程序代写案例-ELEC6217-Assignment 2

ELEC6217 Wireless Transceiver Design and Implementation
Assignment 2 - Introduction to OFDM and
filtered OFDM
https://secure.ecs.soton.ac.uk/notes/e
lec6217/
Mohammed El-Hajjar
Bdg. 53, Room 4011
Tel.: 27032
email: [email protected]
Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
Multicarrier Modulations - Motivation
• In broadband wireless communications, signals transmitted over channels experience
frequency-selective fading, which results in strong inter-symbol interference;
• When single-carrier based transmission schemes are employed, receiver needs to
implement a powerful equalizer for attaining desirable performance;
• However, this kind of equalizer usually has a high-complexity of implementation;
• Multicarrier based transmission is capable of efficiently mitigating this problem, and
provides the flexibilities for attaining other benefits;
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Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
Multicarrier Modulations - Motivation
• In multicarrier (MC) modulation, a transmitted bitstream is divided into many
different substreams, which are sent in parallel over many subchannels;
• The parallel subchannels are typically orthogonal under ideal propagation conditions;
• The data rate on each of the subcarriers is much lower than the total data rate;
• The bandwidth of subchannels is usually much less than the coherence bandwidth
of the wireless channel, so that the subchannels experience flat fading. Thus, the
ISI on each subchannel is small;
• MC modulation can be efficiently implemented digitally using the FFT (Fast
Fourier Transform) techniques, yielding the so-called orthogonal frequency division
multiplexing (OFDM);
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Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
Multicarrier Modulations - Useful Concepts
• Delay spread, Tm: The difference between the time of arrival of the earliest
significant multipath component and the time of arrival of the last multipath
components.
• Channel’s coherence bandwidth, Bc: Bc = 1Tm.
• Frequency-selective fading: Assume that the bandwidth of transmit signals is
W. Then, if W ≥ Bc, then the transmitted signals experience frequency-selective
fading. However, if W < Bc (usually W << Bc), the transmitted signals experience
frequency non-selective fading.
• In the time-domain, if Tm << 1/W , then the transmitted signals experience
frequency non-selective fading. While, if Tm ≥ 1/W , then the transmitted signals
experience frequency-selective fading.
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Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
Time-Frequency Properties of Wireless Channels
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Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
OFDM
Orthogonal frequency division multiplexing (OFDM) applies multicarrier modulation
principle, dividing the data stream into several bit streams, each of which has much
lower bit rate, and using these substreams to modulate several carriers.
• Consider a linearly modulated system with data rate R and bandwidth B;
• The coherence bandwidth of the channel is assumed to be Bc < B, so signals
transmitted over this channel experience frequency-selective fading.
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Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
OFDM
When employing the Multicarrier modulations:
• The bandwidth B is broken into N subbands, each of which has a bandwidth
BN = B/N for conveying a data rate RN = R/N ;
• Usually, it is designed to make BN << Bc, so that the subchannels experience
(frequency non-selective) flat fading.
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Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
OFDM
What is OFDM good for?
1- Combating fading: in a “parallel” transmission, each symbol in a subcarrier has
a much larger symbol duration, equal to N times of the symbol duration in “serial”
transmission.
In a deep fade, several symbols in the single carrier system can be affected seriously
and lost completely. However, in parallel transmission, each of the N symbols is only
slightly affected and can still be recovered correctly.
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Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
OFDM
2- Combating frequency selective: Channel can be severely frequency selective, but
for each sub-carrier, the sub-channel is flat or at least only slightly frequency selective
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Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
OFDM Transceiver
OFDM transmitter/receiver: Let transmitted frequency frame be [S1, S1, · · · , SN−1]
using N subcarriers and the transmitted time frame be [s1, s1, · · · , sN−1]
From discrete Fourier theory: {sm}N−1m=0 ↔ {Sn}N−1n=0 .
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Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
Cyclic Prefix
• If the channel is ideal, at receiver, from N time samples, N frequency samples can
be recovered via FFT.
• If the channel is dispersive, say, the CIR length is NhTs, then the transmitted
length of N time symbols will spread to a length of Nh + N , and N frequency
samples is insufficient.
• A solution is to add some dummy symbols to make it N +Nh frequency samples
or cyclic extension.
• An equivalent and more efficient alternative is to add cyclic extension in a
transmitted time frame as follows: The last Nh time samples is copied back
to the beginning of the frame, and transmitted samples are N +Nh.
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Wireless Transceiver Design and Implementation M. El-Hajjar, [email protected]
Coursework Instructions
• In this part of Assignment 2, you are required to understand the concept of OFDM
and explain it briefly in your report.
• Also, you are expected to read about filtered OFDM (f-OFDM) used in 5G and write
a summary of your understanding. The following papers are good references for
you to understand the concept of f-OFDM: https://arxiv.org/pdf/1508.07387.pdf
and https://arxiv.org/pdf/1807.10371.pdf.
• In your description of f-OFDM, you are expected to explain the basic idea of
f-OFDM and the motivation to use f-OFDM and compare f-OFDM with OFDM.
• Finally, you should simulate an OFDM system using BPSK modulation, 64
subcarriers and 4-tap channel impulse response. Here, you are expected to
plot the BER versus SNR for a variable cyclic prefix length and analyse your results.
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