300952
Wireless and Mobile Networks
Lecture 6. OFDM and Spread Spectrum
Lecture objectives
• Understand the concept and application of orthogonality
• Explain the concepts of OFDM, OFDMA, and SC-FDMA
• Understand the concept and operation of spread spectrum
• Understand the operation of two major forms of spread spectrum: FHSS and DSSS
• Understand how spread spectrum is applied in CDMA
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ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING
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Introduction
• Orthogonal Frequency Division Multiplexing (OFDM) created great expansion in wireless networks
– Greater efficiency in bps/Hz
• Main air interface in the change from 3G to 4G
– Also expanded 802.11 rates
• Critical technology for broadband wireless access
– WiMAX
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How OFDM works
• Also called multicarrier modulation
• Start with a data stream of R bps – Could be sent with bandwidth Nfb – With bit duration 1/R
• OFDM splits into N parallel data streams
– Called subcarriers
– Each with bandwidth fb
– And data rate R/N (bit time N/R)
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Orthogonality (1)
• The spacing of the fb frequencies allows tight packing of signals – Actually with overlap between the signals
– Signals at spacing of fb ,2fb, 3fb ,etc.
• The choice of fb is related to the bit rate to make the signals orthogonal
– Average over bit time of s1(t) × s2(t) = 0
– Receiver is able to extract only the s1(t) signal • If there is no corruption in the frequency spacing
• Traditional FDM makes signals completely avoid frequency overlap – OFDM allows overlap which greatly increases capacity
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Orthogonality (2)
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Orthogonality (2)
• Given an OFDM subcarrier bit time of T – fb must be a multiple of 1/T
• Example: IEEE 802.11n wireless LAN – 20 MHz total bandwidth
– 52 data subcarriers
– fb = 0.3125 MHz
– Signal is translated to 2.4 GHz or 5 GHz bands
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Benefits of OFDM
• Frequency selective fading only affects some subcarriers – Can easily be handled with a forward error-correcting code
• More importantly, OFDM overcomes intersymbol interference (ISI)
– ISI is a caused by multipath signals arriving in later bits
• See Lecture 4
– OFDM bit times are much, much longer (by a factor of N)
• ISI is dramatically reduced
– N is chosen so the root-mean-square delay spread is significantly smaller than
the OFDM bit time
– It may not be necessary to deploy equalizers to overcome ISI • Eliminates the use of these complex and expensive devices.
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OFDM Implementation
• Inverse Fast Fourier Transform (IFFT)
– The OFDM concept would use N oscillators for N different subcarrier frequencies
• Expensive for transmitter and receiver
– Discrete Fourier Transform (DFT) processes digital signals
• If N is a power of two, the computational speed dramatically improves by using the fast version of the DFT (FFT).
– Transmitter takes a symbol from each subcarrier • Makes an OFDM symbol
• Uses the Inverse FFT to compute the data stream to be transmitted • OFDM symbol provides the weights for each subcarrier
• Then it is sent on the carrier using only one oscillator
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IFFT Implementation of OFDM
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Cyclic prefix
• OFDM’s long bit times eliminate most of the ISI
• OFDM also uses a cyclic prefix (CP) to overcome the residual ISI
– Adds additional time to the OFDM symbol before the real data is sent • Called the guard interval
• ISI diminishes before the data starts
– Data from the end of the OFDM symbol is used as the CP • Simplifies the computations
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Cyclic Prefix Diagram
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Difficulties of OFDM (1)
• Peak-to-averagepowerratio(PAPR)
– For OFDM signals, this ratio is much higher than for single-carrier signals
– OFDM signal is a sum of many subcarrier signals • Total can be very high or very low
• Power amplifiers need to amplify all amplitudes equally Vout =KVin
– Should have a linear characteristic with slope K on a V out
– Yet practical amplifiers have limited linear ranges • Causing distortion if outside the linear range
vs. V
in
curve
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Ideal and Practical Amplifier Characteristics
Examples of Linear and Nonlinear Amplifier Output
Difficulties of OFDM (2)
• PAPR problem (continued)
– Expensive amplifiers have wide linear range
• Solutions
– Could reduce the peak amplitude
• Called input backoff
• But this would increase the signal to interference plus noise ratio (SINR) – Noise and interference would be relatively stronger because signal is weaker
– Specific PAPR reduction techniques can be used • Specialized coding, phase adjustments, clipping, etc.
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Difficulties of OFDM (3)
• IntercarrierInterference(ICI)
– OFDM frequencies are spaced very precisely
– Channel impairments can corrupt this
– Cyclic prefix helps reduce ICI
• But CP time should be limited so as to improve spectral efficiency • A certain level of ICI may be tolerated to have smaller CPs
– Doppler spread, mismatched oscillators, or even one subcarrier can cause ICI
• Spacing between subcarriers may need to be increased
• Could also use different pulse shapes, self-interference cancellation, or frequency domain equalizers.
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OFDMA
• Orthogonal Frequency Division Multiple Access (OFDMA) uses OFDM to share the wireless channel
– Different users can have different slices of time and different groups of subcarriers
– Subcarriers are allocated in groups
• Calledsubchannelsorresourceblocks
• Too much computation to allocate every subcarrier separately
• Subchannel allocation
– Adjacent subcarriers – similar SINR
• Must measure to find the best subchannel
– Regularly spaced subcarriers – diverse SINR
– Randomly space subcarriers – diverse SINR and reduced adjacent-cell interference
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OFDM and OFDMA
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Opportunistic scheduling
• Schedule subchannels and power levels based on – Channel conditions
– Data requirements
• Adjust in a dynamic fashion
– Use channel variations as an opportunity to schedule the best choice in users
• Hence the term opportunistic scheduling
– Criteria (maybe more than one used simultaneously)
• System efficiency – pick users with best throughput
• Fairness – proportional fairness considers the ratio of users’ current rates to the users’ average rates to know when a channel is best for them
• Requirements–audio,video
• Priority – public safety, emergency, or priority customers
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Single-carrier FDMA
• SC-FDMA has similar structure and performance to OFDMA – But lower PAPR
– Mobile user benefits – battery life, power efficiency, lower cost
– Good for uplinks
• Uses extra DFT operation and frequency equalization compared to OFDM – DFT prior to IFFT
– Spreads data symbols over all subcarriers
– Every data symbol is carried by every subcarrier
• Multiple access is not possible
– At one time, all subcarriers must be dedicated to one user – Multiple access is provided by using different time slots
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Example of OFDMA and SC-FDMA
SPREAD SPECTRUM
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Introduction
• Different from OFDM, it can be used to transmit analog or digital data using an analog signal
• Developed initially for military and intelligence – Makes jamming and interception difficult
• Two types were developed – Frequency hopping
– Direct sequence
– Both are used in various wireless standards and products
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Spread Spectrum (1)
• Input is fed into a channel encoder
– Produces analog signal with narrow bandwidth
• Signal is further modulated using sequence of digits
– Spreading code or spreading sequence
– Generated by pseudonoise, or pseudo-random number generator
• Effect of modulation is to increase bandwidth of signal to be transmitted
• On receiving end, digital sequence is used to demodulate the spread
spectrum signal
• Signal is fed into a channel decoder to recover data
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Spread Spectrum (2)
• What can be gained from apparent waste of spectrum? – Immunity from various kinds of noise and multipath distortion – Can be used for hiding and encrypting signals
– Several users can independently use the same higher bandwidth with very little interference
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Frequency Hoping Spread Spectrum (FHSS) (1)
• Signal is broadcast over seemingly random series of radio frequencies – A number of channels allocated for the FH signal
– Width of each channel corresponds to bandwidth of input signal
• Signal hops from frequency to frequency at fixed intervals – Transmitter operates in one channel at a time
– Bits are transmitted using some encoding scheme
– At each successive interval, a new carrier frequency is selected
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Frequency Hoping Spread Spectrum (FHSS) (2)
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Frequency Hoping Spread Spectrum
• Channel sequence dictated by spreading code
• Receiver, hopping between frequencies in synchronization with
transmitter, picks up message
• Advantages
– Eavesdroppers hear only unintelligible blips
– Attempts to jam signal on one frequency succeed only at knocking out a few bits
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FHSS Using MFSK
• MFSK signal is translated to a new frequency every T seconds by c
modulating the MFSK signal with the FHSS carrier signal
• For data rate of R:
– duration of a bit: T = 1/R seconds
– duration of signal element: T = LT seconds s
• T ≥ T – slow-frequency-hop spread spectrum cs
• T < T - fast-frequency-hop spread spectrum cs
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FHSS Performance Considerations
• Large number of frequencies used
• Results in a system that is quite resistant to jamming
– Jammer must jam all frequencies
– With fixed power, this reduces the jamming power in any one frequency band
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Direct Sequence Spread Spectrum (DSSS) (1)
• Each bit in original signal is represented by multiple bits in the transmitted signal
• Spreading code spreads signal across a wider frequency band – Spread is in direct proportion to number of bits used
• One technique combines digital information stream with the spreading code bit stream using exclusive-OR
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Direct Sequence Spread Spectrum (DSSS) (2)
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Direct Sequence Spread Spectrum (DSSS) (3)
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Code-Division Multiple Access (CDMA)
• Basic Principles of CDMA
– D = rate of data signal
– Break each bit into k chips
• Chips are a user-specific fixed pattern
– Chip data rate of new channel = kD
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CDMA Example (1)
• If k=6 and code is a sequence of 1s and -1s – For a ‘1’ bit, A sends code as chip pattern
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– For a ‘0’ bit, A sends complement of code
• <-c1, -c2, -c3, -c4, -c5, -c6>
• Receiver knows sender’s code and performs electronic decode function
Su(d)=d1c1+d2c2+d3c3+d4c4+d5c5+d6c6
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CDMA Example (2)
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CDMA Example (3)
• User A code = <1, –1, –1, 1, –1, 1>
– To send a 1 bit = <1, –1, –1, 1, –1, 1>
– To send a 0 bit = <–1, 1, 1, –1, 1, –1>
• User B code = <1, 1, –1, – 1, 1, 1>
– To send a 1 bit = <1, 1, –1, –1, 1, 1>
• Receiver receiving with A’s code
– (A’s code) x (received chip pattern)
• User A ‘1’ bit: 6 -> 1
• User A ‘0’ bit: -6 -> 0
• User B ‘1’ bit: 0 -> unwanted signal ignored
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CDMA in a DSSS Environment
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Categories of Spreading Sequences
• Spreading Sequence Categories – PN sequences
– Orthogonal codes
• For FHSS systems
– PN sequences most common
• For DSSS systems not employing CDMA – PN sequences most common
• For DSSS CDMA systems – PN sequences
– Orthogonal codes
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PN Sequences
• PN generator produces periodic sequence that appears to be random
• PN Sequences
– Generated by an algorithm using initial seed
– Sequence isn’t statistically random but will pass many test of randomness
– Sequences referred to as pseudorandom numbers or pseudonoise sequences – Unless algorithm and seed are known, the sequence is impractical to predict
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Orthogonal Codes
• A set of sequences in which all pairwise cross correlations are zero
– Cross-correlation: similarity between two series as a function of the displacement of one relative to the other
• Walsh codes are the most common orthogonal codes in CDMA
– Requires tight synchronization between all the users in the channel to avoid cross
correlation
– Synchronization to the accuracy of a small fraction of one chip
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Sources for this lecture
Cory Beard, William Stallings. Wireless Communication Networks and Systems, 1st edition. Pearson Higher Education, 2016
(Chapters 8 and 9)
All material copyright 2016
Cory Beard and William Stallings, All rights reserved
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