WIRELESS COMMUNICATION FUNDAMENTALS

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WIRELESS COMMUNICATION FUNDAMENTALS Wednesday, August 07, 2013 1

Wingdings 2:

Wireless communication fundamentals Multiplexing Spread Spectrum DSSS Cellular system CDMA Wednesday, August 07, 2013 2

Verdana:

Multiplexing Several users can share a medium with minimum or no interference. Four dimensions of multiplexing: Space division multiplexing, Frequency division multiplexing, Time division multiplexing, Code division multiplexing. Wednesday, August 07, 2013 3

Calibri:

Space Division Multiplexing Space is represented via circles indicating the interference range The space between the interference ranges is called guard space. Used in FM station Problem arise if two or more channels were established within the same space. Wednesday, August 07, 2013 4 s 2 s 3 s 1 f t c k 2 k 3 k 4 k 5 k 6 k 1 f t c f channels k i

Wingdings:

Frequency Division Multiplexing Subdivide frequency dimension into several non-overlapping frequency bands. Guard spaces are needed to avoid frequency band overlapping called adjacent channel interference. Wednesday, August 07, 2013 5 f t c k 2 k 3 k 4 k 5 k 6 k 1 f t c

Symbol:

Frequency Division Multiplexing (con...) This scheme is used for radio stations within the same region , where each radio station has its own frequency. The receiver only has to tune into the specific sender. Disadvantages : While radio stations broadcast 24 hours a day, mobile communication typically takes part for only a few minutes at a time. Assigning a separate frequency for each possible communication waste of frequency. Wednesday, August 07, 2013 6

Helvetica:

Time Division Multiplexing A channel is given the whole bandwidth for a certain amount of time Guard spaces - represents time gaps If two transmissions overlap in time - called co-channel interference. Advantages: Only one carrier in the medium at any time Throughput high even for many users Disadvantages: Precise synchronization necessary Wednesday, August 07, 2013 7 f t c k 2 k 3 k 4 k 5 k 6 k 1

Verve:

Frequency and Time Division Multiplexing Wednesday, August 07, 2013 8 f t c k 2 k 3 k 4 k 5 k 6 k 1 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 Compared to code multiplex Precise coordination required

WIRELESS COMMUNICATION FUNDAMENTALS:

Code Division Multiplexing Used in military applications due to its inherent security features . Guard spaces - ‘distance’ in code space. Ex : Orthogonal Codes . Advantages: Bandwidth efficient No coordination and Synchronization necessary Good protection against interference and tapping Disadvantages: Lower user data rates More complex for signal regeneration Implemented using spread spectrum technology Wednesday, August 07, 2013 9 k 2 k 3 k 4 k 5 k 6 k 1 f t c

Wireless communication fundamentals:

Spread Spectrum Spreading the bandwidth needed to transmit the data. Converts the narrowband signal into a broadband signal. Drawbacks of Spread Spectrum: Increased complexity of receivers Large frequency is needed Wednesday, August 07, 2013 10

Multiplexing:

Spread Spectrum Spreading and Dispreading Wednesday, August 07, 2013 11 dP/df f i) dP/df f ii) sender dP/df f iii) dP/df f iv) receiver f v) user signal broadband interference narrowband interference dP/df

Space Division Multiplexing:

Narrowband interference without spread spectrum and Spread Spectrum to avoid narrowband interference Wednesday, August 07, 2013 12 frequency channel quality 1 2 3 4 5 6 narrow band signal guard space 2 2 2 2 2 frequency channel quality 1 spread spectrum Narrowband interference without spread spectrum Spread spectrum to avoid narrowband interference

Frequency Division Multiplexing:

DSSS (Direct Sequence Spread Spectrum) Wednesday, August 07, 2013 13 user data chipping sequence resulting signal 0 1 0 1 1 0 1 0 1 0 1 0 0 1 1 1 XOR 0 1 1 0 0 1 0 1 1 0 1 0 0 1 = t b t c t b : bit period t c : chip period XOR of the signal with pseudo-random number (chipping sequence) Sequence 101101110000, a so-called barker code Advantages Chipping sequence Pseudo noise sequence Spreading factor 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

Frequency Division Multiplexing (con...):

DSSS (Direct Sequence Spread Spectrum) cont… Wednesday, August 07, 2013 14 X user data chipping sequence modulator radio carrier spread spectrum signal transmit signal DSSS Transmitter demodulator received signal radio carrier X chipping sequence Low pass filtered signal DSSS Receiver integrator products decision data sampled sums Correlator

Time Division Multiplexing:

FHSS (Frequency Hopping Spread Spectrum) 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 Disadvantages Not as robust as DSSS Simpler to detect Wednesday, August 07, 2013 15

Frequency and Time Division Multiplexing:

Slow and fast frequency hopping Wednesday, August 07, 2013 16 t user data slow hopping (3 bits/hop) fast hopping (3 hops/bit) 0 1 t b 0 1 1 t f f 1 f 2 f 3 t t d f f 1 f 2 f 3 t d t b : bit period t d : dwell time

Code Division Multiplexing:

FHSS Transmitter and Receiver Wednesday, August 07, 2013 17 modulator user data hopping sequence modulator narrowband signal spread transmit signal transmitter received signal receiver demodulator data frequency synthesizer hopping sequence demodulator frequency synthesizer narrowband signal

Spread Spectrum:

Cellular Systems Implements space division multiplex: base station covers a certain transmission area (cell) Mobile stations communicate only via the base station 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 Wednesday, August 07, 2013 18

Spread Spectrum Spreading and Dispreading:

Cellular system with three and seven cell clusters Wednesday, August 07, 2013 19 f 1 f 2 f 3 f 2 f 1 f 1 f 2 f 3 f 2 f 3 f 1 f 2 f 1 f 3 f 3 f 3 f 3 f 3 3 cell cluster 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 3 f 5 f 6 f 7 f 2 f 2 7 cell cluster Cells are combined in clusters Sectorized antennas can be used Borrowing channel allocation(BCA) Fixed channel allocation(FCA) Dynamic channel allocation(DCA) h 2 g 2 g 1 g 2 f 2 h 3 g 3 g 3 f 3 h 1 g 1 g 3 g 1 h 3 g 2 f 3 f 2 f 1 f 1 f 1 f 3 f 2 h 1 h 2 Cellular system with three cell clusters and three sectors per cell

Narrowband interference without spread spectrum and Spread Spectrum to avoid narrowband interference:

Cellular system Cond.. Advantages Higher capacity Less transmission Power Local interference only Robustness Disadvantages: Infrastructure needed Handover needed Frequency planning Wednesday, August 07, 2013 20

DSSS (Direct Sequence Spread Spectrum):

Cell Breathing depending on the current load Wednesday, August 07, 2013 21 CDM cells are commonly said to ‘breathe‘. 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

DSSS (Direct Sequence Spread Spectrum) cont…:

CDMA Code Division Multiplexing(CDMA) Multiple Access in CDMA: Each user is assigned a unique PN code. Each user transmits its information by spreading with unique code. Direct Sequence spread spectrum is used. Users are separated by code not by time slot and freq slot. Wednesday, August 07, 2013 22

FHSS (Frequency Hopping Spread Spectrum):

CDMA in theory Sender A Sends A d = 1, key A k = 010011 (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 = 110101 (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.) C=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 = 2 + 0 + 0 + 2 + 2 + 0 = 6 Result greater than 0, therefore, original bit was “1” Receiving B B e = (-2, 0, 0, -2, +2, 0)  B k = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. “0” Wednesday, August 07, 2013 23

Slow and fast frequency hopping:

CDMA on signal level I Wednesday, August 07, 2013 24 data A key A signal A data  key 1 0 1 0 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1 0 1 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0 A d A k A s Real systems use much longer keys resulting in a larger distance between single code words in code space. 1

FHSS Transmitter and Receiver:

CDMA on signal level II Wednesday, August 07, 2013 25 signal A data B key B key sequence B signal B A s + B s data  key 1 0 0 0 0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1 1 1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1 B d B k B s A s

Cellular Systems :

CDMA Advantages: All terminals can use the same frequency, no planning needed Huge code space (e.g. 2 32 ) compared to frequency space Interferences (e.g. white noise) is not coded Forward error correction and encryption can be easily integrated Disadvantages : Higher complexity of a receiver All signals should have the same strength at a receiver Wednesday, August 07, 2013 26

Cellular system with three and seven cell clusters:

References Jochen H. Schiller – “Mobile Communications” second edition Wednesday, August 07, 2013 27

Cellular system Cond..:

Wednesday, August 07, 2013 28

Cell Breathing depending on the current load:

Thank you Wednesday, August 07, 2013 29

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