Structure of the Lecture

Transcription

1 Structure of the Lecture Chapter 2 Technical Basics: Layer Methods for Medium Access: Layer 2 Channels in a frequency band Static medium access methods Flexible medium access methods Chapter 3 Wireless Networks: Bluetooth, WLAN, WirelessMAN, WirelessWAN Mobile Networks: GSM, GPRS, UMTS Satellites and Broadcast Networks Chapter 4 Mobility on the network layer: Mobile IP, Routing, Ad-Hoc Networks Mobility on the transport layer: reliable transmission, flow control, QoS Mobility support on the application layer Page

2 Multiplexing Lehrstuhl für Informatik 4 Multiplexing in 4 dimensions: space (s i ) time(t) frequency (f) code (c) Channels k i k k 2 k 3 k 4 k 5 k 6 Multiplexing in space: c t c t Goal: multiple use of a shared medium s f s 2 f c t s 3 f Page 2

3 Channels Lehrstuhl für Informatik 4 The whole assigned frequency band is divided somehow in areas (channels) which are used for different transmissions. To avoid interference between the channels, guard spaces are needed. Static schemes: to avoid collisions of several transmission attempts, a transmission gets assigned a certain channel exclusively: Frequency Division Multiple Access (FDMA) Frequency Time Division Multiple Access (TDMA) Frequency Code Division Multiple Access (CDMA) Frequency Code... Time Each transmission is assigned a certain sub-band of the whole frequency band Time Each transmission is assigned the whole frequency band for certain time slots Time All transmissions take place all the time on the whole frequency band, but using a code Page 3

4 Frequency Multiplexing t c k k 2 k 3 k 4 k 5 k 6 f Principle: Separate the whole frequency band into sub-bands One sub-band corresponds to one channel, assigned to one transmission Usable bandwidth depends on the frequency band and the modulation method Guard spaces between the sub-bands are necessary to avoid interference of neighbored channels Advantages: Easy to implement (Radio, TV) No coordination necessary Disadvantages: Waste of bandwidth if the traffic is distributed unevenly Inflexible Page 4

5 Frequency Division Duplex (FDD) FDD is a standard (since early systems of mobile communication) to use frequency multiplex for a duplex communication Principle: For a duplex communication, two sub-bands are used, one for sending, the other for receiving (uplink resp. downlink) On technical reasons to avoid interference between the sent and the received signal in the antennas: a guard space between the uplink- and the downlink frequencies is needed (e.g. 45MHz GSM-System) two separate antennas have to be used, or a duplexer is needed which consists of two band filters which suppress the unwished signals Downlink Uplink D d A d B d C d A u... D u B u C u... X MHZ X45 MHz Frequency Page 5

6 FDD/FDMA in GSM 960 MHz f MHz 95 MHz MHz 200 khz MHz t Page 6

7 Time Multiplexing Principle: A transmission is assigned the whole frequency band exclusively for the duration of a certain time slot Advantages: No guard spaces Throughput high even for many users Low power consumption (deactivation of sending/receiving device for unused time slots) Disadvantage: c k k 2 k 3 k 4 k 5 k 6 f Precise synchronization between all senders Guard times between the time slots t Page 7

8 TDMA Guard Times and Synchronization For avoiding overlapping of the signals of different senders, a precise synchronization is necessary Problem: Assume a central base station to which all mobile stations communicate and which sends out synchronization signals. A mobile station near the base station can start sending earlier as stations far away, because it receives the synchronization signal earlier (speed of signal propagation) Example: Distance mobile station base station 35 km => Duration of synchronization signal: base station mobile station: round-trip-time: 234µs m m / s = 7µ s Solution: insert a guard time between time slots to compensate the difference in the round-trip-times Slot i Slot i guard time Page 8

9 TDMA - Guard Times and Synchronization But: decreasing efficiency, because within guard times no transmission is possible Example: slot duration in GSM: 577µs => guard time of 234µs causes a loss of 40% of transmission capacity! Solution: Timing Advance: the base station measures the round-trip-times to a mobile station and instructs stations far away to start sending earlier => Reduction of the guard time to 30µs Page 9

10 Time and Frequency Multiplexing Combination of FDMA and TDMA A channel gets a certain frequency sub-band for certain time slots Example: GSM Advantages: Protection against tapping Protection against frequency selective interferences High data rates, adaptation to the traffic amount is possible c k k 2 k 3 k 4 k 5 k 6 f But: precise coordination is necessary t Page 0

11 Time Division Duplex (TDD) Principle: Only a single carrier frequency Separation of downlink and uplink in terms of time Flexible: if you need more capacity in the downlink than in the uplink, just spend larger time slots in the downlink part and give the uplink part smaller time slots Downlink - ms 2 ms Uplink - ms D d A d B d C d D u A u B u C u variable sender/receiver separation time Page

12 TDD/TDMA - Example DECT 47 µs Downlink Uplink t Page 2

13 FDD vs. TDD Lehrstuhl für Informatik 4 Flexibility Complexity FDD (-) two carriers per connection, separation between them (-) efficient symmetry only possible with two equal bandwidths of the carriers (-) asymmetric connections need different bandwidths (-) Duplexer needed (antenna) (-) FDD wastes frequencies () easy to implement TDD () one carrier per connection () efficient symmetry by using equal uplink/downlink division () efficient asymmetric connections with variable borderlines between uplink and downlink () no duplexer needed () TDD only needs small frequency bands (-) synchronization is needed Efficiency and Throughput () high spectral efficiency with QAM modulation () two carriers enable a high throughput in both directions () high spectral efficiency with QAM modulation () high capacity Page 3

14 Code Multiplex For an efficient usage of the bandwidth, the whole frequency band is used by all transmissions in parallel The transmissions are separated by codes The code realizes a signal spreading over the whole bandwidth. When choosing codes, it must be guaranteed that they are different enough to separate the transmissions (orthogonal codes) Advantages: Efficient bandwidth usage No coordination/synchronization needed between the stations if appropriate codes are chosen (misleading not really completely true) Good protection against tapping (military) Redundancies protect against interferences Disadvantages: More complex signal regeneration Lower user data rates by coding the data Page 4

15 Code Division Multiplex (CDM) All stations use the same frequency and utilize the whole bandwidth of the transmission channel simultaneously The signal is combined with a pseudo random number (XOR); each sender needs a unique number The receiver is able to reproduce the signal if he knows the sender s random number and performs a correlation function Bit rate r i << Bit rate r c (for optical systems: r i /r c 5000) Signal Bi trate r i X X Code A Code A Bit rate r c Bit rate r c Signal n Bit rate r i X Mixing of the data streams on one carrier frequency X Code B Code B Page 5

16 CDMA - Computation Sender A Sends A d =, Code A k = 000 (set: 0 = -, = ) Sent signal A s = A d * A k = (-,, -, -,, ) Sender B Sends B d = 0, Code B k = 00 (set: 0 = -, = ) Sent signal B s = B d * B k = (-, -,, -,, -) Both signals interfere in the air: A s B s = (-2, 0, 0, -2, 2, 0) Remark: the sequences of the form (-,, -, -,, ) are called chip sequences Receiver wants to receive transmission of A: perform code A k bitwise (inner produkt) (-2, 0, 0, -2, 2, 0) A k = = 6 Result is larger than 0, sent bit must have been a Analogously for B (-2, 0, 0, -2, 2, 0) B k = = -6, thus a 0 Page 6

17 CDMA on Signal Level I data A code A chip sequence A data code A d A k signal A A s Real systems use much longer chip sequences resulting in a larger distance between single code words in code space Page 7

18 CDMA - on Signal Level II signal A A s data B code B chip sequence B data code B d B k signal B B s A s B s Page 8

19 CDMA - on Signal Level III data A 0 A d A s B s A k (A s B s ) * A k integrator output comparator output 0 Page 9

20 CDMA - on Signal Level IV data B 0 0 B d A s B s B k (A s B s ) * B k integrator output comparator output 0 0 Page 20

21 CDMA - on Signal Level V A s B s wrong code K (A s B s ) * K integrator output comparator output (0) (0)? Page 2

22 CDMA - Example Principle: Each station is assigned an 8 bit chip sequence *) For transferring a it sends its chip sequence, for transferring a 0 it sends its complement Bipolar notation with 0 as - and as All chip sequences are pairwise orthogonal, i.e. if S and T are orthogonal chip sequences (S T), then holds: m S T SiTi = 0, S T = 0, S S = m i= If two or more stations are sending at the same time, their signals are added linear Chip sequences of four stations: A: (--- - ) B: ( ) C: ( ) D: ( ) Example transmissions: (in each case exactly one bit is transferred) C E = ( ) -- B C E 2 = ( ) A B E 3 = ( ) 0 - A B C E 4 = ( ) A B C D E 5 = ( ) 0 A B C D E 6 = ( ) *) simplified example, normally at least 0 bits E i = transferred chip sequence for case i Page 22

23 Page 23 CDMA - Example For the six example transmissions one receives: 8 0) / (2 C E 8 2) / (4 C E 8 ) / 3 3 ( C E 0 8 2) / (0 C E 8 2) / (2 C E 8 ) / ( C E = = = = = = = = = = = = Result of orthogonal codes On the receiver side: For filtering out the bit stream of a certain station, the receiver must know the chip sequence of this station For extracting the bits of the station with chip sequence C from the received sequence E, it computes For example: E C 0 0 C C C B C A C C B A C E C B A E = = = = = ) ( i.e. station transmits station transmits station does not transmit anything station transmits station transmits station transmits 0

24 Spread Spectrum Technology Principle: CDMA can also deal with one problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Use of a bandwidth which is much larger than the one of the modulated signal Spread of the signal over the complete bandwidth using a pseudo random sequence power interference spread signal power detection at receiver signal spread interference f f That means: spread a narrow band signal into a broad band signal using a special code to protect against narrow band interference Page 24

25 Effects of Spreading and Interference power Original signal of a sender: Usage of a narrow frequency band f power Spreading of the signal to a broad frequency band; at the same time, the signal power is distributed power f During transmission, narrowband and broadband interference occur User signal Broadband interference f Narrowband interference Page 25

26 Effects of Spreading and Interference power f The receiver reconstructs the disturbed signal; as a result the narrowband interference is spreaded power Using a band pass filter, those parts of the signal can be removed which are outside the bandwidth of the original signal f Page 26

27 Spreading and Frequency-selective Fading channel quality frequency narrowband channels: Signal quality depends on carrier frequency! narrow band signal guard space channel quality spread spectrum channels: Uniform quality for all transmissions spread spectrum frequency Page 27

28 Spread Spectrum Technology Advantages: Several signals can be transferred without coordination within the same bandwidth at the same time Small susceptibility for effects of the multipath transmission: due to the high bandwidth in any case only a small part of the frequency spectrum is affected, so that the typical signal weakenings are weaker than with narrow-band systems Small influence of environmental disturbances Existence of transmissions (and as a result their decoding) is difficult to detect (of special relevance for military systems) Procedures: Direct Sequence Spread Spectrum (DSSS) Frequency Hopping Spread Spectrum (FHSS) Page 28

29 Direct Sequence - - bit code word chip Original signal Code sequence Frequency Spread signal - Frequency Division of the signal into redundant information units (chips), the transmitter sends several (at least 0) bits for one bit of information Both, sender and receiver must know the chip sequence (code) Spreading, i.e. distribution of the chips over a large bandwidth For other users, the transmission appears to be background noise Re-establishment of the original, possibly disturbed signal is possible due to the redundancy Power Power Page 29

30 Direct Sequence Principle: Doubled modulation: a. Modulation of the data to a spread wide-band signal b. Modulation of this signal to the carrier frequency The receiver processes the inverted procedure with identical chip sequences Integration over the bit period user data X spread spectrum signal modulator transmit signal received signal demodulator lowpass filtered signal correlator products X integrator chipping sequence transmitter radio carrier radio carrier receiver chipping sequence sampled sums decision data Page 30

31 Frequency Hopping 60 Frequency Original signal Power 40 Frequency Time Frequency Hopping signal Power Frequency Carrier frequency is changed in certain time intervals in accordance to a code sequence (synchronous change of the frequency by sender and receivers) Frequency hops of the signal in fixed times of approx ms Collisions are possible, if two or more senders use by coincidence the same frequency. Therefore suitable codes must be used. Interference are limited to short periods, simple implementation Not as robust as DSSS, easier to tap Page 3

32 Frequency Hopping Principle: Vary the carrier frequency in discrete levels: Level sequences are determined by pseudo random sequence Receiver must use identical sequence Two categories regarding the number of transferred bits per level: Max. one bit: Fast Frequency Hopping Several bits: Slow Frequency Hopping user data modulator narrowband signal modulator spread signal received signal demodulator narrowband signal demodulator data transmitter frequency synthesizer hopping sequence frequency synthesizer hopping sequence receiver Page 32

33 Frequency Hopping Two variants: Fast change (fast hopping): several frequencies per bit Slow change (slow hopping): several bits per frequency Data Page 33

34 Space Division Multiple Access (SDMA) Space Multiplexing means: base stations serve a certain space (cell) only, different base stations have a distance large enough to supply different regions Mobile stations communicate only with the base station in range Advantages of a cell structure: The same frequencies can be used in different cells for different users, i.e. higher capacity, higher number of users possible Less transmission power More robust against break-down Propagation of signals is (relatively) easy to handle Problems: Fixed network needed for connecting base stations Handover (changing from one cell to another) necessary Interference with neighbored cells at cell borders avoid using the same frequencies in neighbored cells! Cell sizes from 300 m in cities to 35 km on the country side for GSM even less for higher frequencies, e.g m for Wireless LAN Page 34

35 Frequency Planning Frequency reuse only with a certain distance between the base stations Standard model using 7 frequencies: k3 k5 k2 k4 k6 k5 k k4 k3 k7 k k2 An area in which all frequencies are used, is called cluster 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 n More capacity in cells with more traffic Assignment can also be based on interference measurements Page 35

36 Frequency Planning f 3 f f 2 f 3 f 2 f f f 2 f 3 f 2 f 3 f 3 f f 2 3 cell cluster f f 3 f 3 f 3 f 2 f 7 7 cell cluster f 4 f 5 f f 3 f 2 f 6 f 7 f 3 f 2 f 4 f 5 f f 3 f 6 f 5 f 2 f 2 f 2 f 2 f f 3 h f 3 h f 3 h 2 h 2 h g 3 2 h g 3 g 3 g 3 f f g 2 g g 2 g g 3 3 cell cluster with 3 sector antennas Page 36

37 Cell Breathing Lehrstuhl für Informatik 4 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 Page 37

38 Comparison of 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 Page 38

39 Channel Access FDMA and simple TDMA are in general to inflexible for data communication CDMA is very complex DSSS and FHSS do no access control, but increase the robustness of a transmission Maybe we could use proofed mechanisms from data communication in fixed networks instead of or in combination with static mechanisms? Let s try for Ethernet: CSMA/CD Simply send if the medium is free, recognize if a collision occurs Problem in wireless networks The signal strength decreases at least quadratic with the distance CS/CD are used by the sender, but collisions occur at the receiver Possibly, the sender can t recognize this collision, i.e. CD fails Furthermore, CS can fail if a terminal is to far away (Hidden Station) Page 39

40 Hidden Station and Exposed Station Hidden Station 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 A B C Exposed Station B sends to A, C wants to send to D C has to wait CS signals that the medium is in use A is outside the radio range of C waiting is not necessary! C is exposed to B A B C D Page 40

41 Further Problem with CSMA/CD: Power Control Terminals A and B send, C receives The signal strength decreases proportionally with the square of the distance Therefore, the signal from terminal B drowns out A s signal C is not able to receive A Precise power control needed! A B C But: is it possible to design variants for wireless networks? Page 4

42 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 Slotted Aloha Collision t Sender A Sender B Sender C t Page 42

43 DAMA - Demand Assigned Multiple Access Channel efficiency only 8% 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 Examples for reservation algorithms: Explicit Reservation according to Roberts (Reservation-ALOHA) Implicit Reservation (PRMA) Reservation-TDMA Page 43

44 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) 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 Page 44

45 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 Time slot: ACDABA-F AC-ABA-- A---BAF- A---BAFD frame frame 2 frame 3 frame 4 frame 5 A C D A B A F A C A B A A B A F A B A F D A C E E B A F D t collision at reservation attempts Page 45

46 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 Page 46

47 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 IEEE802. as DFWMAC (Distributed Foundation Wireless MAC) Page 47

48 MACA Examples MACA avoids the problem of hidden stations A and C want to send to B A sends RTS first C waits after receiving CTS from B RTS CTS CTS A B C MACA avoids the problem of exposed terminals B wants to send to A, C to some other terminal C does not wait unnecessarily because it cannot receive CTS from A RTS CTS RTS A B C Page 48

49 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) 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 Page 49

50 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) Page 50

51 Lehrstuhl für Informatik 4 Other Mechanisms with Busy Tone Collision avoidance by Out-of-Band Signaling Use an additional channel for signaling information Busy Tone Multiple Access (BTMA) Each station hearing an ongoing transmission on the data channel sends a busy tone on the additional control channel All terminals in the range of 2 hops of a sending terminal will wait No hidden stations, but many exposed stations Receiver initiated Busy Tone Multiple Access (RI-BTMA) Only the receiver sends busy tone Nearly no exposed stations, but the busy tone only can be sent when the receiver has decoded the transmission request Wireless Collision Detect (WCD) Protocol Combination of BTMA and RI-BTMA: two types of busy tones First like BTMA: terminals send a busy tone collision detect After recognizing a transmission request, the receiver sends a feedback-tone, the other terminals stop the collision detect busy tone Page 5

52 SAMA - Spread Aloha Multiple Access 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 Collision Sender A Sender B 0 0 narrow band send for a shorter period with higher power Spread the signel, e.g. using the chipping sequence 00 ( CDMA without CD ) t Problem: find a chipping sequence with good characteristics! Page 52

53 Conclusion Lehrstuhl für Informatik 4 Layer Common modulation techniques GMSK, QPSK, QAM for all wireless networks Signal propagation and robustness depend on the frequency band Layer 2 Static access methods TDMA/FDMA/CDMA: suitable for voice transmission because fixed capacities can be assigned Dynamic access methods for data communication exist in several variants: with reservation, using special signaling packets, polling, busy tones on additional channels, Additionally: CDMA techniques (DSSS, FHSS) can increase the robustness of a transmission this can be combined with dynamic access methods! Page 53