% of the total available bandwidth is essentially wasted due to losses from packet collisions

  47. In pure ALOHA, frames are transmitted at completely arbitrary times

  48. Slotted ALOHA • A method for doubling the capacity of pure ALOHA systems • Divide up time into discrete interval slots • Stations share the same time by synchronizing with a master • Master periodically broadcasts a synchronization pulse (clock tick packet) to all stations • Stations are only allowed to send their packets immediately after receiving a clock tick • Each interval corresponding to one frame time • Maximum channel utilization is 36.8%

  49. ALOHA Protocols • Advantages • Simple (due to distributed control) • Flexible to fluctuations in the number of hosts • Fair • Disadvantages • Low channel efficiency with a large number of hosts • Not good for real-time traffic (e.g., voice) • Cannot support priority traffic

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