cellular communications
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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 - PowerPoint PPT PresentationTRANSCRIPT
CELLULAR COMMUNICATIONS4. 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=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:
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 ) fm
fm 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
F F
Normal Signal Signal 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 802.11n)
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