Cellular Communications

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