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Multiplexing & Multiple Access

Multiplexing & Multiple Access. Multiplexing SDM FDM/FDMA TDM/TDMA CDM CDMA. service location new applications, multimedia adaptive applications congestion and flow control quality of service addressing, routing, device location hand-over authentication media access

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Multiplexing & Multiple Access

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  1. Multiplexing & Multiple Access • Multiplexing • SDM • FDM/FDMA • TDM/TDMA • CDM • CDMA 1

  2. service location new applications, multimedia adaptive applications congestion and flow control quality of service addressing, routing, device location hand-over authentication media access multiplexing media access control encryption modulation interference attenuation frequency MC Influence to the Layer Model Application layer Transport layer Network layer Data link layer Physical layer 2

  3. Multiple transmitters sending signals • at 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. Multiplexing • Capacity of transmission medium usually exceeds capacity required for transmission of a single signal • Multiplexing - carrying multiple signals on a single medium • More efficient use of transmission medium 4

  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. Multiplexing Techniques • Frequency-division multiplexing (FDM) • Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal • Time-division multiplexing (TDM) • Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal 6

  7. Example: 4 users FDM frequency time TDM frequency time Circuit Switching: FDM and TDM 7

  8. Frequency-division Multiplexing 8

  9. Time-division Multiplexing 9

  10. Multiplexing channels ki Multiplexing: Multiple transmitters send signals at the same time Multiplexing in 4 dimensions • space (si) • time (t) • frequency (f) • code (c) Goal: supporting multiple users on a shared medium (more channels) • Maximize channel utilization (higher total bandwidth) Important: guard spaces needed What will be the problem if the separation is small and large? Small, the receiver cannot distinguish signals/noises. Large, a waste of bandwidth k1 k2 k3 k4 k5 k6 c t c s1 t s2 f f c t s3 f Fr. Schiller 10

  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. Frequency Multiplexing Separation 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 users A channel gets a certain band of the spectrum for the whole time Advantages: • Simple • No dynamic coordination necessary Disadvantages: • Waste of bandwidth if the traffic is distributed unevenly • Inflexible • Guard spaces (adjacent channel interference) k1 k2 k3 k4 k5 k6 c f t 12

  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. Time Multiplexing A channel gets the whole spectrum for a certain amount of time Advantages: • Only one carrier in themedium at any time (constant time period) • Throughput high even for many users (RR) Disadvantages: • Time quantum normally very small • Precise synchronization necessary (timing) k1 k2 k3 k4 k5 k6 c f t 14

  15. Time and Frequency Multiplexing Combination of both methods (time & frequency) A channel gets a certain frequency band for a certain amount of time Example: GSM (a 2G cellular network) Advantages: • Better protection against tapping (more complicated) • Protection against frequency selective interference But: precise coordinationrequired k1 k2 k3 k4 k5 k6 c f t 15

  16. Code Multiplexing Each channel has a unique code (encoding and decoding) => d1 -> (encoding function f(d1,key)) -> p1 After encoding, noises can be identified as noises All channels use the same spectrum at the same time Advantages: • Bandwidth efficient • No coordination and synchronization necessary • Good protection against interference and tapping (different coding schemes) Disadvantages: • Lower user data rates • More complex signal regeneration What is the guard space? Keys for coding k1 k2 k3 k4 k5 k6 c f t 16

  17. FDD/FDMA - General SchemeExample GSM f 960 MHz 124 200 kHz 935.2 MHz 1 20 MHz 915 MHz 124 890.2 MHz 1 Fr. Schiller t GSM: 900MHz Uplink: 890.2MHz to 915MHz Downlink: 935.2MHz to 960MHz Each channel 0.2MHz separated. Totally 124 channels for each direction 17

  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. TDD/TDMA - General SchemeExample DECT 10-6 417 µs 1 2 3 11 12 1 2 3 11 12 t downlink uplink 417 x 12 = 5004 Fixed period of 5ms Fr. Schiller 19

  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. 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. 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. 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. CDMA Encoding Fr. Schiller • Each user is assigned a unique signature sequence (or code), denoted by (c1,c2,…,cM). Its component is called a chip • Each bit, di, is encoded by multiplying the bit by the signature sequence: • Zi,m = di cm • XOR of the signal with pseudo-random number (chipping sequence) • One bit is now sent as multiple bit => higher bandwidth is required tb user data 0 1 XOR tc chipping sequence 0 1 1 0 1 0 1 0 1 1 0 1 0 1 = resulting signal 0 1 1 0 1 0 1 1 0 0 1 0 1 0 tb: bit periodtc: chip period tc = 1/m x tb 0 : +1 1: -1 0 (1) X 0 (1) = 1; 0 (1) X 1 (-1) = -1 1 (-1) X 0 (1) = -1; 1 (-1) X 1 (-1) = 1 24

  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. CDMA Decoding • Without interfering users, the receiver would receive the encoded bits, Zi,m, and recover the original data bit, di, by computing: 26

  27. CDMA Decoding Example • (c1,c2,…,c8) = (+1,+1,+1,–1,+1,–1,–1,–1) • (Z1,1,Z1,2,…,Z1,8) = (–1,–1,–1,+1,–1,+1,+1,+1) • (+1)x(-1) (-1)x(+1) • (–1,–1,–1,–1,–1,–1,–1,–1) • di = –1 i = 1, m = 1 i = 1, m = 8 multiply add and divide by M -8/ m, m = 8 27

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  29. Multi-User Scenario • If there are Nusers, the signal at the receiver becomes: • How can a CDMA receiver recover a user’s original data bit? 29

  30. 2-Senders example Multiplied by the signature sequence of user 1 30

  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: • 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. Meaning of Correlation • What is correlation? • It determines how much similarity one sequence has with another • It is defined with a range between –1 and 1 Other values indicate a partial degree of correlation. 32

  33. Orthogonal Codes Orthogonal 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 users Types • Welsh codes • Variable-Length Orthogonal codes 33

  34. Walsh Codes Set of Walsh codes of length n consists of the n rows of an n x n Walsh matrix: • W1 = (0) • n = dimension of the matrix • Every row is orthogonal to every other row • Requires tight synchronization • Cross correlation between different shifts of Walsh sequences is not zero 34

  35. Example 35

  36. Typical Multiple Spreading Approach • Spread data 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) • Provides mutual randomness (low cross correlation) between users in different cells 36

  37. Comparison SDMA/TDMA/FDMA/CDMA 37

  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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