Multiplexing The combining of two or more

Multiplexing • The combining of two or more information channels onto a common transmission medium. • Basic forms of multiplexing: – Frequency-division multiplexing (FDM). – Time-division multiplexing (TDM) – Code-division multiplexing (CDM) 28/01/2003 1

FDM • Frequency Division Multiplexing – The deriving of two or more simultaneous, continuous channels from a transmission medium by assigning a separate portion of the available frequency spectrum to each of the individual channels. • FDMA (frequency-division multiple access): The use of frequency division to provide multiple and simultaneous transmissions. 28/01/2003 2

• Transmission is organized in frequency channels, assigned for an exclusive use by a single user at a time • If the channel is not in use, it remains idle and cannot be used by others • There are channeling frequency plans elaborated to avoid mutual co-channel and adjacent-channel interference among neighboring stations • The use of a radio channel or a group of radio channels requires authorization (license) – for each individual station or for group of stations 28/01/2003 3

FDM (Frequency Division Multiplexing) Frequency FDMA Bc Tim e Power y F u req c en Bm Frequency channel Time Example: Telephony Bm = 3 -9 k. Hz 28/01/2003 4

FDD • Frequency division duplexing – 2 radio frequency channels for each duplex link (1 up-link & 1 down-link or 1 forward link and 1 reverse link) 28/01/2003 5

Transmitter Emissions • Transmitter output components Real Output spectrum Ideal – Fundamental (wanted) signal – Harmonic emissions – Master oscillator (fundamental & harmonics) – Non-harmonically related spurious – Noise Frequency (relative) 28/01/2003 6

Receiver response • Fundamental channel • Spurious channels Response Ideal – Intermediate frequency – Image frequency – Channels received via LO harmonics – Intermodulation channels Frequency 28/01/2003 7

Intermodulation • 2 or more signals, nonlinear circuit • Intermodulation products: Fi = SCk. Fk • {Ck} positive/negative integers or zero • {Fk} frequencies of signals applied • Order of Intermod. Product = S|Ck| • 3 rd order (2 F 1 -F 2, 2 F 2 -F 1), also 5 th and 7 th 28/01/2003 8

• Non-ideal wideband linear systems are frequently treated by expressing the output (Y) of the system as a power series: • where X is the total input signal, and the coefficients a are presumed to be real and independent on X. • Assume, for simplicity, that input consists of three elementary signals: 28/01/2003 9

• Some simple calculations will show that the output of the system Y, in addition to the linearly transposed input signals, contains the following spectral components: • 1 st order Multiplied version of the input signal 28/01/2003 10

• 2 nd order • a) Distorted version of the modulating signals • b) 2 nd harmonics • c) Sum and difference 28/01/2003 11

3 rd order a) Distorted modulating signal b) 3 rd harmonics c) Crossmodulation d) Intermodulation 28/01/2003 12

Non-essential channels F Y 28/01/2003 X 13

F-D Separation Concept 28/01/2003 14

Theoretical Cells & Cell Clusters Various combinations possible 28/01/2003 15

Distance separation Frequency and Distance Separation acceptable L+FDR= Separation unacceptable Frequency separation 28/01/2003 24

F-D Separation: 1 D (Line) 1 2 1 2 Separation by (reuse distance) 1 zone 2 channels 1 2 3 1 2 zones 3 channels n zones (n +1) channels 28/01/2003 25

F-D Separation: 2 D (Surface) Reuse distance = 1 4 channels Reuse distance = 2 9 channels n zones (n +1)2 channels 2 28/01/2003 26

Cell clusters 7 3 Various combinations possible 28/01/2003 27

F-D Separation: 3 D (Space) 4 >8 4 1 zone 8 channels 9 9 > 27 9 2 zone 27 channels n zones (n+1)3 channels 28/01/2003 28

Ideal Lattices • • • Bound-less, regular, plane lattice Each station located at a node All nodes occupied (no "holes") All stations identical (omnidirectional) Uniform propagation (no terrain obstacles) • Uniform EM environment • One set of channels regularly re-used 28/01/2003 29

Equipment deficiency: example Spectrum “blocked” by typical UHF-TV terrestrial transmitter due to receiver’s deficiencies (“FCC Taboos”) Area * No. of channels ideal: 1% co-ch: 23% other: 77% 28/01/2003 31 Dixon 64

F F F 1 2 N Sub-Ch 1 Sub-Ch 2 Sub-Ch N Demodulation Signal Processing Serial-to-Parallel Converter OFDM Digital Modulation Delogne P, Bellanger M: The Impact of Signal Processing on an Efficient Use of the Spectrum, Radio Science Bulletin June 1999, 23 -28 Le. Floch B, Alard M, Berrou C: Coded Orthogonal Frequency Division Multiplex, Proc of IEEE June 1995, 982 -996 28/01/2003 • Orthogonal Frequency Division Multiplexing (OFDM) – The channel is split into a number of sub-channels – Each sub-channel transmits a part of the original information – Each sub-channel adjusted to its environment (S/N) – Reduces multipath & selective fading – Allows for higher speeds – Requires smart signal processing – Used in 802. 11 a(USA), DTTB(Eu), Hyperplan(Eu), Power Line Coms. standards. 32

TDM • Time Division Multiplex: A single carrier frequency channel is shared by a number of users, one after another. Transmission is organized in repetitive “time-frames”. Each frame consists of groups of pulses - time slots. • Each user is assigned a separate time-slot. • TDD – Time Division Duplex provides the forward and reverse links in the same frequency channel. 28/01/2003 33

Frequency TDM Power density Time-frame Tim e y c en u eq Fr Time slot Time Example: DECT (Digital enhanced cordless phone) Frame lasts 10 ms, consists of 24 time slots (each 417 s) 28/01/2003 34

SDM • Space Division Multiple Access controls the radiated energy for each user in space using directive antennas – Sectorized antennas – Adaptive antennas 28/01/2003 35

CDMA or SS • Code Division Multiple Access or Spread Spectrum communication techniques – FH: frequency hoping (frequency synthesizer controlled by pseudo-random sequence of numbers) – DS: direct sequence (pseudo-random sequence of pulses used for spreading) – TH: time hoping (spreading achieved by randomly spacing transmitted pulses) – Other techniques • Hybrid combination of the above techniques (radar and other applications) • Random noise as carrier 28/01/2003 36

Frequency CDMA - FH SS Power density CDMA Bm Bc Tim e Time-frequency slot 28/01/2003 Time y c en u req F Transmission is organized in timefrequency “slots”. Each link is assigned a sequence of the slots, according to a specific code. Used e. g. in Bluetooth system 37

DS SS communications basics Original information Spreading Original signal Propagation effects Spread signal Transmission Unwanted signals + Noise Reconstructed information De-spreading Spread signal+ 28/01/2003 Reconstr. signal 38

SS: basic characteristics • Signal spread over a wide bandwidth >> minimum bandwidth necessary to transmit information • Spreading by means of a code independent of the data • Data recovered by de-spreading the signal with a synchronous replica of the reference code – TR: transmitted reference (separate data-channel and reference-channel, correlation detector) – SR: stored reference (independent generation at T & R pseudo-random identical waveforms, synchronization by signal received, correlation detector) – Other (MT: T-signal generated by pulsing a matched filter having long, pseudo-randomly 28/01/2003 controlled impulse response. Signal detection at R by identical filter & correlation computation) 39

DS SS: transmitter Modulator [A(t), (t)] Information Carrier cos( 0 t) X Antenna [g 1(t)] Modulated signal S 1(t) = A(t) cos( 0 t + (t)) band Bm Hz Spread signal g 1(t)S 1(t) band Bc Hz Bc >> Bm gi(t): pseudo-random noise (PN) spreading functions that spreads the energy of S 1(t) over a bandwidth considerably wider than that of S 1(t): ideally gi(t) gj(t) = 1 if i = j and gi(t) gj(t) = 0 if i j 28/01/2003 40

antenna Linear combination g 1(t)S 1(t) g 2(t)S 2(t) ……. gn(t)Sn(t) N(t) (noise) S’(t) 28/01/2003 X To demodulator DS SS-receiver Correlator & bandpass filter g 1(t)S 1(t) Spreading g (t)S (t) 1 function ……. 2 2 [g 1(t)] g (t)S (t) 1 n n g 1(t) N(t) g 1(t) S’(t) S 1(t) 41

SS-receiver’s Input Unwanted signals W/Hz SS s. : g 2(t)S 2(t); …; gn(t)Sn(t) Other s. : S’(t) Noise: N(t) Wanted (spread) signal: g 1(t)S 1(t) Bc Hz Signal-to-interference ratio (S/ I)in = S/ [I( )*Bc] 28/01/2003 Bc = I( ) = S= Input correlator bandwidth Average spectral power density of unwanted signals in Bc Power of the wanted signal 42

SS-correlator/ filter output Wanted (correlated) signal: de-spread to its original bandwidth as g 1(t)S 1(t) = S 1(t) with g 1(t) = 1 Bm Uncorrelated (unwanted) signals spread & rejected by correlator + noise g 1(t) S’(t); g 1(t) N(t); g 1(t) gj(t)Sj(t) = 0 as gi(t) gj(t) = 0 for i j Signal-to-interference ratio (S/ I)out = S/ [I( )*Bm] Bc Bc = Input correlator bandwidth Bm = Output filter bandwidth I( ) = Average spectral power density of unwanted signals & noise in Bm S = power of the wanted signal at the correlator output Spreading = reducing spectral power density 28/01/2003 43

SS Processing Gain = = [(S/ I)in/ (S/ I)out ] = ~Bc/ Bm Example: GPS signal RF bandwidth Bc ~ 2 MHz Filter bandwidth Bm ~ 100 Hz Processing gain ~20’ 000 (+43 d. B) Input S/N = -20 d. B Output S/N = +23 d. B (signal power = 1% of noise power) (signal power = 200 x noise power) (GPS = Global Positioning System) 28/01/2003 44

SS systems attributes (1) • Low spectral density of the signal – LPI: low probability of intercept – LPPF: low probability of position fix – LPSE: low probability of signal exploitation – Privacy – Covert operations capabilities – Low interference potential 28/01/2003 45

SS systems attributes (2) • • AJ: anti-jamming/ anti-interference capability Security Natural cryptographic capabilities Multiple-user random access communications with selective addressing (CDMA) • High time resolution (~1/B; multi-path suppression) 28/01/2003 46

Summary • To illustrtae the nature of the multiple access techniques consider a number of guests at a cocktail party. The aim is for all the guests to hold an intelligible conversation. In this case the resource available is the house itself • FDMA: each guest has a separate room to talk to their partner • TDMA: everyone is in a common room and has a limited time slot to hold the conversation • FH-CDMA: the guests run from room to talk • DS-CDMA: everyone is in a common room talkim at the same time, but each pair talks in a different language 28/01/2003 47

Access Control to Radio Resources • Distributed wireless networks (e. g. packet radio, ad hoc networks) have no central control. • Centralized wireless networks (e. g. WLAN, Cellular) control the use of radio channel; various approaches exist • Slotted systems (e. g. TDMA) require wide network synchronization for use of discrete time slots 28/01/2003 48

Packet Radio Protocols 28/01/2003 49

Packet Radio • In packet radio access techniques, many user attempt to access a single channel, which may led to collisions. • Protocols aim at limiting collisions • ALOHA is the oldest, classic protocol, developed in 1970 in Hawaii as an extension of TDMA and FDMA 28/01/2003 50

ALOHA • If 2 or more users transmit at the same time so that receiver receives more than one packet, the receiver is unable to separate the packets since they are not orthogonal in time (like in TDMA) or in frequency (like in FDMA). • The vulnerable period is the time interval during which the packets are susceptible to collisions with transmissions from other users Packet B Transmitter 1 Packet A Transmitter 2 t 1 28/01/2003 Packet C T 1+2 51

• In pure ALOHA, the vulnerable period is 2 packet durations. A user transmits whenever it has a packet to deliver. If no acknowledgment (ACK) is received, the user waits a random time and retransmit the packet. The throughput is T = Re-2 R, R being the normalized channel traffic in Erlangs (Tmax = 0. 184 at R = 0. 5) • In slotted ALOHA, time is divided into equal time slots of length greater than the packet duration. The users have synchronized clocks and transmit messages only at the beginning of a new time slot. This prevent partial collisions where one packet collides with a portion of another. The vulnerable period is only one packet duration. The throughput is T = Re-R (Tmax = 0. 368 at R = 1) • ALOHA protocols do not listen to the channel before transmission, and do not exploit information about the other users. 28/01/2003 52

• Carrier Sense Multiple Access (CSMA) protocols base on monitoring the channel. If the channel is idle (no carrier is detected), then the user is allowed to transmit. Important are detection delay and propagation delay. • Reservation protocols – certain packet slots are assigned with priority 28/01/2003 53

References • Coreira LM, Wireless Flexible Personalized Communications, J Wiley • Dunlop J, Smith DG, Telecommunication Engineering, Chapman & Hall • Reed JH, Software Radio, Prentice Hall • Taub H, Shilling DL, Principles of Communication Systems, Mc. Graw Hill 28/01/2003 54