Multipath n Ofdm

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Satellite Communications

ELEM026 Professor Clive Parini

Lecture 6 Multipath & OFDM 2011

THE PROBLEM OF MULTIPATH DELAY SPREAD

Multipath delay spread

•  An impulse is transmitted at time t=0, assuming there are a multitude of reflected paths present a receiver located say 1Km away would detect a series of pulses, or delay spread.

Time,t µs

0 1 2 3 4 5

power

Δt

Direct path

Useful symbol duration Tu

Intersymbol Interference (ISI) •  If Δt is significant compared to one symbol period

then ISI can occur. –  Symbols arriving later than their own symbol period can

corrupt trailing symbols •  For a fixed path difference and given delay spread a

higher data rate system will be more prone to ISI.

–  For a GSM system operating at 270Kbit/s will the delay spread shown previously cause ISI?

–  Symbol period = 1/270K = 3.7µs –  Answer is ?

EQUALISATION-1 •  ISI can be overcome using Equalisation

–  In its most simplest form ISI in a channel has resulted from the addition of the data stream plus a delayed version of this data stream.

–  The principle of EQUALISATION is that by taking this received signal, delaying part of it, and subtracting it from itself the original signal can be recovered.

–  Take for example the decision feedback equaliser •  First need to know what reflections there were and what the signal

strength was from each of these. i.e. it needs to determine

Time,t µs

0 1 2 3 4 5

power

Δt

EQUALISATION-e.g. GSM

•  Knowledge about the channels multipath delay spread is obtained by sending periodically a blip out to the mobile. –  To work, the channel needs to send nothing for a

moment, then send the blip and then another wait period. The mobile then receives the multipath delay spread.

–  This blip must be frequently sent since even a slight movement can change the multipath in a channel.

–  For GSM a blip is sent to each mobile every 4ms –  In practice a blip is not sent (too sharp leading to wide

spectral range) –  Instead a special binary sequence called the channel

sounding sequence is sent

CDMA & multipath •  CDMA is well matched to a multipath channel.

–  If signals arrive more than one chip apart from each other the receiver can resolve them. The cross-correlation between the spreading code and a copy of it delayed by one chip is very near to zero. Hence multipath is treated like any other interfering channel. But there is more……

•  Instead of ignoring these delayed versions of the desired signal they can be received with a delayed spreading code and combined. This is the RAKE receiver.

CDMA RAKE receiver –  Consider a channel receiving a direct signal of

amplitude a1 and delay t1 plus two multipath signals having amplitudes a2, a3 and delays t2, t3.

Binary data

Code generator

modulator

t1 a1

t2 a2

t3 a3

Multipath channel

De- modulator

a1

a2

a3

RAKE receiver C(t-t1)

C(t-t2)

C(t-t3)

Multipath delays less than one chip cannot be recovered

Intersymbol interference (ISI) and the multicarrier approach

•  The problem lies in the fact that over the transmission bandwidth (determined by the symbol rate) the channel frequency characteristics are non linear.

•  We could reduce the symbol rate so channel characteristics more linear over the bandwidth

•  If we had an 8 bit word to send we could reduce the symbol rate by a factor of 8 BUT use 8 different sub carriers as shown

B Pulse length ~1/B

– Data are transmited over only one carrier

Channel

Guard bands NEEDED

Channelization

N carriers

B Pulse length ~ N/B

Similar to FDM technique

– Data are shared among several carriers and simultaneously transmitted

Modulation techniques: monocarrier vs. multicarrier

To improve the spectral efficiency:

To use orthogonal carriers (allowing overlapping) Eliminate band guards between carriers

– Selective Fading

– Very short pulses

– ISI is compartively long

– Poor spectral efficiency because of band guards

Drawbacks

– It is easy to exploit Frequency diversity

– Flat Fading per carrier

– N long pulses

– ISI is comparatively short

– Poor spectral efficiency because of band guards

Advantages Furthermore

– It allows to deploy 2D coding techniques

– Dynamic signalling

N carriers

B Pulse length ~ N/B

Similar to FDM technique

– Data are shared among several carriers and simultaneously transmitted

B Pulse length ~1/B

– Data are transmited over only one carrier

Channel

Guard bands

Channelization

ORTHOGONAL FREQUENCY DIVISON MULTIPLEX – OFDM – A digital multicarrier modulation method

•  Orthogonal frequency-division multiplexing (OFDM), also sometimes called discrete multitone modulation (DMT), is based upon the principle of frequency division FDM, but is utilized as a digital modulation scheme.

•  The bit stream that is to be transmitted is split into several parallel bit streams, typically dozens to thousands. "

•  The available frequency spectrum is divided into several sub-channels, and each low-rate bit stream is transmitted over one sub-channel by modulating a sub-carrier using a standard modulation scheme, for example QPSK"

•  The sub-carrier frequencies are chosen so that the modulated data streams are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated."

Orthogonality •  Orthogonality requires that the sub-carrier spacing is Δf=k/Tu Hz, where Tu seconds is the useful symbol duration (the receiver side window size), and k is a positive integer. (often=1)

•  Thus, with N sub-carriers, the total pass-band bandwidth will be B ≈ N·Δf (Hz).

•  Example: A useful symbol duration TU = 1 ms. –  N = 1,000 sub-carriers would result in a total passband

bandwidth of NΔf = 1 MHz. –  For this symbol time, the required bandwidth in theory

according to Nyquist is N/2TU = 0.5 MHz (i.e., half of the achieved bandwidth required by this method).

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OFDM –the principal

Data coded in frequency domain

N carriers

B

Transformation to time domain: each frequency is a sine wave in time, all added up.

f

Transmit

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Symbol: 8 periods of f0




+

Receive time

B

Decode each frequency bin separately


Channel frequency response

f

f

Time-domain signal Frequency-domain signal

OFDM uses multiple carriers to modulate the data

N carriers

B

Modulation technique

A user utilizes all carriers to transmit its data as coded quantity at each frequency carrier, which can be BPSK or QPSK.

Intercarrier Separation = k/(symbol duration)

– No intercarrier guard bands – Controlled overlapping of bands – Maximum spectral efficiency (Nyquist rate)

– Very sensitive to freq. synchronization – Easy implementation using IFFTs

Features

Data

Carrier

T=1/f 0 Time

f 0 B

Freq

uenc

y

One OFDM symbol

Time-frequency grid

• Spectrum for a single BPSK signal modulated with random signal with nulls at symbol rate

• For 8 bit code word use 8 sub band frequencies spaced the symbol rate apart and modulate each one with one bit of the word using BPSK. Spectrum is shown, note at each sub carrier frequency no interference from other sub carriers -they are orthogonal

• This gives the following time response for the symbol 10110001

Recovered phase of sub carriers (blue), sent (black). It can be seen that the orthogonality is maintained and all that is required for correct decoding is to equalize the phase shift.

OFDM: simple 8bit BPSK example

OFDM TRANSMITTER

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EG For BPSK, the map is +1, -1; for QPSK the map is 1+j1, 1-j1, -1-j1, -1+j1

UP CONVERT ONTO RF CARRIERS (SINE AND COSINE WAVES)

•  Rather than use 8 separate BPSK modulators can create the time domain symbol using constellation mapping plus IFFT.

OFDM RECEIVER

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DOWN CONVERTED TO BASEBAND SINE AND COSINE WAVES

USE FFT TO GET BACK TO FREQUENCY DOMAIN

CONVERT BPSK OR QPSK CONSTELATION INTO PARELLEL BIT STREAMS AND RESTORE TO SINGLE HIGH DATA RATE SERIAL DIGITAL SIGNAL

e.g. for QPSK: 1+j1 converts to 11 1-j1 converts to 10 -1-j1 converts to 01 -1+j1 coverts to 00

OFDM SUMMARY •  ISI limits the symbol time, so for data rate R, symbol period is

Ts=1/R. •  By splitting data into N streams, each sub-stream has rate R/N

and symbol time of N/R, i.e its N times longer and so is more immune to ISI.

•  As a design criteria N is chosen such that NTs = Tu significantly greater than rms delay spread of channel.

•  Typical system : –  64 sub channels QPSK modulated –  Each channel symbol rate=0.25Mps –  48 subcarriers devoted to information transmission –  4 subcarriers used for pilot tone (synchronisation) –  12 for other purposes –  Occupied BW=20MHz, 312.5Khz/subchannel –  Usable data rate =12Mbs –  Subchannel symbol duration=4000ns –  Guard time between 2 transmitted symbols =800ns

OFDM issues •  Since the duration of each symbol is long, it is feasible to insert a

guard interval between the OFDM symbols, thus eliminating any intersymbol interference.

•  OFDM requires very accurate frequency synchronization between the receiver and the transmitter; with frequency deviation the sub-carriers will no longer be orthogonal, causing inter-carrier interference (ICI) (i.e., cross-talk between the sub-carriers).

•  Frequency offsets are typically caused by mismatched transmitter and receiver oscillators, or by Doppler shift due to movement.

•  While Doppler shift alone may be compensated for by the receiver, the situation is worsened when combined with multipath, as reflections will appear at various frequency offsets, which is much harder to correct.

•  This effect typically worsens as speed increases and is an important factor limiting the use of OFDM in high-speed vehicles. 19

OFDM applications

•  DAB - OFDM forms the basis for the Digital Audio Broadcasting (DAB) standard in the European market.

•  ADSL - OFDM forms the basis for the global ADSL (asymmetric digital subscriber line) standard.

•  UWB

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Multipath n Ofdm