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=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 Spread Spectrum F Normal Signal F Signal with 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 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 crosscorrelation 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