Basics of Wireless and Mobile Communications

Transcription

1 Basics of Wireless and Mobile Communications Wireless Transmission Frequencies Signals Antenna Signal propagation Multiplexing Modulation Spread spectrum Cellular systems Media Access Schemes Motivation SDMA, FDMA, TDMA, CDMA Comparison Basic Functions in Mobile Systems Location management Handover Roaming

2 References Jochen Schiller: Mobile Communications (German and English), 2nd edition, Addison- Wesley, 2003 (most of the material covered in this chapter is based on the book) Holma, Toskala: WCDMA for UMTS. 3rd edition, Wiley,

3 Mobile Communication Systems the Issues: What does it require? Provide telecommunition services voice (conversation, messaging) data (fax, SMS/MMS, internet) video (conversation, streaming, broadcast) anywhere coverage anytime ubiquitous connectivity, reachability wireless without cord/wire mobile in motion, on the move (terrestrial) secure integrity, identity, privacy, authenticity, non-repudiation (Unleugbarkeit) reliable guaranteed quality of service 3

4 Frequencies for communication (spectrum) twisted pair coax cable GSM, DECT, UMTS, WLAN optical transmission 1 Mm 300 Hz 10 km 30 khz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible light UV VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency UHF = Ultra High Frequency SHF = Super High Frequency EHF = Extra High Frequency UV = Ultraviolet Light Frequency and wave length: = c / f wave length, speed of light c 3 x 10 8 m/s, frequency f 4

5 Electromagnetic Spectrum 100 MHz: UKW Radio, VHF TV 400 MHz: UHF TV 450 MHz: C-Netz 900 MHz: GSM MHz: GSM MHz: DECT 2000 MHz: UMTS (3G) 2400 MHz: WLAN, Bluetooth 2450 MHz: Mikrowellenherd 3500 MHz: WiMax 5 o

6 Frequencies for mobile communication VHF-/UHF-ranges for mobile radio simple, small antennas good propagation characteristics (limited reflections, small path loss, penetration of walls) typically used for radio & TV (terrestrial+satellite) broadcast, wireless telecommunication (cordless/mobile phone) SHF and higher for directed radio links, satellite communication small antenna, strong focus larger bandwidth available no penetration of walls Mobile systems and 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. 6

7 Frequencies and regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Examples of assigned frequency bands (in MHz): Cellular Phones (licensed) Cordless Phones (unlicensed) Wireless LANs (unlicensed) Others WiMax (IEEE , licensed) Europe USA Japan GSM , / , , / , / UMTS (FDD) , UMTS (TDD) , CT , CT DECT IEEE b a/HIPERLAN , RF-Control 27, 128, 418, 433, GHz, 2.5GHz and 3.5GHz AMPS, TDMA, CDMA , TDMA, CDMA, GSM , PACS , PACS-UB IEEE , RF-Control 315, GHz, 2.5GHz and 3.5GHz PDC , , , PHS JCT IEEE RF-Control 426, GHz, 2.5GHz and 3.5GHz Abbreviations: AMPS Advanced Mobile Phone System CDMA Code Division Multiple Access CT Cordless Telephone DECT Digital Enhanced Cordless Telecommunications GSM Global System for Mobile Communications HIPERLAN High-Performance LAN IEEE Institute of Electrical and Electronics Engineers JCT Japanese Cordless Telephone NMT Nordic Mobile Telephone PACS Personal Access Communications System PACS-UB PACS- Unlicensed Band PDC Pacific Digital Cellular PHS Personal Handyphone System TDMA Time Division Multiple Access WiMAX Worldwide Interoperability for Microwave Access 7 o

8 UMTS Frequency Bands (FDD mode only) Operating Band Frequency Band UL Frequencies UE transmit (MHz) DL Frequencies UE receive (MHz) Typically used in region... I EU, Asia II America III EU (future use) IV Japan V America, Australia, Brazil VI Japan VII Extension Band VIII EU (future use) IX Japan X America/US 8 o

9 UMTS Frequency Bands (FDD mode only), Germany Operator Uplink (MHz) Downlink (MHz) Carriers Auction Price Vodafone 1920,3 1930,2 2110,3 2120,2 2x10 MHz 16,47 Mrd. DM (8,42 Mrd. ) Currently spare 1930,2 1940,1 2120,2 2130,1 2x10 MHz 16,45 Mrd. DM Group 3G (Marke Quam) E-Plus 1940,1 1950,0 2130,1 2140,0 2x10 MHz 16,42 Mrd. DM (8,39 Mrd. ) Currently spare 1950,0 1959,9 2140,0 2149,9 2x10 MHz (16,37 Mrd. DM Mobilcom; returned) O2 1959,9 1969,8 2149,9 2159,8 2x10 MHz 16,52 Mrd. DM (8,45 Mrd. ) T-Mobile 1969,8 1979,7 2159,8 2169,7 2x10 MHz 16,58 Mrd. DM (8,48 Mrd. ) In 2000, the UMTS frequency bands were auctioned in Germany. 6 operators won 10 MHz each, for total 50 B 9 o

10 Basic Lower Layer Model for Wireless Transmission Transmit direction Receive direction Data link layer media access fragmentation reassembly frame error protection frame error detection multiplexing demultiplex Physical layer encryption decryption coding, Digital forward error Signal decoding, protection Processing bit error correction interleaving deinterleaving modulation demodulation D/A conversion, signal generation transmit receive Wireless Channel (path loss) A/D conversion; (signal equalization) Intersymbol- Interference (distortion of own signal) Intercell-Interference (multiple users) Intracell-Interference (multiple users) Thermal Noise 10 o

11 Signals in general physical representation of data function of time and location 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 sine wave as special periodic signal for a carrier: s(t) = A t sin(2 f t t + t ) amplitude frequency phase shift 11

12 Signal representations amplitude (time domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase in polar coordinates) A [V] A [V] Q = M sin t[s] I= M cos f [Hz] Composed signals transferred into frequency domain using Fourier transformation Digital signals need infinite frequencies for perfect transmission modulation with a carrier frequency for transmission (analog signal!) 12

13 Fourier representation of periodic signals Every periodic signal g(t) can be constructed by g( t) 1 2 c n1 a n sin(2nft) n1 b n cos(2nft) ideal periodic signal t real composition (based on harmonics) t 13

14 Signal propagation Propagation in free space always like light (straight line, line of sight) Receiving power proportional to 1/d² (ideal), 1/d α (α=3...4 realistically) (d = distance between sender and receiver) Receiving power additionally influenced by fading (frequency dependent) shadowing reflection at large obstacles scattering at small obstacles diffraction at edges shadowing reflection scattering diffraction 14

15 Radio Propagation: Received Power due to Pathloss Ideal line-of sight 1m 10m 100m (d -2 ): 1 1:100 1:10000 Realistic 1 1:3000 to 1:10 Mio to propagation (d ): 1:10000 db 1:100 Mio db 15

16 Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction signal at sender signal at receiver Time dispersion: signal is dispersed over time 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 Delayed signal rec d via longer path Signal received by direct path 16

17 Effects of mobility Fading Channel characteristics change over time and location signal paths change different delay variations of different signal parts (frequencies) different phases of signal parts quick changes in the power received (short-term fading or fast fading) Additional changes in distance to sender obstacles further away slow changes in the average power received (long-term fading or slow fading) power long-term fading short-term fading t 17

18 Fast Fading simulation showing time and frequency dependency of Rayleigh fading V = 110km/h 900MHz 18

19 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 sender transmission detection distance signal adds to the background noise interference 19

20 Interference 20

21 Carrier to Interference Ratio (CIR, C/I) (Uplink Situation) Ratio of Carrier-to-Interference power at the receiver CIR C I j N The minimum required CIR depends on the system and the signal processing potential of the receiver technology Typical in GSM: C/I=15dB (Factor 32) 21

22 Range limited systems (lack of coverage) Mobile stations located far away from BS (at cell border or even beyond the coverage zone) C at the receiver is too low, because the path loss between sender and receiver is too high C/I is too low No signal reception possible 22

23 Interference limited systems (lack of capacity) Mobile station is within coverage zone C is sufficient, but too much interference I at the receiver C/I is too low No more resources / capacity left 23

24 Information Theory: Channel Capacity (1) Bandwidth limited Additive White Gaussian Noise (AWGN) channel Gaussian codebooks Single transmit antenna Single receive antenna (SISO) Shannon (1950): Channel Capacity <= Maximum mutual information between sink and source Signal-to-noise ratio SNR 24 o

25 Information Theory: Channel Capacity (2) For S/N >>1 (high signal-to-noise ratio), approximate Observation: Bandwidth and S/N are reciproke to each other This means: With low bandwidth very high data rate is possible provided S/N is high enough Example: higher order modulation schemes With high noise (low S/N) data communication is possible if bandwidth is large Example: spread spectrum Shannon channel capacity has been seen as a unreachable theoretical limit, for a long time. However: Turbo coding (1993) pushs practical systems up to 0.5 db to Shannon channel bandwidth 25 o

26 Link Capacity for Various Rate-Controlled Technologies achievable rate (bps/hz) Shannon bound Shannon bound with 3dB margin (3GPP) HSDPA (3GPP2) EV-DO (IEEE) required SNR (db) The link capacity of current systems is quickly approaching the Shannon limit (within a factor of two). Future improvements in spectral efficiency will focus on IA/SDMA techniques and/or coordination between base stations. Link Performance of OFDM & 3G Systems are Similar and Approaching the (Physical) Shannon Bound 26 o

27 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 y z z x y x ideal isotropic radiator 27

28 Antennas: simple dipoles Real antennas are not isotropic radiators but, e.g. dipoles with lengths /4 on car roofs or /2 as Hertzian dipole shape of antenna proportional to wavelength /4 /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) 28

29 Antennas: directed and sectorized Often used for microwave connections (narrow directed beam) or base stations for cellular networks (sectorized cells) 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 29

30 Antenna 3-sectorized downtilt UMTS Networks Andreas Mitschele-Thiel, Jens Mückenheim 19 October

31 Real world propagation examples 31

32 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 cophasing needed to avoid cancellation /4 /2 /4 /2 /2 /2 + + ground plane 32

33 Multiplexing Goal: multiple use of a shared medium Multiplexing in 4 dimensions space (s i ) time (t) frequency (f) code (c) Multiple use is possible, if resource (channel) is different in at least one dimension channels k i s 1 k 1 k 2 k 3 k 4 k 5 k 6 c t f c s 2 t c t f s 3 f Important: guard spaces needed! 33

34 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 needed applicable to analog signals k 1 k 2 k 3 k 4 k 5 k 6 Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard space c f t 34

35 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 needed c f t 35

36 Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM (frequency hopping) Advantages: some (weak) protection against tapping protection against frequency selective interference k 1 k 2 k 3 k 4 k 5 k 6 but: precise coordination required c f t 36

37 Code multiplex Each channel has a unique code All channels use the same spectrum at the same time k 1 k 2 k 3 k 4 k 5 k 6 Advantages: bandwidth efficient no coordination and synchronization necessary good protection against interference and tapping Disadvantages: complex receivers (signal regeneration) c f Implemented using spread spectrum technology t 37

38 Cellular systems: Space Division Multiplex 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 Disadvantages: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells Cell sizes vary from 10s of meters in urban areas to many km in rural areas (e.g. maximum of 35 km radius in GSM) 38

39 Cellular systems: Frequency planning I Frequency reuse only with a certain distance between the base stations Typical (hexagon) model: f 4 f 5 f 6 reuse-3 cluster: f 3 f 1 reuse-7 cluster: f 1 f 7 f 3 f 1 f 2 f 2 f 3 f 1 f 2 f 4 f 5 f 1 f 3 f 2 f 6 f 7 f 3 f 2 f 4 f 5 f 1 f 6 f 3 f 2 f 7 Other regular pattern: reuse-19 the frequency reuse pattern determines the experienced CIR 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 39

40 Cellular systems: frequency planning II f 3 f 1 f 2 f 3 f 2 f 1 f 3 f 2 f 1 f 2 f 3 f 1 f 1 f 3 f 2 3 cell cluster f 3 f 3 f 3 7 cell cluster f 2 f 4 f 5 f 1 f 3 f 2 f 3 f 2 f 6 f 7 f 4 f 5 f 3 f 7 f 1 f 6 f 5 f 2 f 2 f 2 f 2 f f 1 3 h f 3 h f 3 h 2 h 2 1 h g h 3 g 1 g 3 g 3 f 1 f 1 g 2 g g 2 1 g g cell cluster with 3 sector antennas 40

41 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 protection against narrow band interference power interference spread signal power detection at receiver signal (despreaded) spread interference Side effects: f coexistence of several signals without dynamic coordination tap-proof f Alternatives: Direct Sequence (UMTS) Frequency Hopping (slow FH: GSM, fast FH: Bluetooth) 41

42 Effects of spreading and interference i) narrow band signal dp/df ii) spreaded signal (broadband signal) dp/df f sender f user signal broadband interference narrowband interference iii) addition of interference iv) despreaded signal v) application of bandpass filter dp/df dp/df dp/df f receiver f f 42

43 Spreading and frequency selective fading channel quality narrowband interference without spread spectrum frequency narrow band signal guard space channel quality spread spectrum to limit narrowband interference spread spectrum frequency 43

44 DSSS (Direct Sequence Spread Spectrum) I 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 needed t b 0 1 t c t b : bit period t c : chip period user data XOR chipping sequence = resulting signal 44

45 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 45 o

46 Modulation The shaping of a (baseband) signal to convey information. Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) Digital modulation digital data is translated into an analog signal (baseband) ASK, FSK, PSK differences in spectral efficiency, power efficiency, robustness Motivation for modulation smaller antennas (e.g., /4) medium characteristics Frequency Division Multiplexing spectrum availability 46

47 Modulation and demodulation analog baseband digital signal data digital analog modulation modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data radio receiver radio carrier 47 o

48 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 48

49 Advanced Frequency Shift Keying bandwidth needed for FSK depends on the distance between the carrier frequencies Idea: special pre-computation avoids sudden phase shifts MSK (Minimum Shift Keying) MSK technique: bit stream is separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen the frequency of one carrier is twice the frequency of the other, eliminating abrupt phase changes even higher bandwidth efficiency using a Gaussian low-pass filter GMSK (Gaussian MSK), used for GSM and DECT 49

50 Example of MSK data even bits odd bits low frequency high frequency bit Transformation scheme even odd signal h l l h value h: high frequency l: low frequency +: original signal -: inverted signal MSK signal t No phase shifts! 50

51 Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used e.g. in satellite systems QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK 00 more complex used in UMTS and EDGE (8-PSK) often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS) Puls filtering of baseband to avoid sudden phase shifts => reduce bandwidth of modulated signal Q Q 0 I 11 I 01

52 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: e.g. 16-QAM, 64-QAM n=2: 4-QAM identical to QPSK bit error rate increases with n, but less errors compared to comparable PSK schemes Example: 16-QAM (1 symbol = 16 levels = 4 bits) Symbols 0011 and 0001 have the same phase, but different amplitude 0000 and 1000 have different phase, but same amplitude also: 64-QAM (1 symbol = 64 levels = 6 bits) Q QAM is used in UMTS HSDPA (16-QAM) UMTS LTE (64-QAM) standard 9600 bit/s modems I

53 Media Access Schemes Motivation limits of CSMA/CD hidden and exposed terminals near-far problem TDD vs. FDD TDMA Aloha, slotted Aloha Demand Assigned Multiple Access (DAMA) CDMA theory and practice Comparison

54 Media Access: Motivation The problem: multiple users compete for a common, shared resource (medium) Can we apply media access methods from fixed networks? Example CSMA/CD Carrier Sense Multiple Access with Collision Detection (IEEE 802.3) send as soon as the medium is free (carrier sensing CS) listen to the medium, if a collision occurs stop transmission and jam (collision detection CD) Problems in wireless networks signal strength decreases (at least) 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 54

55 Motivation - hidden and exposed terminals 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 detect 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 55 A B C

56 Motivation - near and far terminals 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 A B C Severe problem for CDMA-networks precise power control needed! 56

57 Access methods SDMA/FDMA/TDMA SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure 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 previously are now used to control medium access! 57

58 Communication link types Each terminal needs an uplink and a downlink Types of communication links: Simplex unidirectional link transmission Half Duplex Bi-directional (but not simultaneous) Duplex simultaneous bi-directional link transmission, two types: Frequency division duplexing (FDD) Time division duplexing (TDD) 58

59 Duplex modes T d T u F d T d T u F u Frequency Division Duplex (FDD) Separate frequency bands for up- and downlink + separation of uplink and downlink interference - no support for asymmetric traffic Examples: UMTS, GSM, IS-95, AMPS Time Division Duplex (TDD) Separation of up- and downlink traffic on time axis + support for asymmetric traffic - mix of uplink and downlink interference on single band Examples: DECT, UMTS (TDD) 59

60 FDD/FDMA - general scheme, example GSM 960 f khz t 60

61 TDD/TDMA - general scheme, example DECT 417 µs downlink uplink t 61

62 Aloha/slotted aloha Mechanism random, distributed (no central arbiter), time-multiplex Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries Aloha collision sender A sender B sender C t Slotted Aloha collision sender A sender B sender C t 62

63 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 typical scheme for satellite links application to packet data, e.g. in GPRS and UMTS Examples for reservation algorithms: Explicit Reservation (Reservation-ALOHA) Implicit Reservation (PRMA) Reservation-TDMA 63

64 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 successful reserved slots (no collisions possible) synchronisation: 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 collision Aloha reserved Aloha reserved Aloha reserved Aloha t 64

65 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 Advantages: all terminals can use the same frequency, less planning needed huge code space (e.g ) compared to frequency space interference (e.g. white noise) is not coded forward error correction and encryption can be easily integrated 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 (power control) 65

66 CDMA Principle sender (base station) receiver (terminal) Code 0 Code 0 data 0 Code 1 Code 1 data 0 data 1 Transmission via air interface data 1 Code 2 Code 2 data 2 data 2 66

67 CDMA by example data stream A & B spreading spreaded signal Source 1 Code 1 Source 1 spread Source 2 Code 2 Source 2 spread 67

68 CDMA by example Despread Source 1 + Sum of Sources Spread Sum of Sources Spread + Noise decoding and despreading Despread Source 2 overlay of signals transmission and distortion (noise and interference) 68

69 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 69

70 CDMA on signal level I data A key A key sequence A data key 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 70

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

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

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

74 CDMA on signal level V A s + B s wrong key K (A s + B s ) * K integrator output comparator output (0) (0)? Assumptions orthogonality of keys neglectance of noise no differences in signal level => precise power control 74

75 Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea Terminals Signal separation Advantages Disadvantages Comment segment space into cells/sectors only one terminal can be active in one cell/one sector cell structure, directed antennas very simple, increases capacity per km² inflexible, antennas typically fixed only in combination with TDMA, FDMA or CDMA useful 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 established, fully digital, flexible guard space needed (multipath propagation), synchronization difficult standard in fixed networks, together with FDMA/SDMA used in many mobile networks segment the frequency band into disjoint sub-bands every terminal has its own frequency, uninterrupted filtering in the frequency domain simple, established, robust inflexible, frequencies are a scarce resource typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse) spread the spectrum using orthogonal codes all terminals can be active at the same place at the same moment, uninterrupted code plus special receivers 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 75

76 Basic Functions in Mobile Systems Location management Handover Roaming Authentication (see later)

77 Location Management The problem: locate a mobile user from the network side (mobile-terminated call) Two extreme solutions: Mobile registers with each visited cell (e.g. direct call to the hotel room to reach a person) signaling traffic to register mobile when cell is changed network has to maintain location information about each mobile + low signaling load to page mobile (i.e. in one cell only) Page mobile using a network- or worldwide broadcast message (e.g. broadcast on TV or radio to contact a person) heavy signaling load to page the mobile (i.e. in all cells) + no signaling traffic while mobile is idle 77

78 Location Management The issue: Compromise between minimizing the area where to search for a mobile minimizing the number of location updates Solution 1: Large paging area RA RA Solution 2: Small paging area Location RA Update TOTAL Signalling Cost + = Paging Signalling Cost Paging Area Update Signalling Cost RA RA Location RA Update RA Location Update RA Location Update Location Update RA 78

79 Handover The problem: Change the cell while communicating Reasons for handover: Quality of radio link deteriorates Communication in other cell requires less radio resources Supported radius is exceeded (e.g. Timing advance in GSM) Overload in current cell Maintenance Link quality cell 1 cell 2 cell 1 cell 2 Handover margin (avoid ping-pong effect) Link to cell 1 Link to cell 2 time 79

80 Roaming The problem: Use a network not subscribed to Roaming agreement needed between network operators to exchange information concerning: Authentication Authorisation Accounting Examples of roaming agreements: Use networks abroad Use of T-Mobile network by O 2 (E2) subscribers in area with no O 2 coverage 80