1. TLEN 5830 Wireless Systems Lecture Slides 28-Sep-2017
Signal Modulation and Encoding (continued)

2. Additional reference materials
Required Textbook: Antennas and Propagation for Wireless Communication Systems, by Simon R. Saunders and Alejandro Aragon-Zavala, ISBN ; March 2007 (2nd edition). Optional References: Wireless Communications and Networks, by William Stallings, ISBN , 2002 (1st edition); Wireless Communication Networks and Systems, by Corey Beard & William Stallings (1st edition); all material copyright 2016 Wireless Communications Principles and Practice, by Theodore S. Rappaport, ISBN (2nd edition) Acknowledgements:

3. Signal Encoding Criteria
What determines how successful a receiver will be in interpreting an incoming signal?
Signal-to-noise ratio (or better Eb/N0)
Data rate
Bandwidth
An increase in data rate increases bit error rate
An increase in SNR decreases bit error rate
An increase in bandwidth allows an increase in data rate
Importantly, another factor can be utilized to improve performance
and that is the encoding scheme

4. Factors Used to Compare Encoding Schemes
Signal spectrum
With lack of high-frequency components, less bandwidth required
Clocking
Ease of determining beginning and end of each bit position
Signal interference and noise immunity
Certain codes exhibit superior performance in the presence of noise (usually expressed in terms of a BER)
Cost and complexity
The higher the signal rate to achieve a given data rate, the greater the cost

5. Basic Encoding Techniques
Digital data to analog signal
Amplitude-shift keying (ASK)
Amplitude difference of carrier frequency
Frequency-shift keying (FSK)
Frequency difference near carrier frequency
Phase-shift keying (PSK)
Phase of carrier signal shifted

6. Modulation of Analog Signals for Digital Data

7. Phase-Shift Keying (PSK)
Two-level PSK (BPSK)
Uses two phases to represent binary digits

8. Phase-Shift Keying (PSK)
Differential PSK (DPSK)
Phase shift with reference to previous bit
Binary 0 – signal burst of same phase as previous signal burst
Binary 1 – signal burst of opposite phase to previous signal burst

9. Differential Phase-Shift Keying

10. Quadrature Phase-Shift Keying (PSK)
Four-level PSK (QPSK)
Each element represents more than one bit

11. QPSK Constellation Diagram

12. QPSK and OQPSK Modulators

13. Phase-Shift Keying (PSK)
Multilevel PSK
Using multiple phase angles with each angle having more than one amplitude, multiple signal elements can be achieved
D = modulation rate, baud or symbols/sec
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element

14. Performance Bandwidth of modulated signal (BT) ASK, PSK BT = (1+r)R
FSK BT = 2Δf+(1+r)R
R = bit rate
0 < r < 1; related to how signal is filtered
Δf = f2 – fc = fc - f1

15. Performance Bandwidth of modulated signal (BT) MPSK MFSK
L = number of bits encoded per signal element
M = number of different signal elements

16. Bit Error rate (BER)
Performance must be assessed in the presence of noise
“Bit error probability” is probably a clearer term
BER is not a rate in bits/sec, but rather a probability
Commonly plotted on a log scale in the y-axis and Eb/N0 in dB on the x-axis
As Eb/N0 increases, BER drops
Curves to the lower left have better performance
Lower BER at the same Eb/N0
Lower Eb/N0 for the same BER

17. Theoretical Bit Error Rate for Various Encoding Schemes

18. Theoretical Bit Error Rate for Multilevel FSK, PSK, and QAM

19. Quadrature Amplitude Modulation
QAM is a combination of ASK and PSK
Two different signals sent simultaneously on the same carrier frequency

20. QAM Modulator

21. 16-QAM Constellation Diagram

22. Spread Spectrum 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

23. General Model of Spread Spectrum Digital Communication System

24. Spread Spectrum
On receiving end, digital sequence is used to demodulate the spread spectrum signal
Signal is fed into a channel decoder to recover data

25. Spread Spectrum 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

26. Frequency Hoping Spread Spectrum (FHSS)
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

27. Frequency Hopping Example

28. Frequency Hopping Spread Spectrum System

29. 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

30. FHSS Using MFSK
MFSK signal is translated to a new frequency every Tc seconds by 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: Ts = LT seconds
Tc ≥ Ts - slow-frequency-hop spread spectrum
Tc < Ts - fast-frequency-hop spread spectrum

31. Slow-Frequency-Hop Spread Spectrum Using MFSK 1M = 4, k = 22

32. Frequency-Hop Spread Spectrum Using MFSK 1M = 4, k = 22

33. 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

34. SPREAD SPECTRUM – a little history FYI
Spread Spectrum is now usually associated with digital communications using radio. (Note that wireless = radio for this course as we are talking about basic technologies, not commercial offerings.)
Spread Spectrum techniques can be employed for analog communications over guided media, although most spread spectrum systems are via radio.

35. SPREAD SPECTRUM
The general concept of many communications schemes— including SS—is simple but profound: we can trade bandwidth for noise reduction. We are familiar with this concept by the use of 200 kHz channels for FM broadcasting 15 kHz stereo audio and for that matter using digital techniques to require a 64 kbps link for a 3 kHz telephone call.
The original use of the SS techniques was to make eavesdropping and/or locating military radio communications extremely difficult. But the immunity to other similar signals noise as well as to single frequency (non-spread signal) interference was immediately manifest.
The original refinement of these techniques was to completely obscured the existence of the communications to conventional narrow band receivers.

36. Three Basic Spread Spectrum Techniques and Purposes
PRIMARY PRACTICAL ATTRIBUTES
1. Frequency Hopping
Unable to be received other than by intended recipients
Effectively eliminates single frequency interference and/or jamming
Reciprocal from outlook of single channel users
Used with analog and digital communications
Signals are not completely occult (hidden).
2. Direct Sequence
Encrypts digital data at very high level
Significant noise reduction from other spectrum users, including discrete channels as well as other DSCC users.
Also reciprocal to channelized radio signals
3. Code Division Multiple Access
Specific type of DSSS scheme widely used specifically for multiplexing multiple users rather than for encryption or for noise reduction

37. SPREAD SPECTRUM TECHNIQUES - Frequency Hopping Spread Spectrum (FHSS)
This is a very old technique dating back to the 1940s (WW II) and earlier but said to be invented by movie star Hedy Lamarr*.
FHSS is applicable to analog radio communications and groups of bits of digital data, even one bit—or less—at a time modulating a radio carrier.
The transmitter and receiver simply jump (hop) in synchronism between a number of different narrow band frequency—conventional AM or FM—channels.
The channel occupancy of the result can be over many channels or even entire frequency bands, e.g. the 3 – 30 MHz short wave band.
The participating transmitter and receiver need to have the exact correct sequence of frequencies and be synchronized; other receivers tuned to single channels can detect only occasional very small segments of the communications--rendering eavesdropping virtually impossible.
The existence of the communications may be discerned due to the detection of "blips" of RF energy on any given channel.
*A most fascinating story.

38. 1. Frequency Hopping Spread Spectrum was invented during WW II by movie star Hedy Lamarr.

39. THE INVENTION OF FREQUENCY HOPPING STREAD SPECTRUM
The invention of spread spectrum is quite a story involving Nazis, a future Hollywood star, attempted murders, a number of druggings (more than one), a brothel (perhaps more than one), a Hollywood mogul (decidedly one), a player piano, and a piano player (a particular one).

40. Picture: millikansbend.org
Original spread spectrum scheme used perforated paper reels similar to those used in player pianos to control secure synchronous transmitter and receiver frequency hopping. Besides preventing eavesdropping, frequency hopping minimized detection by enemy direction finders to locate clandestine transmitters.

41. Two pages of drawings from Lamarr (name on patent is Markey, the name of Hedy’s 2nd of 6 husbands) and Antheil's patent*.
Note the player-piano-like slotted paper on the second sheet at right below.

42. FREQUENCY HOPPING SPREAD SPECTRUM
Points to Remember
Single frequency jamming and/or multipath distortion that occurs only on a few of the channels used have only minimal effects on the bit error rates. This takes a little explaining. The idea is that only syllables are lost for voice transmission where humans can “fill in the missing data”, hopefully correctly. For digital transmission, error correcting can apply. Bits do get lost unless retransmissions and/or FEC is used.
The hopping sequence must be “pseudo random” otherwise interference would be periodic and decodable. This is very important for digital transmission employing FHSS.
Pseudo Random means complicated enough to have a very long periodicity (if at all) and with statistics that appear to mimic those of true random distributions.
The effect of FHSS is to minimize interference or detection of the transmitting location. That a “hopper” is near-by can often be determined.

43. FHSS Continued…
When used with digital communications, the pseudo random hopping sequence will ensure that the bit errors will occur in what appears to be a random way. This is more easily handled by error correction operating at the data link or higher layers.
BERs will be low if enough "good" channels are employed in the sequence: bad if many of the channels are in use.
Frequency Hopping is also a multiplex and encryption technique.
Background noise (for analog signals) or BER (for digital transmission) increase as the number of users increase. The general result of many users is similar to having a high random noise within the “channel”.
If the occupancy time of any given in-use channel is very low--a fraction of a second--intelligible communications can still occur or BER is tolerable when FEC schemes are used.
FHSS is widely used for 2.4 GHz cordless telephones and was part of the original IEEE 802 wireless LAN standard.

45. Direct Sequence Spread Spectrum (DSSS)
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 (Figure 9.6)

46. Example of Direct Sequence Spread Spectrum

47. Direct Sequence Spread Spectrum System

48. DSSS Using BPSK Multiply BPSK signal,
sd(t) = A d(t) cos(2π fct)
by c(t) [takes values +1, -1] to get
s(t) = A d(t)c(t) cos(2π fct)
A = amplitude of signal
fc = carrier frequency
d(t) = discrete function [+1, -1]
At receiver, incoming signal multiplied by c(t)
Since, c(t) x c(t) = 1, incoming signal is recovered

49. Example of Direct Sequence Spread Spectrum Using BPSK

50. Approximate Spectrum of Direct Sequence Spread Spectrum Signal

51. 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

52. CDMA Example If k=6 and code is a sequence of 1s and -1s
For a ‘1’ bit, A sends code as chip pattern
<c1, c2, c3, c4, c5, c6>
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
<d1, d2, d3, d4, d5, d6> = received chip pattern
<c1, c2, c3, c4, c5, c6> = sender’s code

53. CDMA Example

54. CDMA Example 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

55. CDMA in a DSSS Environment

56. Rake receiver (“digital multipath”)
Multiple versions of a signal arrive more than one chip interval apart
RAKE receiver attempts to recover signals from multiple paths and combine them
This method achieves better performance than simply recovering dominant signal and treating remaining signals as noise

57. Principle of RAKE Receiver

58. Categories of Spreading Sequences
Spreading Sequence Categories
PN sequences
Orthogonal codes
For FHSS systems
PN sequences most common
For DSSS systems not employing CDMA
For DSSS CDMA systems

59. 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

60. Additional reference materials
Required Textbook: Antennas and Propagation for Wireless Communication Systems, by Simon R. Saunders and Alejandro Aragon-Zavala, ISBN ; March 2007 (2nd edition). Optional References: Wireless Communications and Networks, by William Stallings, ISBN , 2002 (1st edition); Wireless Communication Networks and Systems, by Corey Beard & William Stallings (1st edition); all material copyright 2016 Wireless Communications Principles and Practice, by Theodore S. Rappaport, ISBN (2nd edition)