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Cellular Communications

Cellular Communications. 4. Modulation. 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

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Cellular Communications

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  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 • 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 802.11n)

  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

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