Wireless Transmission & Media Access

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1 Wireless Transmission & Media Access Signals and Signal Propagation Multiplexing Modulation Media Access 1 Significant parts of slides are based on original material by Prof. Dr.-Ing. Jochen Schiller, FU-Berlin

2 Signals 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 ϕ sine wave as special periodic signal for a carrier: s(t) = A t sin(2 π f t t + ϕ t ) 2

3 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 3

4 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) 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 4

5 Real World Examples 5

6 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 long term fading short term fading t 6

7 Time Division Multiplexing Statistical (temporal) alignment of multiply sourced packet streams Shared use of medium with one carrier at any time High throughput, efficient use of medium Synchronisation necessary Typical use: wired LANs 7

8 Frequency Division Multiplexing Frequency spectrum divided in smaller bands Any user (channel) gets a certain band for the whole time No coordination/synchronisation Works for analog signals as well Guarantied throughput, inefficient use of bandwidth Typical use: telephone system (standard 12 x Hz between 12 and 60 khz) User Frequency 8 Time

9 Optical: Wavelength Division Multiplexing Colours of different sources are added to different carrier frequencies and combined Subtype of frequency division multiplexing Allows combined use of fibers Processing in pure optical components Spectrum: λ + λ + λ + λ Fiber 1 Fiber 2 Fiber 3 Fiber 4 Combiner Splitter 9

10 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 k 1 k 2 k 3 k 4 k 5 k 6 c f t 10

11 Multiplexing in Wirless Networks Additional dimension: Space division multiplexing Attempts to combine multiplexing technologies Goal: multiple use of a (limited) shared medium Example: Time and frequency multiplexing in GSM Caveat: precise coordination/guarding of spaces required 11

12 Modulation and demodulation digital data digital modulation analog baseband signal analog modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data radio receiver radio carrier Sinus baseband signal: s(t) = A sin(2 π ft + ϕ) 12

13 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 spread interference f protection against narrowband interference Side effects: coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence, Frequency Hopping f 13

14 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 necessary t b 0 1 t c t b : bit period t c : chip period user data XOR chipping sequence = resulting signal 14

15 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 15

16 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 16

17 Frequency planning 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 17

18 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 18

19 Media Access How does the shared media air differ? Example: Think of CSMA/CD But: Signal propagation is different Signal strength decreases proportional to the square of distance Senders may not hear or drown each other Collisions occur at receiver, CS & CD at the sender 19 Different access methods needed (taken from multiplexing techs): SDMA (Space Division Multiple Access) FDMA (Frequency Division Multiple Access) TDMA (Time Division Multiple Access) CDMA (Code Division Multiple Access)

20 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 receive the collision (CD fails) A is hidden for C 20 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 A B C

21 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 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! 21

22 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) 22

23 MACA examples 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 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 23

24 MACA variant: DFWMAC in IEEE sender receiver idle idle ACK RxBusy time-out NAK; RTS packet ready to send; RTS wait for the right to send CTS; data time-out; RTS data; ACK time-out data; NAK RTS; CTS wait for ACK wait for data ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy RTS; RxBusy 24

25 Access method CDMA 25 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

26 Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea Terminals Signal separation Advantages Disadvantages Comment 26 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

Chapter 3 : Media Access. Mobile Communications. Collision avoidance, MACA

Chapter 3 : Media Access. Mobile Communications. Collision avoidance, MACA Mobile Communications Chapter 3 : Media Access Motivation Collision avoidance, MACA SDMA, FDMA, TDMA Polling Aloha CDMA Reservation schemes SAMA Comparison Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/