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CELLULAR COMMUNICATIONS
4. Modulation
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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
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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
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Radio Carrier
Single alternated waveform. If carries no information appears at
receiver:
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Amplitude Modulation(AM)
Change amplitude of the signal according to information
Simplest digital form is “on-off keying”(telegraph Morse code)
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Amplitude Modulation
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Fully modulated signal
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AM efficiency Carrier: w=2f 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:
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AM Energy usage
Fully modulated A=2M Energy at carrier and one of sideband is wasted 33% of the transmitted energy carries information
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Audio AM
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Frequency Modulation
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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
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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
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Digital Version of FM
Frequency Shift Keying (FSK)
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Phase Modulation
Another form of FM
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Binary Phase Shift Keying (BPSK)
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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
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Quadrature Amplitude Modulation
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16-QAM
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Circular 16-QAM
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Other QAMs
HSPA+ (aka high speed GSM+) is 64QAM HDTV is 256QAM ADSL 16/64 QAM
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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
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F F
Normal Signal Signal with Spread Spectrum
Spread Spectrum
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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
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Frequency Hopping
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Adaptive Frequency Hopping Don’t transmit on a bad
frequencies/channels Measure error rate on each channel
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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
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White Noise
Completely random signal, alternates widely
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Spectrum of white noise
Same average power at each frequency
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Filtered (Bandlimited) Noise
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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
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Example :DSSS with PN
Transmitter/Receiver should be able to generate same synchronized Pseudo Random Noise sequences
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DSSS-PN Receiver/Transmitter
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Spreading
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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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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. ...
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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
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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
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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)
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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
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OFDM
Select orthogonal carriers Reach maximum at different times Can pack close without much
interference More carriers within the same
bandwidth
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More on OFDM