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MULTIPLEXING Space, Frequency, Time, Code

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Multiplexing

q Multiplexing: Ø Goal: multiple use of a shared medium

q Multiplexing in 4 dimensions: Ø Space (s), Frequency (f), Time (t), Code (c)

q Task: Ø Assign space, time, frequency, code to each communication channel Ø Minimize interference § Using “guard spaces” Ø Maximize medium utilization

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Space Division Multiplex

channels k q A separate sender for each i

communication channel k1 k2 k3 k4 k5 k6 Ø With wide enough distance c between senders t c § Guard spaces t q Used by FM radio stations s 1 f Ø Many stations around the s 2 f world use same frequency c without interference t q What if several radio

stations want to broadcast s3 in same city? f

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Frequency Division Multiplex

q Subdivide the frequency dimension into several non- overlapping frequency bands Ø Senders can use their assigned band continuously

q Used for radio stations within same region

q Pros: Ø No dynamic coordination k1 k2 k3 k4 k5 k6

q Cons: c Ø Waste of bandwidth if f usage is non-uniform Ø Limits # of senders Ø Inflexible assignment t 25

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Time Division Multiplex

q A sender gets the whole bandwidth for a certain amount of time

q Advantages Ø Only one carrier in the medium at any time Ø Throughput high even for many users

q Disadvantages

Ø Precise synchronization k1 k2 k3 k4 k5 k6 necessary c f

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Frequency and Time Multiplex

q Combination: Ø A sender can use a frequency band for a certain amount of time Ø Used by GSM

q Pros: Ø (weak) protection against tapping Ø protection against frequency- k k k k k k selective interference 1 2 3 4 5 6 c q Cons: f Ø Precise coordination required

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Code Division Multiplex

k1 k2 k3 k4 k5 k6 q Each channel has a unique code Ø All channels use same spectrum at same time c Ø “guard spaces” realized by using orthogonal code

q Pros: Ø Good protection against interference f and tapping Ø Bandwidth efficient

q Cons:

Ø Relatively complex receiver t Ø Precise synchronization required Ø Precise power control required § All signals should reach receiver with nearly equal strength

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DIGITAL MODULATION Amplitude, Frequency, Phase

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Modulation

analog baseband digital signal data digital analog 101101001 modulation modulation

radio Radio Transmitter carrier

analog baseband digital signal analog synchronization data demodulation decision 101101001

radio carrier Radio Receiver

q Digital modulation: Ø Digital data is translated into an analog (baseband) signal

q Analog modulation: Ø Shifts center frequency up to that of radio carrier 30

Analog Modulation analog digital baseband data digital signal analog 101101001 modulation modulation radio Radio Transmitter carrier

q Why analog modulation?

Ø Smaller carrier wavelength for smaller antennas

Ø To enable FDM

Ø To exploit/avoid frequency-dependent medium characteristics

q 3 basic schemes: AM (amplitude), FM (frequency), PM (phase) 31

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Digital Modulation

q Modulation of digital signals known as Shift Keying

q Issues:

Ø Spectral efficiency: § How efficiently is the frequency spectrum utilized

Ø Power efficiency: § How much power is needed to transfer bits § Very important for portable devices

Ø Robustness to multi-path propagation, noise, interference

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Digital Modulation Methods

1 0 1 q Amplitude Shift Keying Ø Binary values are represented by different amplitudes t Ø Low bandwidth requirements Ø Very susceptible to interference § Used only in directed infra-red beams 1 0 1

q Frequency Shift Keying Ø Needs larger bandwidth t Ø Less susceptible to errors

q Phase Shift Keying 1 0 1 Ø Uses shifts in the signal phase § 180o, if change in data Ø Synchronization required (freq, phase) t Ø Most resistant to interference Ø Complex receiver/transmitter 33

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Advanced Frequency Shift Keying

q Minimum Shift Keying (MSK): avoids abrupt phase changes Ø Bits durations doubled and divided into even, odd bits § Depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen Ø Higher frequency is twice the lower frequency

1 0 1 1 0 1 0 data bit even 0 1 0 1 even bits odd 0 0 1 1

odd bits signal h n n h value - - + +

low h: high frequency frequency n: low frequency +: original signal -: inverted signal high frequency

MSK signal t 34

Advanced Phase Shift Keying Q o q Binary PSK: phase shift of 180 I Ø Very simple, but low efficiency 1 0

o q Quadrature PSK: phase shift of 45 10 Q 11 Ø Codes two bits into one phase shift § Relative to phase of a reference signal I – Requires frequent synchronization § Relative to phase of previous two bits 00 01 – More efficient and less complex Ø Achieves high bit-rates for same bandwidth

A q Can be extended to: Ø More angles Ø Combine with ASK t

Ø Leads to more receiver complexity 11 10 00 01

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Multi-carrier modulation

q Splits high bit-rate stream into many with lower bit-rates Ø Each is sent using an independent carrier frequency

q Pros: Ø Independent frequencies do not interfere with each other Ø Frequency-selective fading doesn’t influence whole signal Ø Guard spaces used between symbol (groups) § Helps handle multi-path propagation, ISI mitigation Ø Computationally efficient based on FFT

q DAB uses 192-1536 sub-carriers

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