1. Cellular Communications
4. Modulation

2. Modulation Radio signals can be used to carry information
Audio, data, video
Information is used to modify (modulate) a single frequency known as carrier
Modified(modulated) signal is transmitted to receiver
At the receiver the information is removed from the radio signal
Information is reconstructed into original format through in a process of demodulation

3. Some key points Spectrum is scarce Efficient use of energy
Spectrum is scarce natural resource.
There is only limited range of wavelength that can be used for communications
Regulated by government (FCC)
Modulation techniques should make effective use of spectrum, i.e. transmit as much as possible information using given amount of spectrum
Efficient use of energy
Mobile devices has limited battery
Transmitting unnecessary energy on a radio carrier may interfere with other transmitters
Reliably Transmit information with minimal possible amount of energy

4. Radio Carrier Single alternated waveform.
If carries no information appears at receiver:

5. Amplitude Modulation(AM)
Change amplitude of the signal according to information
Simplest digital form is “on-off keying”(telegraph Morse code)

6. Amplitude Modulation

7. Fully modulated signal

8. AM efficiency Carrier: w=2f Message: m(t), Signal y(t)=m(t)*c(t)
Let consider highest frequency in a message wc and its maximum/minimum amplitude M
Modulated Signal:
After some trigonometry:

9. AM Energy usage Fully modulated A=2M
Energy at carrier and one of sideband is wasted
33% of the transmitted energy carries information

10. Audio AM

11. Frequency Modulation

12. FM efficiency
Modulation index (max change in carrier frequency due to modulation): M
Bandwidth of FM signal is BW = 2 (M + 1 ) fm
fm maximum modulating frequency used
Energy efficiency increased by increasing bandwidth

13. AM vs FM FM is more resilient to noise
FM: signal level variation does not affect quality provided the signal is strong enough to recover its frequency
Used for 1G analogue mobile phone systems

14. Digital Version of FM
Frequency Shift Keying (FSK)

15. Phase Modulation
Another form of FM

16. Binary Phase Shift Keying (BPSK)

17. Quadrature Phase Shift Keying(QPSK)
BPSK, 180% change in phase represent change in bit
QPSK 90% change in phase represent change in 2 bit sequence

18. Quadrature Amplitude Modulation

19. 16-QAM

20. Circular 16-QAM

21. Other QAMs HSPA+ (aka high speed GSM+) is 64QAM HDTV is 256QAM
ADSL 16/64 QAM

22. Spread Spectrum Techniques
Conserve spectrum by keeping transmission as narrow as possible
Sometimes it’s beneficial to spread transmission over wide frequency range (spread spectrum)
Fading and noise might be different for different frequencies
Spreading over wide range of frequencies will help to reduce errors/signal noise
Spreading power over many frequencies result in very low power transmission at each frequency
Reduce interference to other transmitter , single frequency transmission appears as a noise

23. Spread Spectrum F F
Normal Signal
Signal with Spread Spectrum

24. Frequency Hopping
Transmitter sends a signal at each frequency during very short period of time
Transmit next piece of data on other frequency
Hop hundreds of time per second between different frequencies
To receive the signal, receiver must be able to follow the hop sequence of the transmitter
Both receiver and transmitter must know hop sequence and be synchronized in time

25. Frequency Hopping

26. Adaptive Frequency Hopping
Don’t transmit on a bad frequencies/channels
Measure error rate on each channel

27. Direct Sequence Spread Spectrum
AM/FM transmit around single carrier
Frequency Hopping transmit at wide range of carriers but one carrier at the time
DSSS transmit at wide range of carriers simultaneously
Very low power at each carrier
Appears as a noise at each carrier
Transmission across carriers is “synchronized” so signal can be recovered
Several transmissions on the same set of carriers(spectrum) as looks as noise for each other
Different transmissions use different “synchronization” methods/codes

28. White Noise
Completely random signal, alternates widely

29. Spectrum of white noise
Same average power at each frequency

30. Filtered (Bandlimited) Noise

31. How to make a carrier to look like band limited noise?
Make it look randomly alternating
Modulate it with randomly alternating signal (analog) or bits (digital)
Represent data that we want to transmit with a longer sequence of bits that “looks like random” (pseudo-random)
Use less time to modulate each bit (e.g. BPSK)
Transmit modulate rapidly alternating signal
Same total energy
Speeded over wide ranges of frequencies

32. Example :DSSS with PN
Transmitter/Receiver should be able to generate same synchronized Pseudo Random Noise sequences

33. DSSS-PN Receiver/Transmitter

34. Spreading

35. 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

36. Some Properties of PN sequences
Balance property
The number of "1"s in the sequence is one greater than the number of "0"s.
Run property: Of all the "runs" in the sequence of each type (i.e. runs consisting of "1"s and runs consisting of "0"s):
One half of the runs are of length 1.
One quarter of the runs are of length 2.
One eighth of the runs are of length 3.
... etc. ...

37. Autocorrelation property
Autocorrelation is large when signal/mask perfectly synchronized
Synchronization between rx/tx
Hopefully does not give a large peak when there is no signal

38. Orthogonal Sequences
Cross correlation: same as autocorrelation but among different sequences
Several different sequences with zero cross- correlation between them allow several transmissions at the same channel (“range of carriers”)
Base for Code Division Multiple Access method (CDMA)
3G/UMTS use version of CDMA(WCDMA)
Will talk about it later

39. Orthogonal Frequency Division Multiplex(OFDM)
OFDM/COFDM Used in
WiFi (802.11)
ADSL
WiMax 4G
More
Provide very high data rates (e.g. up to 150Mbps n)

40. Multichannel Communications
Transmit bits in parallel using several carriers (frequencies)
Transmission over each carriers take certain amount of bandwidth around this carrier
Carriers need to be separated from each other to avoid interference
Relatively small amounts of parallel transmissions can be fitted in a given spectrum

41. OFDM Select orthogonal carriers Reach maximum at different times
Can pack close without much interference
More carriers within the same bandwidth

42. More on OFDM