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  At the receiver the information is removed from the radio signal  Information is reconstructed into original format through in a process of demodulation

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

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

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

Amplitude Modulation

Fully modulated signal

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

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

Audio AM

Frequency Modulation

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

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

Digital Version of FM  Frequency Shift Keying (FSK)

Phase Modulation  Another form of FM

Binary Phase Shift Keying (BPSK)

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

Quadrature Amplitude Modulation

16-QAM

Circular 16-QAM

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

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

FF Normal SignalSignal with Spread Spectrum Spread Spectrum

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

Frequency Hopping

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

Spread Spectrum Illustration

Transmit 3 Pictures to 3 Destinations

XOR each image with a mask

Result

Add Them Up and send to all dest.

Each recipient decodes using his mask

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

White Noise  Completely random signal, alternates widely

Spectrum of white noise  Same average power at each frequency

Filtered (Bandlimited) Noise

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

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

DSSS-PN Receiver/Transmitter

Spreading

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

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....

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

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

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)

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

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

More on OFDM