ELEC6217 Wireless Transceiver Design and Implementation Assignment 2 - Introduction to OFDM and filtered OFDM https://secure.ecs.soton.ac.uk/notes/elec6217/ 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; 1 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); 2 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. 3 Wireless Transceiver Design and Implementation M. El-Hajjar,
[email protected] Time-Frequency Properties of Wireless Channels 4 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. 5 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. 6 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. 7 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 8 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 . 9 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. 10 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. 11
欢迎咨询51作业君