1 Mobile Networks Module A (Part A2) Introduction Prof. JP Hubaux

2 Modulation and demodulation (reminder) synchronization decision digital data analog demodulation radio carrier analog baseband signal radio receiver digital modulation digital data analog modulation radio carrier analog baseband signal radio transmitter

3 About CSMA/CD Can we apply media access methods from fixed networks? Example of CSMA/CD  Carrier Sense Multiple Access with Collision Detection  send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Problems in wireless networks  a radio can usually not transmit and receive at the same time  signal strength decreases proportionally to the square of the distance or even more  the sender would apply CS and CD, but the collisions happen at the receiver  it might be the case that a sender cannot “hear” the collision, i.e., CD does not work  furthermore, CS might not work if, e.g., a terminal is “hidden”

4 Hidden terminals  A sends to B, C cannot receive A  C wants to send to B, C senses a “free” medium (CS fails)  collision at B, A cannot receive the collision (CD fails)  A is “hidden” for C Exposed terminals  B sends to A, C wants to send to another terminal (not A or B)  C has to wait, CS signals a medium in use  but A is outside the radio range of C, therefore waiting is not necessary  C is “exposed” to B Hidden and exposed terminals BAC

5 Terminals A and B send, C receives  signal strength decreases proportional to the square of the distance  the signal of terminal B therefore drowns out A’s signal  C cannot receive A If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA-networks - precise power control needed! Motivation - near and far terminals ABC

6 Access methods SDMA/TDMA/FDMA/CDMA SDMA (Space Division Multiple Access)  segment space into sectors, use directed antennas  cell structure TDMA (Time Division Multiple Access)  assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time FDMA (Frequency Division Multiple Access)  assign a certain frequency to a transmission channel between a sender and a receiver  permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) CDMA (Code Division Multiple Access)  assign an appropriate code to each transmission channel (DSSS, Direct Sequency Spread Spectrum)  frequency hopping over separate channels (FHSS, Frequency Hopping Spread Spectrum)

7 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA Used in GSM Slotted Non-persistent p-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in (optional) MACAW MACA-BIFAMA CARMA Used in (mandatory) Used in (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS Used in GSM Fixed Used in Bluetooth Used in UMTS

8 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA Used in GSM Slotted Non-persistent p-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in (optional) MACAW MACA-BIFAMA CARMA Used in (mandatory) Used in (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS Used in GSM Fixed Used in Bluetooth

9 f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 Time multiplex A channel gets the whole spectrum for a certain amount of time. Advantages:  only one carrier in the medium at any time  throughput high for many users Disadvantages:  precise synchronization necessary

10 Frequency multiplex Separation of the whole spectrum into smaller frequency bands. A channel gets a certain band of the spectrum for the whole time. Advantages:  no dynamic coordination necessary  works also for analog signals Disadvantages:  waste of bandwidth if the traffic is distributed unevenly  inflexible  guard spaces k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c

11 f Time and frequency multiplex Combination of both methods. A channel gets a certain frequency band for a certain amount of time. Example: GSM Advantages:  better protection against tapping  protection against frequency selective interference But: precise coordination required t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1

12 Code multiplex Each channel has a unique code All channels use the same spectrum at the same time Advantages:  bandwidth efficient  no coordination and synchronization necessary  good protection against interference and tapping Disadvantages:  lower user data rates  more complex signal regeneration Implemented using spread spectrum technology k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c

13 TDMA/TDD – example: DECT t downlinkuplink 417 µs DECT: Digital Enhanced Cordless Telecommunications TDD: Time Division Duplex DECT: Digital Enhanced Cordless Telecommunications TDD: Time Division Duplex

14 FDMA/FDD – example: GSM f t MHz 200 kHz MHz MHz 915 MHz 960 MHz downlink uplink FDD: Frequency Division Duplex

15 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA Used in GSM Slotted Non-persistent p-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in (optional) MACAW MACA-BIFAMA CARMA Used in (mandatory) Used in (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS Used in GSM Fixed Used in Bluetooth

16 Mechanism  random, distributed (no central arbiter), time-multiplex  Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries Aloha Slotted Aloha Aloha/slotted aloha sender A sender B sender C collision sender A sender B sender C collision t t

17 Performance of Aloha (1/3) First transmission Retransmission (if necessary) t0t0 t 0 +X t 0 -X Vulnerable period t 0 +X+2t prop Time-out t 0 +X+2t prop +B Backoff period t prop : maximum one-way propagation time betwwen 2 stations Information about the outcome of the transmission is obtained after the reaction time 2 t prop B: backoff time

18 Performance of Aloha (2/3) S: new packetsS: throughput of the system { G

19 Performance of Aloha (3/3) Computation of the average packet transmission time

20 Performance of Slotted Aloha First transmission Retransmission (if necessary) t 0 =kX (k+1)X Vulnerable period t 0 +X+2t prop Time-out t 0 +X+2t prop +B Backoff period

21 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA Used in GSM Slotted Non-persistent p-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in (optional) MACAW MACA-BIFAMA CARMA Used in (mandatory) Used in (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS Used in GSM Fixed Used in Bluetooth

22 Carrier Sense Multiple Access (CSMA)  Goal: reduce the wastage of bandwidth due to packet collisions  Principle: sensing the channel before transmitting (never transmit when the channel is busy)  Many variants:  Collision detection (CSMA/CD) or collision avoidance(CSMA/CA)  Persistency (in sensing and transmitting) Station A begins transmission at t=0 A Station A captures the channel at t=t prop A

23 1-Persistent CSMA  Stations having a packet to send sense the channel continuously, waiting until the channel becomes idle.  As soon as the channel is sensed idle, they transmit their packet.  If more than one station is waiting, a collision occurs.  Stations involved in a collision perform a the backoff algorithm to schedule a future time for resensing the channel  Optional backoff algorithm may be used in addition for fairness Consequence : The channel is highly used (greedy algorithm).

24 Non-Persistent CSMA  Attempts to reduce the incidence of collisions  Stations with a packet to transmit sense the channel  If the channel is busy, the station immediately runs the back-off algorithm and reschedules a future sensing time  If the channel is idle, then the station transmits Consequence : channel may be free even though some users have packets to transmit.

25 p-Persistent CSMA  Combines elements of the above two schemes  Stations with a packet to transmit sense the channel  If it is busy, they persist with sensing until the channel becomes idle  If it is idle: With probability p, the station transmits its packet With probability 1-p, the station waits for a random time and senses again

26 Throughput expression Pure ALOHA Slotted ALOHA Unslotted 1-persistent CSMA Slotted 1-persistent CSMA Unslotted nonpersistent CSMA Slotted nonpersistent CSMA Protocol Throughput

27 Throughput plot Normalized propagation delay is a =0.01

28 CSMA/CD (reminder) Operating principle  Check whether the channel is idle before transmitting  Listen while transmitting, stop transmission when collision  If collision, one of the 3 schemes above (1-persistent, non- persistent or p-persistent) CS: Carrier Sense (Is someone already talking ?) MA: Multiple Access (I hear what you hear !) CD: Collision Detection (We are both talking !!) Three states for the channel : contention, transmission, idle Station Repeater Terminator

29 Why CSMA/CD is unfit for WLANs  Collision Detection requires simultaneous transmission and reception operations (which a radio transceiver is usually unable to do)  detecting a collision is difficult  Carrier Sensing may be suitable to reduce interference at sender, but Collision Avoidance is needed at receiver  CSMA/CD does not address the hidden terminal problem

30 CSMA/CA Is described in the module devoted to IEEE

31 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA Used in GSM Slotted Non-persistent p-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in (optional) MACAW MACA-BIFAMA CARMA Used in (mandatory) Used in (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS Used in GSM Fixed Used in Bluetooth

32 DAMA - Demand Assigned Multiple Access Channel efficiency only 18% for Aloha, 36% for Slotted Aloha Reservation can increase efficiency to 80%  a sender reserves a future time-slot  sending within this reserved time-slot is possible without collision  reservation also causes higher delays  typical scheme for satellite links Examples for reservation algorithms:  Explicit Reservation (Reservation-ALOHA)  Implicit Reservation (PRMA)  Reservation-TDMA

33 DAMA / Explicit Reservation Explicit Reservation (Reservation Aloha):  two modes: ALOHA mode for reservation: competition for small reservation slots, collisions possible reserved mode for data transmission within successful reserved slots (no collisions possible)  it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time AlohareservedAlohareservedAlohareservedAloha collision t

34 DAMA / Packet reservation (PRMA) Implicit reservation  based on slotted Aloha  a certain number of slots form a frame, frames are repeated  stations compete for empty slots according to the slotted aloha principle  once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send  competition for a slot starts again as soon as the slot was empty in the last frame frame 1 frame 2 frame 3 frame 4 frame time-slot collision at reservation attempts ACDABA F AC ABA A BAF A BAF D ACEEBAFD t ACDABA-F AC-ABAF- A---BAFD ACEEBAFD reservation

35 DAMA / Reservation-TDMA Reservation Time Division Multiple Access  every frame consists of N mini-slots and x data-slots  every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k).  other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic) N mini-slots N * k data-slots reservations for data-slots other stations can use free data-slots based on a round-robin scheme e.g. N=6, k=2

36 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA Used in GSM Slotted Non-persistent p-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in (optional) MACAW MACA-BIFAMA CARMA Used in (mandatory) Used in (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS Used in GSM Fixed Used in Bluetooth

37 MACA - collision avoidance MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance  Designed especially for packet radio networks (Phil Karn, 1990)  Principle: RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive Signaling packets contain  sender address  receiver address  packet size Variants of this method can be found in IEEE as DFWMAC (Distributed Foundation Wireless MAC)

38 MACA avoids the problem of hidden terminals  A and C want to send to B  A sends RTS first  C waits after receiving CTS from B MACA avoids the problem of exposed terminals  B wants to send to A, C to another terminal  now C does not have to wait for it cannot receive CTS from A MACA principle A B C RTS CTS A B C RTS CTS RTS

39 MACA example AB C D E AB C D E AB C D E RTS CTS DATA : blocked from Tx 1 2 3

40 MACA variant: application in IEEE idle wait for the right to send wait for ACK senderreceiver packet ready to send; RTS time-out; RTS CTS; data ACK RxBusy idle wait for data RTS; RxBusy RTS; CTS data; ACK time-out  Data with errors; NAK ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy time-out  NAK; RTS

41 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA Used in GSM Slotted Non-persistent p-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in (optional) MACAW MACA-BIFAMA CARMA Used in (mandatory) Used in (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS Used in GSM Fixed Used in Bluetooth

42 Polling mechanisms If one terminal can be heard by all others, this “central” terminal (e.g., base station) can poll all other terminals according to a certain scheme  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 a 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

43 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 (Cellular Digital Packet Data)  Similar approach was proposed for Packet Radio Networks (Kleinrock + Tobagi, 1975)

44 Spread Spectrum principle power density spectrum of the original signal power density of the original signal bandwidth of the original signal power density spectrum of the jamming signal

45 Spread Spectrum principle

46 Spread Spectrum principle

47 Spread Spectrum principle

48 Spread Spectrum principle Processing gain: Increase in received signal power thanks to spreading

49 Spread Spectrum principle

50 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA Used in GSM Slotted Non-persistent p-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in (optional) MACAW MACA-BIFAMA CARMA Used in (mandatory) Used in (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS Used in GSM Fixed Used in Bluetooth

51 Frequency Hopping Spread Spectrum (FHSS) (1/2)  Signal broadcast over seemingly random series of frequencies  Receiver hops between frequencies in sync with transmitter  Eavesdroppers hear unintelligible blips  Jamming on one frequency affects only a few bits  Rate of hopping versus Symbol rate  Fast Frequency Hopping: One bit transmitted in multiple hops.  Slow Frequency Hopping: Multiple bits are transmitted in a hopping period  Example: Bluetooth (79 channels, 1600 hops/s)

52 Frequency Hopping Spread Spectrum (FHSS) (2/2) tbtb tctc Fast Frequency Hopping: t b : duration of one bit t c : duration of one chip

53 Some medium access control mechanisms for wireless TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance Polling Pure CSMA Used in GSM Slotted Non-persistent p-persistentCSMA/CA Copes with hidden and exposed terminal RTS/CTS Used in (optional) MACAW MACA-BIFAMA CARMA Used in (mandatory) Used in (optional) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS Used in GSM Fixed Used in Bluetooth Used in UMTS

54 Direct Sequence Spread Spectrum (DSSS) (1/2) XOR of the signal with pseudo-random number (chipping sequence)  many chips per bit (e.g., 128) result in higher bandwidth of the signal Advantages  reduces frequency selective fading  in cellular networks base stations can use the same frequency range several base stations can detect and recover the signal soft handover Disadvantages  precise power control necessary  complexity of the receiver user data chipping sequence resulting signal XOR = tbtb tctc t b : bit period t c : chip period

55 Direct Sequence Spread Spectrum (DSSS) (2/2) X user data chipping sequence modulator radio carrier spread spectrum signal transmit signal transmitter demodulator received signal radio carrier X chipping sequence lowpass filtered signal receiver integrator products decision data sampled sums correlator

56 Categories of spreading (chipping) sequences  Spreading Sequence Categories  Pseudo-random Noise (PN) sequences  Orthogonal codes  For FHSS systems  PN sequences most common  For DSSS beside multiple access  PN sequences most common  For DSSS CDMA systems  PN sequences  Orthogonal codes

57 Generating a Pseudo-random Noise chip sequence with a linear feedback shift-register (LFSR) Properties of PN sequences:  Property 1: In a PN sequence we have:  Property 2: For a window of length n slid along output for N (=2 n -1) shifts, each n-tuple appears once, except for the all zeros sequence  Property 3: The periodic autocorrelation of a PN sequence is: source: Wikipedia, number of registers: n period:

58 Orthogonal Codes  Orthogonal codes  All pairwise 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  Walsh codes  Variable-Length Orthogonal codes

59 Walsh Codes  Set of Walsh codes of length n consists of the n rows of an n x n Hadamard matrix:  Sylvester's construction:  Every row is orthogonal to every other row and to the logical not of every other row  Requires tight synchronization  Cross correlation between different shifts of Walsh sequences is not zero

60 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 PN sequence (scrambling code)  Provides mutual randomness (low cross correlation) between users in different cells

61 CDMA (Code Division Multiple Access) Principles  all terminals send on the same frequency and can use the whole bandwidth of the transmission channel  each sender has a unique code  The sender XORs the signal with this code  the receiver can “tune” into this signal if it knows the code of the sender  tuning is done via a correlation function Disadvantages:  higher complexity of the receiver (receiver cannot just listen into the medium and start receiving if there is a signal)  all signals should have approximately the same strength at the receiver Advantages:  all terminals can use the same frequency, no planning needed  huge code space (e.g., 2 32 ) compared to frequency space  more robust to eavesdropping and jamming (military applications…)  forward error correction and encryption can be easily integrated

62 CDMA: principle (very simplified) AkAk X AsAs AdAd BkBk X BsBs BdBd A s + B s AkAk X BkBk X C+D AdAd BdBd C+D: Correlation and Decision Spreading Despreading

63 CDMA: example Sender A  sends A d = 1, key A k = (assign: „0“= -1, „1“= +1)  sending signal A s = A d * A k = (-1, +1, -1, -1, +1, +1) Sender B  sends B d = 0, key B k = (assign: „0“= -1, „1“= +1)  sending signal B s = B d * B k = (-1, -1, +1, -1, +1, -1) Both signals superimpose in space  interference neglected (noise etc.)  A s + B s = (-2, 0, 0, -2, +2, 0) Receiver wants to receive signal from sender A  apply key A k bitwise (inner product) A e = (-2, 0, 0, -2, +2, 0)  A k = = 6 result greater than 0, therefore, original bit was „1“  receiving B B e = (-2, 0, 0, -2, +2, 0)  B k = = -6, i.e. „0“

64 Spreading of signal A data A d signal A s key sequence A k Real systems use much longer keys resulting in a larger distance between single code words in code space. A d +A k 1

65 Spreading of signal B signal A s A s + B s data B d signal B s key sequence B k B d +B k

66 Despreading of signal A AkAk (A s + B s ) * A k correlator output decision output A s + B s data A d Note: the received signal is inverted

67 Despreading of signal B correlator output decision output BkBk (A s + B s ) * B k A s + B s data B d Note: the received signal is inverted

68 Despreading with a wrong key decision output wrong key K correlator output (A s + B s ) * K A s + B s

69 Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time Idea: use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing according to aloha SAMA - Spread Aloha Multiple Access 1 sender A 0 sender B 0 1 t narrow band send for a shorter period with higher power spread the signal e.g. using the chipping sequence („CDMA without CD“) Problem: find a chipping sequence with good characteristics 1 1 collision

70 Comparison SDMA/TDMA/FDMA/CDMA In practice, several access methods are used in combination Example :SDMA/TDMA/FDMA for GSM and IS-54

71 References  T. Rappaport: Wireless Communications, Principles and Practice (2 nd edition), Prentice Hall, 2002  M. Schwartz: Mobile Wireless Communications, Cambridge University Press, 2005  J. Schiller: Mobile Communications (2 nd edition), Addison- Wesley, 2004  Leon-Garcia & Widjaja: Communication Networks, McGrawHill, 2000