Physical Layer Issues

Transcription

1 Physical Layer Issues

2 twisted pair coax cable Frequencies for communication optical transmission 1 Mm 300 Hz 10 km 30 khz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 µm 3 THz 1 µm 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible light UV VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Frequency and wave length: λ = c/f wave length λ, speed of light c 3 x 10 8 m/s, frequency f

3 Frequency Bands

4 Frequencies for mobile communication VHF-/UHF-ranges for mobile radio simple, small antenna for cars deterministic propagation characteristics, reliable connections SHF and higher for directed radio links, satellite communication small antenna, focusing large bandwidth available Wireless LANs use frequencies in UHF to SHF spectrum some systems planned up to EHF limitations due to absorption by water and oxygen molecules (resonance frequencies) weather dependent fading, signal loss caused by heavy rainfall etc. WLAN uses unlicensed spectrum in ISM-bands (Industrial, Scientific, Medical) in the 2.4 GHz and 5.2 to 5.8 GHz range)

5 Frequencies and regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Europe USA Japan Cellular Phones Cordless Phones Wireless LANs Others GSM , / , , / , / UMTS (FDD) , UMTS (TDD) , CT , CT DECT IEEE HIPERLAN , RF-Control 27, 128, 418, 433, 868 AMPS, TDMA, CDMA , TDMA, CDMA, GSM , PACS , PACS-UB IEEE , RF-Control 315, 915 PDC , , , PHS JCT IEEE RF-Control 426, 868

6 physical representation of data function of time and location Signals I signal parameters: parameters representing the value of data classification continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values signal parameters of periodic signals: period T, frequency f = 1/T, amplitude A, phase shift ϕ sine wave as special periodic signal for a carrier: s(t) = A t sin(2 π f t t + ϕ t )

7 Fourier representation of periodic signals g( t) = 1 2 c + n= 1 a n sin(2πnft ) + n= 1 b n cos(2πnft ) ideal periodic signal t 0 real composition (based on harmonics) t

8 Signals II Different representations of signals amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase ϕ in polar coordinates) A [V] A [V] Q = M sin ϕ t[s] ϕ ϕ Composed signals transferred into frequency domain using Fourier transformation Digital signals need infinite frequencies for perfect transmission f [Hz] modulation with a carrier frequency for transmission (analog signal!) I= M cos ϕ

9 Antennas: isotropic radiator Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna Real antennas always have directive effects (vertically and/or horizontally) Radiation pattern: measurement of radiation around an antenna Is used as reference for measuring of antennas (EIRP= Equivalent Isotropic Radiated Power) y z z x y x ideal isotropic radiator

10 Antennas: simple dipoles Real antennas are not isotropic radiators but, e.g., dipoles with lengths λ/4 on car roofs or λ/2 as Hertzian dipole shape of antenna proportional to wavelength Metallic Surface λ/4 λ/2 Example: Radiation pattern of a simple Hertzian dipole y y z x z x simple dipole side view (xy-plane) side view (yz-plane) top view (xz-plane) Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power) Gain measure in dbi ( 10*log 10 P1/P2)

11 Antennas: directed and sectorized Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley) y y z x z x directed antenna side view (xy-plane) side view (yz-plane) top view (xz-plane) z z x x sectorized antenna top view, 3 sector top view, 6 sector

12 (passive) Antennas: diversity Grouping of 2 or more antennas multi-element antenna arrays Antenna diversity switched diversity, selection diversity receiver chooses antenna with largest output diversity combining combine output power to produce gain co-phasing needed to avoid cancellation λ/4 λ/2 λ/4 λ/2 λ/2 λ/2 + + ground plane

13 Signal propagation ranges Transmission range communication possible low error rate Detection range detection of the signal possible no communication possible Interference range signal may not be detected signal adds to the background noise sender transmission detection interference distance

14 Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d² (d = distance between sender and receiver) Receiving power additionally influenced by fading (frequency dependent; H 2 O resonance at 2.5 GHz; O 2 Resonance at 60 GHz) shadowing reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges shadowing reflection refraction scattering diffraction

15 Real world example

16 Friis free-space equation in logarithmic form P rcvd (d)= P tx +G t +G r +PL in db PL= 10*log 10 (4*π*d/λ) 2 path loss in free space First Fresnel Zone considerations for antenna highs and reference distance d 0

17 P rcvd (d)= P tx +G t +G r +PL PL= ( L 0 + L 1 ) Link Budget Calculation L 0 =20 log ( 4 π Ρ / λ)

18 Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction LOS pulses multipath pulses signal at sender signal at receiver Time dispersion: signal is dispersed over time (delay spread) interference with neighbor symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different parts

19 Effects of mobility Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts quick changes in the power received (short term fading) Additional changes in distance to sender obstacles further away slow changes in the average power received (long term fading) power short term fading long term fading t

20 Multiplexing Multiplexing in 4 dimensions space (s i ) time (t) frequency (f) code (c) channels k i k 1 c t k 2 k 3 k 4 k 5 k 6 SM c t Goal: multiple use of a shared medium s 1 f s 2 f Important: guard spaces needed! c t s 3 f

21 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 c k 1 k 2 k 3 k 4 k 5 k 6 Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard spaces t f

22 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 even for many users k 1 k 2 k 3 k 4 k 5 k 6 Disadvantages: precise synchronization necessary c f t

23 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 higher data rates as compared to code multiplex but: precise coordination required c k 1 k 2 k 3 k 4 k 5 k 6 f t

24 Code multiplex Each channel has a unique code k 1 k 2 k 3 k 4 k 5 k 6 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 t c f

25 Digital modulation Modulation digital data is translated into an analog signal (baseband) ASK, FSK, PSK - main focus in this chapter differences in spectral efficiency, power efficiency, robustness Analog modulation shifts center frequency of baseband signal up to the radio carrier Motivation smaller antennas (e.g., λ/4) Frequency Multiplexing medium characteristics Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

26 Modulation and demodulation analog baseband digital signal data digital analog modulation modulation radio transmitter radio carrier analog demodulation analog baseband signal Synchronization/ Demodulation/decision digital data radio receiver radio carrier

27 Digital modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference t Frequency Shift Keying (FSK): needs larger bandwidth t Phase Shift Keying (PSK): more complex robust against interference t

28 Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): Q bit value 0: sine wave bit value 1: inverted sine wave very simple PSK 1 0 I low spectral efficiency robust, used e.g. in satellite systems 10 Q 11 QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol I symbol determines shift of sine wave needs less bandwidth compared to BPSK more complex Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS) A t

29 Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation it is possible to code n bits using one symbol 2 n discrete levels, n=2 identical to QPSK bit error rate increases with n, but less errors compared to comparable PSK schemes Q Example: 16-QAM (4 bits = 1 symbol) I 1000 Symbols 0011 and 0001 have the same phase, but different amplitude and 1000 have different phase, but same amplitude. used in standard 9600 bit/s modems

30 Hierarchical Modulation DVB-T modulates two separate data streams onto a single DVB-T stream High Priority (HP) embedded within a Low Priority (LP) stream Multi carrier system, about 2000 or 8000 carriers (OFDM) QPSK, 16 QAM, 64QAM Example: 64QAM good reception: resolve the entire 64QAM constellation poor reception, mobile reception: resolve only QPSK portion 6 bit per QAM symbol, 2 most significant determine QPSK HP service coded in QPSK (2 bit), LP uses remaining 4 bit Q I

31 Multi Carrier Modulation (MCM) With Multi Carrier Modulation (MCM) the data stream is spilt into several concurrent communication streams using different frequencies Example of MCM are ADSL where each frequency is further modulated using BPSK or QAM For IEEE802.11a/g and Hiperlan-2 OFDM is used OFDM uses orthogonal frequencies to avoid inter carrier interference It uses long symbols to reduce ISI and to avoid complex equalization The initial symbol rate n can be divided onto m carriers such that the symbol rate/carrier is n/m. The distance between symbols (in the time domain) becomes larger and thus the ISI smaller.

32 OFDM model for transmission T s = T = N(2 (k-1) /R b ) R b bit rate (bps) FFT

33 Fourier transform of a single puls F -T/2 +T/2 F*T rect(t) si(t) = sin(t)/t

34 OFDM model for transmission (cont d) Modulation factor Constant phase ourier ransform s( t) S( f ) = = e j2π f N 1 2 N n= 2 o t a n N 1 2 N n= 2 T a sin n T t Π 2 T e j2π n f t (( f n f ) π T) ( f n f ) π T How do we select an appropiate value for f?

35 OFDM model for transmission (cont d) f = 0.8/T f T We find ICI (Inter-Carrier-Interference) f = 1.2/T f T

36 OFDM model for transmission (cont d) f = 1/T f T No ICI We have orthogonality between the different subcarriers

37 Components of a real system Transmitter Channel Coding Modulation Interleaving IFFT D/A RF- Transmission Receiver Channel Decoding Demodulation Deinterleaving FFT A/D RF- Reception Synchronizer

38 5 GHz Analog-Transceiver Blocks

39 Spread spectrum technology Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code power interference spread signal power detection at receiver signal spread interference f f Side effects: coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence, Frequency Hopping

40 Effects of spreading and interference dp/df dp/df i) dp/df ii) f sender dp/df f user signal broadband interference narrowband interference dp/df iii) iv) v) f receiver f f

41 channel quality Spreading and frequency selective fading narrowband channels frequency narrow band signal guard space channel quality spread spectrum frequency spread spectrum channels By spreading the effect of the fading channel is equally distributed to all users! How can we avoid interference of the chips?

42 DSSS (Direct Sequence Spread Spectrum) I XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128, best known 11) result in higher bandwidth of the Spreading Factor s = t b /t c 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 Precise synchronization necessary (multi correlators can take advantage from multi-path propagation (Rakereceiver) t b 0 1 t c t b : bit period t c : chip period user data XOR chipping sequence = resulting signal

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

44 Example Barker Code Good autocorrelation properties Minimal sequence allowed by FCC Coding Robust gain against 10.4 time db delay spread

45 FHSS (Frequency Hopping Spread Spectrum) I Discrete changes of carrier frequency sequence of frequency changes determined via pseudo random number sequence Two versions Fast Hopping: several frequencies per user bit Slow Hopping: several user bits per frequency Advantages frequency selective fading and interference limited to short period simple implementation uses only small portion of spectrum at any time Disadvantages not as robust as DSSS simpler to detect

46 FHSS (Frequency Hopping Spread Spectrum) II t b user data f f 3 f 2 f 1 f f 3 f 2 f t d t d t b : bit period t t t t d : dwell time slow Hopping (3 bits/hop) fast Hopping (3 hops/bit) t b <t d t b >t d

47 FHSS (Frequency Hopping Spread Spectrum) III user data modulator narrowband signal modulator spread transmit signal transmitter frequency synthesizer hopping sequence received signal demodulator narrowband signal demodulator data hopping sequence frequency synthesizer receiver

48 Example Bluetooth Frequency Hopping Bluetooth uses a slow frequency hopping scheme The frequency is changed every slot (625 µs) so approximately 1600 hops/s For multi-slot packets the frequency is changed with the next packet The hopping sequence is determined by the master (derived from the bluetooth MAC address) During inquiry and paging the the master MAC and timing offset is exchanged with the slaves The slot are enumerated from 0 to The master uses always the even slot The slot size is 1, 3 or 5

49 Master / Slave Communication B Ma D A C E SCO ACL f(k) f(k+2) f(k+4) f(k+6) f(k+8) f(k+10) f(k+12) f(k+14) f(k+16) f(k+18) f(k+20) f(k+22) f(k+24) A B C D E t SCO 1 ACL SCO 2 ACL SCO 1 ACL SCO 2 ACL SCO 1

50 UWB-System Definition: A signal is considered to be Ultra Wide Band if the Bandwidth of the signal is at least 25 % of the carrier frequency Special definition of FCC: For the UWB-Bands it is sufficient if the channel bandwidth is 500 MHz in the spectrum between 3.1 and 10.6 GHz Currently most systems use narrow band or wide band channels UWB spread the signal power over a very broad band and interferes therefore minimally with existing narrowband/wideband systems Spreading makes the system more stable against fading channel influences More bandwidth allow more data-rate More bandwidth allows more accurate location determination

51 UWB Spectrum FCC ruling permits UWB spectrum overlay Emitted Signal Power GPS PCS Bluetooth, b Cordless Phones Microwave Ovens a -41 dbm/mhz UWB Spectrum Part 15 Limit Frequency (Ghz) 10.6 FCC ruling issued 2/14/2002 after ~4 years of study & public debate FCC believes current ruling is conservative

52 Theoretical Data Rates over Range UWB shows significant throughput potential at at short range

53 What is Ultra Wideband? Radio technology that modulates impulse based waveforms instead of continuous carrier waves Time-domain behavior Frequency-domain behavior Ultrawideband Communication Impulse Modulation time 3 frequency 10 GHz (FCC Min=500Mhz) Narrowband Communication Frequency Modulation GHz

54 Information Modulation Pulse length ~ 200 ps; Energy concentrated in 2 6 GHz band; Voltage swing ~100 mv; Power ~ 10 µw Pulse Position Modulation (PPM) Pulse Amplitude Modulation (PAM) On-Off Keying (OOK) Bi-Phase Modulation (BPSK)

55 Related Standards IEEE : Wireless Personal Area Network (WPAN) IEEE : Bluetooth, 1 Mbps IEEE : WPAN/high rate, 50 Mbps IEEE a: WPAN/Higher rate, 500 Mbps, UWB IEEE c: WPAN Ultra High Data Rates 2-10 Gb/s IEEE : WPAN/low-rate, low-power, mw level, 200 kbps IEEE a: WPAN/low-rate, low-power, distance measurement; UWB

56 TOA (Time of Arrival) & RTD (Round Trip Delay) Three Principles of Positioning TDOA (Time Difference of Arrival) AOA (Angle of arrival)

57 Cell structure Implements space division multiplex: base station covers a certain transmission area (cell) Mobile stations communicate only via the base station Advantages of cell structures: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

58 Frequency planning I Frequency reuse only with a certain distance between the base stations Standard model using 7 frequencies: f 3 f 2 f 4 f 5 f 1 f 3 f 2 f 6 f 7 f 4 f 5 f 1 Fixed frequency assignment: certain frequencies are assigned to a certain cell problem: different traffic load in different cells Dynamic frequency assignment: base station chooses frequencies depending on the frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference measurements

59 Frequency planning II f 3 f 1 f 2 f 3 f 2 f 1 f 3 f 1 f 2 f 3 f 3 f 2 f 3 f 3 f 1 f 1 f 2 3 cell cluster f 3 f 4 f 2 f 5 f 1 f 3 f 2 f 3 f 2 f 6 f 7 f 7 f 4 f 5 f 1 f 6 f 5 f 3 f 2 7 cell cluster f 2 f 2 f 2 f f 1 3 h f 3 h f 3 h 2 h 2 1 h g h g 3 1 g 3 g 3 f 1 f 1 g 2 g g 2 1 g g cell cluster with 3 sector antennas

60 Cell breathing CDM systems: cell size depends on current load Additional traffic appears as noise to other users If the noise level is too high users drop out of cells

61 Mobile Communications Chapter 3 : Media Access Motivation SDMA, FDMA, TDMA Aloha Reservation schemes Collision avoidance, MACA Polling CDMA SAMA Comparison

62 Motivation Can we apply media access methods from fixed networks? Example 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 signal strength decreases proportional to the square of the distance 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

63 Hidden terminals Motivation - hidden and exposed 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 A B C 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

64 Motivation - near and far terminals Terminals A and B send, C receives signal strength decreases (at least) proportional to the square of the distance the signal of terminal B therefore drowns out A s signal C cannot receive A A B C 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!

65 Access methods SDMA/FDMA/TDMA SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure MIMO, Beam steering 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) 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 The multiplexing schemes presented in chapter 2 are now used to control medium access!

66 FDD/FDMA - general scheme, example GSM 960 MHz f MHz 915 MHz MHz 200 khz MHz f u = 890 MHz + i*0.2 MHz f d = f u + 45 MHz 1 FDD (frequency division duplex) t

67 TDD/TDMA - general scheme, example DECT 417 µs downlink uplink t 10ms

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

69 Aloha performance

70 Slotted Aloha performance

71 DAMA - Demand Assigned Multiple Access Channel efficiency only 18 % for Aloha, 36 % for Slotted Aloha (assuming Poisson distribution for packet arrival and packet length) 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 (only in low load situations, the delay jitter increases dramatically in high load situations) typical scheme for satellite links Examples for reservation algorithms: Explicit Reservation according to Roberts (Reservation-ALOHA) Implicit Reservation (PRMA Packet reservation multiple access) Reservation-TDMA

72 Access method 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 successfully 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. (Synchronization intervals strongly dependant on Clock accuracy) collision Aloha reserved Aloha reserved Aloha reserved Aloha t

73 Access method DAMA: PRMA Implicit reservation (PRMA - Packet Reservation MA): 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 this slots starts again as soon as the slot was empty in the last frame reservation ACDABA-F ACDABA-F AC-ABAF- A---BAFD ACEEBAFD frame 1 frame 2 frame 3 frame time-slot A C D A B A F A C A B A A B A F collision at reservation A B A F D attempts frame 5 A C E E B A F D t

74 Access method 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 roundrobin sending scheme (best-effort traffic) N mini-slots N * k data-slots e.g. N=6, k=2 reservations for data-slots other stations can use free data-slots based on a round-robin scheme

75 MACA - collision avoidance MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance 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)

76 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 examples RTS CTS CTS A B C 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 RTS CTS RTS A B C

77 MACA variant: DFWMAC in IEEE sender receiver idle idle ACK RxBusy time-out NAK; RTS wait for ACK packet ready to send; RTS wait for the right to send CTS; data time-out; RTS data; ACK time-out data; NAK wait for data RTS; CTS ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy RTS; RxBusy

78 Polling mechanisms If one terminal can be heard by all others, this central terminal (a.k.a. base station) can poll all other terminals according to a certain scheme now all schemes known from fixed networks can be used (typical mainframe - terminal scenario) This scheme is used in in PCF (Point coordination function) mode Example: Randomly Addressed Polling (no unique MAC address required) 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

79 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 (using a special signaling frequency) 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) (USA, integrated into AMPS)

80 Access method CDMA CDMA (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 each sender has a unique random number, the sender XORs the signal with this random number the receiver can tune into this signal if it knows the pseudo random number, tuning is done via a correlation function 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 ) compared to frequency space interferences (e.g. white noise) is not coded forward error correction and encryption can be easily integrated

81 CDMA in theory 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

82 data A key A key sequence A data key CDMA on signal level I A d A k signal A A s Real systems use much longer keys resulting in a larger distance between single code words in code space.

83 CDMA on signal level II signal A A s data B B d key B key sequence B data key B k signal B B s A s + B s

84 data A CDMA on signal level III A d A s + B s A k (A s + B s ) * A k integrator output comparator output 1 0 1

85 data B CDMA on signal level IV B d A s + B s B k (A s + B s ) * B k integrator output comparator output 1 0 0

86 CDMA on signal level V A s + B s wrong key K (A s + B s ) * K integrator output comparator output (0) (0)?

87 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 sender A sender B SAMA - Spread Aloha Multiple Access narrow band spread the signal e.g. using the chipping sequence ( CDMA without CD ) 1 1 collision send for a shorter period with higher power Problem: find a chipping sequence with good characteristics t

88 Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea Terminals Signal separation segment space into cells/sectors only one terminal can be active in one cell/one sector cell structure, directed antennas segment sending time into disjoint time-slots, demand driven or fixed patterns all terminals are active for short periods of time on the same frequency synchronization in the time domain segment the frequency band into disjoint sub-bands every terminal has its own frequency, uninterrupted filtering in the frequency domain spread the spectrum using orthogonal codes all terminals can be active at the same place at the same moment, uninterrupted code plus special receivers Advantages very simple, increases capacity per km² Disadvantages Comment inflexible, antennas typically fixed only in combination with TDMA, FDMA or CDMA useful established, fully digital, flexible guard space needed (multipath propagation), synchronization difficult standard in fixed networks, together with FDMA/SDMA used in many mobile networks simple, established, robust inflexible, frequencies are a scarce resource typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse) flexible, less frequency planning needed, soft handover complex receivers, needs more complicated power control for senders still faces some problems, higher complexity, lowered expectations; will be integrated with TDMA/FDMA