CSE 4215/5431: Mobile Communications Winter 2010 Suprakash Datta datta@cs.yorku.ca Office: CSEB 3043 Phone: 416-736-2100 ext 77875 Course page: http://www.cs.yorku.ca/course/4215 Some slides are adapted from the book website and that of the text by Stallings 1/18/2019 CSE 4215, Winter 2010

Next Channel effects (e.g. fading), noise Spread spectrum Cellular system basics 1/18/2019 CSE 4215, Winter 2010

Effect of Noise On upper layer designs: Time variability of loss-rates Value of loss rates Effect on radio range Effect on medium access control Effect on real-time designs Many others 1/18/2019 CSE 4215, Winter 2010

Categories of Noise Thermal Noise Other noise Intermodulation noise Crosstalk Impulse Noise 1/18/2019 CSE 4215, Winter 2010

Thermal Noise Thermal noise due to agitation of electrons Present in all electronic devices and transmission media Cannot be eliminated Function of temperature Particularly significant for satellite communication 1/18/2019 CSE 4215, Winter 2010

Thermal Noise Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: N0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann's constant = 1.3803 ´ 10-23 J/K T = temperature, in Kelvin (absolute temperature) 1/18/2019 CSE 4215, Winter 2010

Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz (in watts): or, in decibel-watts 1/18/2019 CSE 4215, Winter 2010

Other Noise Intermodulation noise – occurs if signals with different frequencies share the same medium Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies Crosstalk – unwanted coupling between signal paths Impulse noise – irregular pulses or noise spikes Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults and flaws in the communications system 1/18/2019 CSE 4215, Winter 2010

Expression Eb/N0 Ratio of signal energy per bit Eb to noise power density per Hertz (N0) The bit error rate for digital data is a function of Eb/N0 Given a value for Eb/N0 to achieve a desired error rate, parameters of this formula can be selected As bit rate R increases, transmitted signal power must increase to maintain required Eb/N0 S=signal power R= bit rate 1/18/2019 CSE 4215, Winter 2010

Fading channels Fading: Time variation of received signal power Mobility makes the problem of modeling fading difficult Multipath propagation is a key reason Most challenging technical problem for Mobile Communications 1/18/2019 CSE 4215, Winter 2010

Types of Fading Short term (fast) fading Long term (slow) fading Flat fading – across all frequencies Selective fading – only in some frequencies Rayleigh fading – no LOS path, many other paths Rician fading – LOS path plus many other paths 1/18/2019 CSE 4215, Winter 2010

Fading models 1/18/2019 CSE 4215, Winter 2010

Dealing with fading channels Error correction Adaptive equalization attempts to increase signal power as needed can be done with analog circuits or DSP 1/18/2019 CSE 4215, Winter 2010

Spread Spectrum What? Why? How? 1/18/2019 CSE 4215, Winter 2010

Spread spectrum technology Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 Spread spectrum technology Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code protection against narrow band interference Side effects: coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence, Frequency Hopping signal power interference spread signal power spread interference detection at receiver f f 1/18/2019 CSE 4215, Winter 2010 Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 15

Effects of spreading and interference Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 Effects of spreading and interference dP/df dP/df user signal broadband interference narrowband interference i) ii) f f sender dP/df dP/df dP/df iii) iv) v) f f f receiver 1/18/2019 CSE 4215, Winter 2010 Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 16

Spreading and frequency selective fading Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 Spreading and frequency selective fading channel quality 2 1 5 6 narrowband channels 3 4 frequency narrow band signal guard space 2 frequency channel quality 1 spread spectrum spread spectrum channels 1/18/2019 CSE 4215, Winter 2010 Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 17

Spread Spectrum 1/18/2019 CSE 4215, Winter 2010

Spread Spectrum - sender Input is fed into a channel encoder Produces analog signal with narrow bandwidth Signal is further modulated using sequence of digits Spreading code or spreading sequence Generated by pseudonoise, or pseudo-random number generator Effect of modulation is to increase bandwidth of signal to be transmitted 1/18/2019 CSE 4215, Winter 2010

Spread Spectrum - receiver At the receiving end, digit sequence is used to demodulate the spread spectrum signal Signal is fed into a channel decoder to recover data 1/18/2019 CSE 4215, Winter 2010

Frequency Hoping Spread Spectrum (FHSS) Signal is broadcast over seemingly random series of radio frequencies A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth of input signal Signal hops from frequency to frequency at fixed intervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval, a new carrier frequency is selected 1/18/2019 CSE 4215, Winter 2010

FHSS - contd Channel sequence dictated by spreading code Receiver, hopping between frequencies in synchronization with transmitter, picks up message Advantages Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed only at knocking out a few bits 1/18/2019 CSE 4215, Winter 2010

FHSS - illustration 1/18/2019 CSE 4215, Winter 2010

FHSS details - 1 Discrete changes of carrier frequency Two versions Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 FHSS details - 1 Discrete changes of carrier frequency sequence of frequency changes determined via pseudo random number sequence Two versions Fast Hopping: several frequencies per user bit Slow Hopping: several user bits per frequency Advantages frequency selective fading and interference limited to short period simple implementation uses only small portion of spectrum at any time Disadvantages not as robust as DSSS simpler to detect 1/18/2019 CSE 4215, Winter 2010 Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 24

FHSS - iIIustration tb: bit period td: dwell time tb user data 1 1 1 t Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 FHSS - iIIustration tb user data 1 1 1 t f td f3 slow hopping (3 bits/hop) f2 f1 t td f f3 fast hopping (3 hops/bit) f2 f1 t tb: bit period td: dwell time 1/18/2019 CSE 4215, Winter 2010 Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 25

FHSS Performance Considerations Large number of frequencies used Results in a system that is quite resistant to jamming Jammer must jam all frequencies With fixed power, this reduces the jamming power in any one frequency band 1/18/2019 CSE 4215, Winter 2010

FHSS and Retransmissions What happens when a packet is corrupt and has to be retransmitted? IEEE 802.11: max time of each hop: 400ms, max packet length: 30 ms. 1/18/2019 CSE 4215, Winter 2010

FHSS and WLAN access points IEEE 802.11 FHSS WLAN specifies 78 hopping channels separated by 1 MHz in 3 groups (0,3,6,9,…, 75), (1,4,7,…, 76), (2,5,8,…,77) Allows installation of 3 AP’s in the same area. 1/18/2019 CSE 4215, Winter 2010

Direct Sequence Spread Spectrum (DSSS) Each bit in original signal is represented by multiple bits in the transmitted signal Spreading code spreads signal across a wider frequency band Spread is in direct proportion to number of bits used One technique combines digital information stream with the spreading code bit stream using exclusive-OR 1/18/2019 CSE 4215, Winter 2010

DSSS illustration

DSSS Using BPSK

Spectral view of DSSS 1/18/2019 CSE 4215, Winter 2010

Code-Division Multiple Access (CDMA) Basic Principles of CDMA D = rate of data signal Break each bit into k chips Chips are a user-specific fixed pattern Chip data rate of new channel = kD 1/18/2019 CSE 4215, Winter 2010

CDMA Example If k=6 and code is a sequence of 1s and -1s For a ‘1’ bit, A sends code as chip pattern <c1, c2, c3, c4, c5, c6> For a ‘0’ bit, A sends complement of code <-c1, -c2, -c3, -c4, -c5, -c6> Receiver knows sender’s code and performs electronic decode function <d1, d2, d3, d4, d5, d6> = received chip pattern <c1, c2, c3, c4, c5, c6> = sender’s code 1/18/2019 CSE 4215, Winter 2010

CDMA Example User A code = <1, –1, –1, 1, –1, 1> To send a 1 bit = <1, –1, –1, 1, –1, 1> To send a 0 bit = <–1, 1, 1, –1, 1, –1> User B code = <1, 1, –1, – 1, 1, 1> To send a 1 bit = <1, 1, –1, –1, 1, 1> Receiver receiving with A’s code (A’s code) x (received chip pattern) User A ‘1’ bit: 6 -> 1 User A ‘0’ bit: -6 -> 0 User B ‘1’ bit: 0 -> unwanted signal ignored 1/18/2019 CSE 4215, Winter 2010

CDMA for DSSS