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Multiplexing and Multiple Access 이화여자대학교 김 낙 명 교수 본 강의록은 육군사관학교 최영윤 교수님께서 만드신 원본을 최교수님의 허락 하에 , 대부분 그대로 활용하고 있음을 알려드립니다. Terms. Multiplexing Communication resource(CR) sharing is fixed or slowly changing Resource allocation is assigned a priori(mixing user information before transmission)

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  1. Multiplexing andMultiple Access이화여자대학교김 낙 명 교수본 강의록은 육군사관학교 최영윤 교수님께서 만드신 원본을 최교수님의 허락 하에, 대부분 그대로 활용하고 있음을 알려드립니다.

  2. Terms • Multiplexing • Communication resource(CR) sharing is fixed or slowly changing • Resource allocation is assigned a priori(mixing user information before transmission) • Sharing takes place within the confines of a local site • Multiple Access • The remote sharing of a resource, such as a satellite • Dynamically changing multiple access scheme • Sharing and accessing spectrum resource during transmission

  3. Freq. Division(FDM/FDMA) • Specified subbands of frequency • Time Division(TDM/TDMA) • Periodically recurring time slots • Code Division(CDM/CDMA) • A set of orthogonal or nearly orthogonal spreading code

  4. Space Division(SDMA) • Multiple beam frequency reuse • Polarization Division(PDMA) • Dual polarization frequency reuse

  5. FDM/FDMA • Freq. band assignment is long term or permanent • Use heterodyning or mixing

  6. FDMA of Satellite System • Typically, 12 transponders with 36-MHz bandwidth • Common transponder operate in an FDM/FM/FDMA • FDM : each one having single-sideband 4-KHz spectrum forms a multichannel composite signal • FM : composite signal is frequency-modulated • FDMA : Subdivisions of the 36-MHz bandwidth assigned to different users, each user receives a specific bandwidth allocation * Composite FDM channels are FM modulated and transmitted to the satellite within the bandwidth allocation of an FDMA Plan * The major advantage of FDMA is its simplicity - No synchronization or central timing

  7. TDM/TDMA • The full spectral occupancy of the system • A short duration of time called time slot • The unused time region between time slot called guard time

  8. Fixed-Assignment TDM/TDMA • Efficient when the source traffic is predictable and heavy • For bursty or sporadic traffic, the scheme is wastefull

  9. Dynamic assignment of time slots known as packet-switched system

  10. Combined FDMA/TDMA • M disjoint freq. bandwidth W/M Hz • mth slot within the nth frame • Intersection of the time slot (n,m) and the band (j)

  11. Comparison of FDMA & TDMA • Bit rate • System can support a total of R bps • FDMA : M orthogonal freq. Band with R/M bps each • TDMA : M orthogonal time slots with R bps each, (1/M)th time slot

  12. * b-bit groups or packets • Total Bit Rate FDMA : b-bit packets in T seconds over M disjoint channels bps bps TDMA : b-bit packets in T/M seconds

  13. : Average packet waiting time : Packet transmission time, 1 packet = b (bits) : Frame time for transmitting b bits • Message Delay

  14. CDMA(FH-CDMA) Frequency Hopping - PN code generator - Slow & Fast hopping cf, Direct Sequence

  15. DS(Direct Sequence) Spread Spectrum Data Source Channel Spreading code Interferences Despreading code

  16. DS-CDMA Scheme Carrier Modulated signal . . . Modulator To Conventional demodulator Code Code Data : Modulated signals of user 1,2, …N : Spreading PN Code : Orthogonal Code

  17. Advantages of CDMA • Privacy : Exclusive code for authorized users • Fading channels : Users experience only in the short hopping time period • Jamming Resistance : Hopping over a wide frequency band, avoid spot jamming • Flexibility : No precise time coordination among the various simultaneous users since uncorrelatedness between different PN code

  18. Comparisons of FDMA, TDMA & CDMA FDMA - Transmission is continuous - Tx. rate : Peak = Average - Tranceiver size(G/T) small - 1 tranceiver for each circuit - Voice activity(VA) not exploited - Resource allocated per traffic - usually good for point-to-point TDMA - Transmission is bursty - Tx. rate : Peak >> Average - Tranceiver size(G/T) large - 1 tranceiver for all circuit - VA exploited only in trunks - Full resource allocated at start - usually good for mesh & star CDMA - Transmission is bursty - Tx. rate : Peak = Average - Tranceiver size(G/T) small - 1 tranceiver for all circuit - VA exploited - Resource allocated per traffic - Good for all topologies

  19. Comparisons of DS & FH DS(Direct Sequence) - Baseband spread - Transmitted on single freq. - Performance degrades gracefully with load - Near-far problem : Power control - Multipath Immunity - Path diversity could be exploited - Better suited for distributed resource access algorithm FH(Frequency Hopping) - Baseband spread - Transmitted on a set freq. - Two types : Fast FH & Slow FH - FFH needs more bandwidth than SFH - Stringent Sync. Requirement than DS - Less near-far & power control - Less equalization requirements

  20. SDMA & PDMA

  21. Multiple Access Algorithm/Protocol • Centralized • Distributed

  22. Multiple Access Information Flow • Channelization • Network state • Service request • Assignment • Data transmission

  23. DAMA(Demand-Assignment MA) • Dynamic assignment • More efficient than fixed-assignment • Bandwidth reduction

  24. FDMA/TDMA/CDMA for Mobile Cellular • Freq. Division Multiple Access(FDMA) • Single channel per carrier(SCPC) • Low transmission power from handset • Multiple radio units at Base Station(Poor economics) • need a new radio subsystem for each carrier • Difficult to support multiple bit rates • Difficult to support high speed applications • Not easy to exploit voice or data activity • Possible using complex protocols • Hard capacity limit by limited freq. band

  25. Time Division Multiple Access(TDMA) • Multiple channels per carrier(MCPC-typically 4 to 10) • High peak transmission power from handset • Single radio unit at Base Station(better economics) • Configurable to support multiple bit rate • Configurable to support high speed applications • Can not easily exploit voice or data activity • Hard capacity limit by limited time slots • Code Division Multiple Access(CDMA) • Multiple channels per carrier • Single radio unit at Base Station(better economics) • Much easily configurable to support multiple bit rate • Configurable to support high speed applications • Exploits voice or data activity • Power control algorithm required • Soft capacity limit by limited power interference

  26. Capacity of Cellular FDMA and TDMA : Total system Bandwidth : Channel Bandwidth : Total channels / System : Channels / Cell : Channel Reuse Pattern = Required Carrier to Interference Ratio

  27. CDMA Cellular Networks • Forward Channel (Base Station to Mobile Subscriber) • Point to multi-point transmission • Synchronize the signals to reduce outer-cell interferences by using PN code offset • To separate data channels, use orthogonal spreading code, i.e. Walsh 64 code • Reverse Channel(Mobile Subscriber to Base Station) • Multi-point to point transmission • Asynchronous transmission, use zero-offset PN code • Inner-cell and outer-cell interferences are present • Power control is much more important to provide a sufficient performance • Walsh function is used for modulation • DS/BPSK modulation

  28. One Cell System : • To maximize capacity the SNR at the receiver should be equal for all signals; i.e. careful power control is required. • Each terminal adjusts its transmitter power so that it is received at the cell site with a constant power • Under the perfect power control assumption, system capacity is = Number of users in the 1-cell system = Received signal power from each terminal at the cell cite = Processing Gain = Required C/I ratio for satisfactory speech quality ; depends on the link used

  29. Multiple Cell System : • Assume ideal case(no multi-path fading, no shadow fading) • Assume propagation power loss law is proportional to , where r is the link distance How many users can we have in the above system? Depends on the positions of the mobiles. E.g. if mobiles are close to their respective base stations then there is little outer-cell interference: System Capacity = However, if most mobiles are close to the cell boundaries then outer-cell interference is large and the capacity is reduced considerably from to and System Capacity = In general, the capacity of each cell in a multi-cell system is less than

  30. Suppose that there are many users and they are distributed uniformly over all cells in the network. Assume that the number of users per cell is , and that terminals adjust power so that the power at the receiver is 1. Then, interference at a given cell = inner-cell interference + outer-cell interference = +outer-cell interference • One Solution :  Outer-Cell Interference : Assume that terminals positions are randomly distributed in the cell. E.g. in previous diagram user in cell B interferes with user in cell A as follows : At A : -Power received form is (due to power control); not dependent on position of in cell A. -Interference form in cell B depends on distance d and transmitter power of as follows : interference = If we assume the to be random and take the expected value of the interference power we get approximately 0.06. Similarly for an interference in a cell two cells away from A the average interference (over all positions) is less than 0.01. Similar values may be computed for cells that are 3,4,5,… cells away from cell A. Let the resulting sequence of values be , ,...

  31. Let cells yielding average interference per user equal to , e.g. = 0.06, = 6, etc. Then the expected value of the total interference is , where = # users per cell The preceding sum is approximately equal to outer-cell interference =0.5 . Therefore, total expected interference in a given cell = 1 = Therefore, as a result of outer-cell interference the one-cell capacity is reduced by the factor 1/1.5=0.66. If we normalize the total interference in a given cell to 1 then we have 1 = + Total interference Inner-cell interference Outer-cell interference Denote the total interference factor (1.5) by 1/F. Then each inner-cell terminal we have a contribution of interference of 1/F. The capacity per cell of the multi-cell system is then :

  32. On the average each voice circuit os active for only a fraction of time (say V=0.4) due to listening and pauses in speech. Assume that we have a scheme where the carrier is tuned off during idle speech periods. Then the interference is reduces by the factor V. For a large number of users the capacity increases by the factor 1/V. In practice the power is reduced instead to a non-zero value to facilitate synchronization. • Speech Activity Factor

  33. So far we have assumed omnidirectional antennas. The interference at a given cell may be considerably reduced by employing sectorized antennas. Example : use beamwidth antennas • Sectorization Now, each transmitter interferes with only 1/3 of the number of users as compared the omnidirectional case. Thus, the expected value of the total interference is reduced by a factor of G=3.

  34. CAPACITY EQUATION The preceding equation may be written in a related form (Qualcomm) with an appropriate interpretation of the SNR. Where - is the maximum number of calls per cell - is the system bandwidth (e.g. 1.23 MHz). - is the information data rate of a channel. - V is the voice activity factor (e.g. V=0.4). - F is the frequency re-use efficiency (e.g. F=1/1.5=0.66) - G is the sectorization gain (e.g. G=3 for beamwidth - is the required minimum (e.g. 3-15dB) Example : assume =7(dB) (calls/cell)

  35. Capacity 비교 for CDMA for AMPS for 3 Channel DAMPS 1)AMPS (FM) M = K = 7 (Channels/cell)

  36. 2)DAMPS (TDMA, IS-54) M = K = 4 (Channels/cell) 3)CDMA (CDMA, IS-95) K = 1, F = 0.66, V = 0.4, G = 3 (Channels/cell) Capacity Gain =

  37. ALOHA System • Users organize data into b-bits packets(80 char. + control+parity) • Uses two broadcast channels • Inbound channels(users to central) • Statistically multiplexed, random access channel • Ready users transmit independently at random times as soon as they have data to send • Outbound channels(central to users) • Time multiplexed, well organized channel single source • Operation Mode • Transmission Mode : Users transmit at any time they desire • Listening Mode : After a message transmission, a user listens for an ACK, when collision occurs the users receive NAK • Retransmission Mode : When a NAK is received, the messages are simply retransmitted with random delay • Timeout Mode : After a transmission, if the user does not receive either ACK or NAK within a specified time, the user retransmit the message

  38. Packet Structure 88 bytes @ 8 bits = 704 bits/packet Header(ID & control Info.) 4 bytes Text(80 char.) 80 bytes Parity 4 bytes Pilot Header(4) Parity(2) Text(80 bytes) Parity(2) 5 msec Inbound channel operates at 2400 bps Time to transmit a packet is 704 bits @ 2.4 kbps = 29 msec pilot = 5 msec 34 msec overhead (8 bytes)(8 bits/byte)(1/2400) = 2.67 msec pilot = 5 msec 7.67 msec Parity is CRC-32, Prob. Of undetected error

  39. Geostationary orbit ; Round-trip Delay 1 1 User #1 R Random delay 2 2’ User #2 R Random delay 3 3’ User #3 #2 & #3 1 c 2’ 3’ Satellite

  40. Traffic on Satellite on the Network : Message or packet arrival rate of the total system : retransmission rate : total traffic arrival rate Normalized throughput (sec/packet) : Tx time of a packet R (bps) : Channel capacity Normalized total traffic

  41. Assume large number of users who transmit independently and at random on inbound channel. On inbound channel, there are two types of packet : new and retransmitted. Combined average arrival rate of packets/sec Prob. Of packet on inbound channel succeeding No Packet No Packet = Prob. Of no two packet arrive in sec from start time of given packet. Assuming message arrival statistics follow a Poisson process

  42. Rate at which packets succeed at steady state : new packet generation rate New packets ALOHA CHANNEL + Retransmitted packets S : Normalized throughput G : Normalized total channel traffic

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