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  • Two simple multiple access control techniques. Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling. As we will see, used in AMPS, GSM, IS-54/136
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    1. 1. Multiplexing & Multiple Access Multiplexing SDM FDM/FDMA TDM/TDMA CDM CDMA 1
    2. 2. MC Influence to the Layer Model  service locationApplication layer  new applications, multimedia  adaptive applicationsTransport layer  congestion and flow control  quality of service  addressing, routing,Network layer device location  hand-over  authenticationData link layer  media access  multiplexing  media access control  encryptionPhysical layer  modulation  interference  attenuation  frequency 2
    3. 3. Multiple transmitters sending signalsat the same time through the shared medium “air”How to share the medium (common channel) with other transmitters? Multiplexing Goal: Minimize the degree of interferences and maximize the bandwidth for data transmissions 3
    4. 4. Multiplexing•Capacity of transmission medium usually exceedscapacity required for transmission of a single signal•Multiplexing - carrying multiple signals on a singlemedium •More efficient use of transmission medium 4
    5. 5. Reasons for Widespread Use of Multiplexing Cost per kbps of transmission facility declines with an increase in the data rate Cost of transmission and receiving equipment declines with increased data rate Most individual data communicating devices require relatively modest data rate support 5
    6. 6. Multiplexing Techniques Frequency-division multiplexing (FDM)  Takesadvantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal Time-division multiplexing (TDM)  Takesadvantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal 6
    7. 7. Circuit Switching: FDM and TDM Example:FDM 4 users frequency timeTDM frequency time 7
    8. 8. Frequency-division Multiplexing 8
    9. 9. Time-division Multiplexing 9
    10. 10. Multiplexing channels kiMultiplexing: Multiple transmitters send signals at the same time k1 k2 k3 k4 k5 k6Multiplexing in 4 dimensions  space (si) c  time (t)  frequency (f) t c  code (c) tGoal: supporting multiple users on a s1 shared medium (more channels) f  Maximize channel utilization s2 f (higher total bandwidth) cImportant: guard spaces needed tWhat will be the problem if the separation is s3 small and large? Small, the receiver cannot distinguish signals/noises. Large, a waste of bandwidth f Fr. Schiller 10
    11. 11. Space Division Multiple Access Use space division multiplexing  Frequency reuses to increase the total system bandwidth Segment space into sectors Use directed antennas or limited communication range signals from base stations Mobile stations may receive signals from base stations with different quality (select the best one => it is the closet one) May combine with other schemes, i.e., FDM 11
    12. 12. Frequency MultiplexingSeparation of the whole spectrum into smaller frequency bands (consider the whole spectrum as the multiple lanes of a road)The same station uses different frequencies for sending signals for different usersA channel gets a certain band of the spectrum for the whole timeAdvantages:  Simple  No dynamic coordination necessary k1 k2 k3 k4 k5 k6Disadvantages: c  Waste of bandwidth f if the traffic is distributed unevenly  Inflexible  Guard spaces (adjacent channel interference) t 12
    13. 13. Frequency Division Multiple Access Assign a certain frequency to a transmission channel between a sender and a receiver (use frequency division multiplexing) Channels can be assigned to the same frequency at all times (permanent), i.e., in radio broadcast Channel frequency may change (hopping) according to certain pattern Slow hopping (e.g., GSM) and fast hopping (FHSS, Frequency Hopping Spread Spectrum) Frequency division duplex (FDD): simultaneous access to medium by base station and mobile station using different frequencies  Uplink: from a mobile station to a base station  Downlink: from a base station to a mobile station 13
    14. 14. Time MultiplexingA channel gets the whole spectrum for a certain amount of timeAdvantages:  Only one carrier in the medium at any time (constant time period)  Throughput high even k1 k2 k3 k4 k5 k6 for many users (RR) cDisadvantages:  Time quantum normally very small f  Precise synchronization necessary (timing) t 14
    15. 15. Time and Frequency MultiplexingCombination of both methods (time & frequency)A channel gets a certain frequency band for a certain amount of timeExample: GSM (a 2G cellular network)Advantages:  Better protection against tapping (more complicated)  Protection against frequency k1 k2 k3 k4 k5 k6 selective interference cBut: precise coordination required f t 15
    16. 16. Code MultiplexingEach channel has a unique code (encoding and decoding) => d1 - > (encoding function f(d1,key)) -> p1 k1 k2 k3 k4 k5 k6After encoding, noises can be identified as noisesAll channels use the same spectrum at the same time cAdvantages:  Bandwidth efficient  No coordination and synchronization necessary  Good protection against interference and tapping (different coding schemes)Disadvantages: f  Lower user data rates  More complex signal regenerationWhat is the guard space? Keys for coding t 16
    17. 17. FDD/FDMA - General Scheme Example GSM f 960 MHz 124 935.2 MHz 1 200 kHz 20 MHz 915 MHz 124 1 890.2 MHz Fr. Schiller tGSM: 900MHzUplink: 890.2MHz to 915MHzDownlink: 935.2MHz to 960MHzEach channel 0.2MHz separated. Totally 124 channels foreach direction 17
    18. 18. Time Division Multiple Access Assign a fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time The receiver and transmitter use the same frequency all the times (simplified the design of receivers) How to do the time synchronization is the problem? Fixed time slot or assigned dynamically Fixed TDM:  Allocating time slots for channels in a fixed pattern (fixed bandwidth for each channel)  Fixed time to send and get data from a channel  Fixed bandwidth is good for constant data traffic but not for bursty traffic TDD (time division duplex): assign different slots for uplink and downlink using the same frequency Dynamic TDM requires coordination but is more flexible in bandwidth allocation 18
    19. 19. TDD/TDMA - General Scheme Example DECT 10-6417 µs 1 2 3 11 12 1 2 3 11 12 t downlink uplink 417 x 12 = 5004 Fr. Schiller Fixed period of 5ms 19
    20. 20. Polling Mechanisms If one terminal can be heard by all others, this “central” terminal can poll all other terminals according to a certain scheme, i.e. round-robin or random  Now all schemes known from fixed networks can be used (typical mainframe - terminal scenario) Example: Randomly Addressed Polling  Base station signals readiness to all mobile terminals  Terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as dynamic address)  The base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address)  The base station acknowledges correct packets and continues polling the next terminal  This cycle starts again after polling all terminals of the list 20
    21. 21. ISMA (Inhibit Sense Multiple Access) Current state of the medium is signaled via a “busy tone”  The base station signals on the downlink (base station to terminals) if the medium is free or not  Terminals must not send if the medium is busy  Terminals can access the medium as soon as the busy tone stops  The base station signals collisions and successful transmissions via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach)  Mechanism used, e.g., for CDPD (USA, integrated into AMPS) 21
    22. 22. Code Division Multiple Access All terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel So, how the receivers identify the data/signals for them? Each sender has a unique random number (code), the sender XORs the signal with this random number  Different senders use different codes  The codes separate the signals from different senders The encoded signals are concatenated together for sending, i.e., as a signal stream of signals The receiver “tunes” into this signal stream if it knows the pseudo random number. Tuning is done via a correlation function The received decodes the signal stream using the known code to identify the data for it Different receivers received different data as they use different codes 22
    23. 23. Code Division Multiple Access Disadvantages:  Higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal)  All signals should have the same strength at a receiver Advantages:  All terminals can use the same frequency, no planning needed  Huge code space (e.g. 232) compared to frequency space  Interferences is not coded  Forward error correction and encryption can be easily integrated 23
    24. 24. CDMA Encoding Fr. Schiller Each user is assigned a unique tb signature sequence (or code), denoted by (c1,c2,…,cM). Its user data component is called a chip 0 1 XOR tc Each bit, di, is encoded by multiplying chipping sequence the bit by the signature sequence: 01101010110101 = Zi,m = di cm resulting signal 01101011001010 XOR of the signal with pseudo- random number (chipping sequence) tb: bit period tc: chip period tc = 1/m x tb 0 : +1 1: -1 One bit is now sent as multiple bit => 0 (1) X 0 (1) = 1; 0 (1) X 1 (-1) = higher bandwidth is required -1 1 (-1) X 0 (1) = -1; 1 (-1) X 1 (-1) =1 24
    25. 25. Encoding Example Data bit d1 = –1 (0: +1; 1 = -1) Signature sequence (c1,c2,…,c8) = (+1,+1,+1,–1,+1,–1,–1,–1) Zi,m = di cm = (-1) x (+1), (-1) x (+1), …, (-1) x (-1) Encoder Output (Z1,1,Z1,2,…,Z1,8) = (–1,–1,–1,+1,–1,+1,+1,+1) 25
    26. 26. CDMA Decoding Without interfering users, the receiver would receive the encoded bits, Zi,m, and recover the original data bit, di, by computing: M 1 di = M ∑Z m =1 c i ,m m 26
    27. 27. CDMA Decoding Example M 1(c1,c2,…,c8) = (+1,+1,+1,–1,+1,–1,–1,–1) di = M ∑Z m =1 c i ,m m(Z1,1,Z1,2,…,Z1,8) = (–1,–1,–1,+1,–1,+1,+1,+1)i = 1, m = 1 i = 1, m = 8 multiply(+1)x(-1) (-1)x(+1) (–1,–1,–1,–1,–1,–1,–1,–1) add and -8/ m, m = 8 divide by M di = –1 27
    28. 28. 28
    29. 29. Multi-User Scenario If there are N users, the signal at the receiver becomes: N Z * i ,m = ∑Z n i ,m n =1 How can a CDMA receiver recover a user’s original data bit? 29
    30. 30. 2-Senders exampleMultiplied by the signaturesequence of user 1 N Z i*,m = ∑ Z inm , n =1 30
    31. 31. Signature Sequences In order for the receiver to be able to extract out a particular sender’s signal, the CDMA codes must be of low correlation Correlation of two codes, (cj,1,…, cj,M) and (ck,1,…, ck,M) , are defined by inner product: M 1 M ∑c m =1 c j ,m k ,m Code 1: 1, 1, 1, -1, 1, -1, -1, -1 Code 2: 1, -1, 1, 1, 1, -1, 1, 1 Inner product: 1 + (-1) + 1 + (-1) + 1 + 1 + (-1) + (-1) /8 = 0 31
    32. 32. Meaning of CorrelationWhat is correlation?  It determines how much similarity one sequence has with another  It is defined with a range between –1 and 1 Correlation Value Interpretation 1 The two sequences match each other exactly. 0 No relation between the two sequences –1 The two sequences are mirror images of each other. Other values indicate a partial degree of correlation. 32
    33. 33. Orthogonal CodesOrthogonal codes  All pair wise cross correlations are zero  Fixed- and variable-length codes used in CDMA systems  For CDMA application, each mobile user uses one sequence in the set as a spreading code  Provides zero cross correlation among all usersTypes  Welsh codes  Variable-Length Orthogonal codes 33
    34. 34. Walsh CodesSet of Walsh codes of length n consists of the n rows of an n x n Walsh matrix:  Wn Wn   W1 = (0) W2 n = W   n Wn   n = dimension of the matrix  Every row is orthogonal to every other row  Requires tight synchronization  Crosscorrelation between different shifts of Walsh sequences is not zero 34
    35. 35. Example 35
    36. 36. Typical Multiple Spreading Approach Spreaddata rate by an orthogonal code (channelization code)  Provides mutual orthogonality among all users in the same cell Further spread result by a Pseudo-Noise (PN) sequence (scrambling code)  Providesmutual randomness (low cross correlation) between users in different cells 36
    37. 37. Comparison SDMA/TDMA/FDMA/CDMAApproach SDMA TDMA FDMA CDMAIdea segment space into segment sending segment the spread the spectrum cells/sectors time into disjoint frequency band into using orthogonal codes time-slots, demand disjoint sub-bands driven or fixed patternsTerminals only one terminal can all terminals are every terminal has its all terminals can be active be active in one active for short own frequency, at the same place at the cell/one sector periods of time on uninterrupted same moment, the same frequency uninterruptedSignal cell structure, directed synchronization in filtering in the code plus specialseparation antennas the time domain frequency domain receiversAdvantages very simple, increases established, fully simple, established, flexible, less frequency capacity per km² digital, flexible robust planning needed, soft handoverDis- inflexible, antennas guard space inflexible, complex receivers, needsadvantages typically fixed needed (multipath frequencies are a more complicated power propagation), scarce resource control for senders synchronization difficultComment only in combination standard in fixed typically combined still faces some problems, with TDMA, FDMA or networks, together with TDMA higher complexity, CDMA useful with FDMA/SDMA (frequency hopping lowered expectations; will used in many patterns) and SDMA be integrated with mobile networks (frequency reuse) TDMA/FDMA 37
    38. 38. References Schiller: Ch. 2.1, 2.2, 2.4, 2.5, 2.6.1-2.6.4 Schiller: Ch 3.1, 3.2, 3.3, 3.4.1, 3.4.8 ,3.4.9, 3.4.10, 3.5, 3.6 Schiller, Mobile Communications, sections 4.1 (except 4.1.7) and 4.4.2, 4.4.4 – 4.4.6 (except the protocol stack) Wireless Communications & Networks, 2Edition, Pearson, William Stallings (Ch 7) 38

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