Chapter 9 Science and Technology Tutorials 2018/9/16 Peter Yeh

Multiple Access Frequency Division Multiple Access (FDMA) AMPS and CT2 Time Division Multiple Access (TDMA) Hybrid FDMA/TDMA Code Division Multiple Access a physical channel corresponds to a binary code 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

CDMA Each station has its own unique chip sequence (CS) All CS are pairwise orthogonal For example :(codes A, B, C and D are pairwise orthogonal) A: 00011011 => (-1-1-1+1+1-1+1+1) B: 00101110 => (-1-1+1-1+1+1+1-1) C: 01011100 => (-1+1-1+1+1+1-1-1) D: 01000010 => (-1+1-1-1-1-1+1-1) 2018/9/16 Peter Yeh

CDMA A·B = (1+1-1-1+1-1+1-1) = 0 B·C = (1-1-1-1+1+1-1+1) = 0 EX: if station C transmits 1 to station E, but station B transmits 0 and station A transmits 1 simultaneously then the signal received by station E will become S = (-1+1-3+3-1-1-1+1). E can convert the signal S to S·C = (1+1+3+3+1-1+1-1)/8 = 1 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Mobile Radio Signals Four Main Effects of Signals Attenuation that increase with distance Random variation due to environmental features Signal fluctuations due to the motion of a terminal Distortion due to the signal travel along different path from a transmitter to a receiver 2018/9/16 Peter Yeh

Attenuation Due to Distance the signal strength decreases with distance according to the relationship Preceive = Ptransmit const/x^ (In general,  = 2, 3 or 4) 2018/9/16 Peter Yeh

Slow (Shadow) Fading Random Environmental Effects As a terminal moves, the signal strength gradually rises and falls with significant changes occurring over tens of meters Sreceive = 10 log10(1000Preceive) dBm = Stransmit + const -10  log10(x) dBm The standard deviation of Sreceive is a quantity  dB (4 dB <=  <= 10 dB) 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

What is a Decibel- dB Decibel is the unit used to express relative differences in signal strength. It is expressed as the base 10 logarithm of the ratio of the powers of two signals: dB = 10 log (P1/P2) Logarithms are useful as the unit of measurement because signal power tends to span several orders of magnitude signal attenuation losses and gains can be expressed in terms of subtraction and addition 2018/9/16 Peter Yeh

For example: Suppose that a signal passes through two channels is first attenuated in the ratio of 20 and 7 on the second. The total signal degradation is the ratio of 140 to 1. Expressed in dB, this become 10 log 20 + 10 log 7 = 13.01 + 8.45 = 21.46 dB The following table helps to indicate the order of magnitude associated with dB: 1 dB attenuation means that 0.79 of the input power survives. 3 dB attenuation means that 0.5 of the input power survives. 10 dB attenuation means that 0.1 of the input power survives. 20 dB attenuation means that 0.01 of the input power survives. 30 dB attenuation means that 0.001 of the input power survives. 40 dB attenuation means that 0.0001 of the input power survives. 2018/9/16 Peter Yeh

Fast (Rayleigh) Fading Fast (Rayleigh) Fading Due to Motion of Terminals As the terminal moves, each ray undergoes a Doppler shift, causing the wavelength of the signal to either increase or decrease Doppler shifts in many rays arriving at the receiver cause the rays to arrive with different relative phase shifts At some locations, the rays reinforce each other. At other locations, the ray cancel each other These fluctuations occur much faster than the changes due to environmental effects 2018/9/16 Peter Yeh

Multipath Propagation There are many ways for a signal to travel from a transmitter to a receiver (see Fig 9.5) Multiple path propagation is referred to as intersymbol interference (see Fig. 9.6) Path delay = the maximum delay difference between all the paths 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Technology Implications Systems employ power control to overcome the effects of slow fading Systems use a large array of techniques to overcome the effects of fast fading and multi-path propagation Channel coding (Section 9.4) Interleaving (Section 9.5) Equalization (Section 9.6) PAKE receivers (Section 6.3) Slow frequency hopping (Section 7.3.3) Antenna diversity 2018/9/16 Peter Yeh

Channel Reuse Reuse Planning A channel plan is a method of assigning channels to cells in a way that guarantees a minimum reuse distance between cells using the same channel N > = 1/3(D/R)^2 where D is the distance between a BS and the nearest BS that use the same channel and R is radius of a cell Practical value of N range from 3 to 21 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Spectrum Efficiency Compression Efficiency and Reuse Factor Compression Efficiency = C conversations/per MHz (one-cell system) If N is the number of reuse factor, spectrum efficiency E = C/N conversations per base station per MHz A measure of this tolerance is the signal-to-interference ratio S/I A high tolerance to interference promotes cellular efficiency S/I is an increasing function of the reuse factor N 2018/9/16 Peter Yeh

Slow Frequency Hopping The signal moves from one frequency to another in every frame The purpose of FH is to reduce the transmission impairments Without FH, the entire signal is subject to distortion whenever the assigned carrier is impaired 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

RAKE Receiver Synchronization is a major task of a SS receiver Difficulty: multi-path propagation Solution: Multiple correlator (demodulator) in each receiver Each correlator operates with a digital carrier synchronized to one propagation path 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Channel Coding Channel codes protect information signals against the effects of interference and fading Channel coding decrease the required signal-to-interference ratio (S/I)req and the reuse factor N Channel coding will decrease the compression efficiency C The net effect is to increase the overall spectrum efficiency Channel codes can serve two purposes: error detection and forward error correction 2018/9/16 Peter Yeh

Block Codes Block code (n, k, dmin) Used to Protect The Control Information n is the total number of transmitted bits per code word k is the number of information bits carried by each code word dmin the minimum distance between all pairs of code word ex: n = 3, k = 2, dmin = 2 (000, 011, 101, 110) 2018/9/16 Peter Yeh

Block Codes When dmin = 5, there are three possible decoder actions The decoder can correct no errors and detect up to four errors It can correct one error and detect two or three errors It can correct two errors, three or more bit errors in a block produce a code word error 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Convolutional Codes Each time a new input bit arrives at the encoder, the encoder produces m new output bits the encoder obtains m output bits by performing m binary logic operations on the k bits in the shift register The code rate is r = 1/m 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Example: V1 = R1 V2 = R1  R2  R3 V3 = R1  R3 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Interleaving Most error-correcting codes are effective only when transmission error occur randomly in time To prevent errors from clustering, cellular systems permute the order of bits generated by a channel coder Receivers perform the inverse permutation 2018/9/16 Peter Yeh

Interleaving Example: WHAT I TELL YOU THREE TIMES IS TRUE If there are four consecutive errors in the middle, the result is WHAT I TELL YBVOXHREE TIMES IS TRUE Alternatively, it is possible to interleave the symbol using a 5 x 7 interleaving matrix (See pp. 364-365) WHOT I XELL YOU THREE TIMEB IS VRUE 2018/9/16 Peter Yeh

Adaptive Equalization An adaptive equalizer operates in two modes Training mode: Modem transmits a signal, referred to as a training sequence, that is known to receiver. The receiving modem process the distorted version of training sequence to obtain a channel estimate Tracking mode: The equalizer uses the channel estimate to compensate for distortions in the unknown information sequence 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Walsh Hadamard Matrix The CDMA system uses a 64 x 64 WHM in two ways: In down-link transmissions, it used as an orthogonal code, which is equivalent to an error-correcting block code with (n, k; dmin) = (64, 6; 32) In up-link transmissions, the matrix serve as a digital carrier due to its orthogonal property 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Walsh Hadamard Matrix W 1 = | 0 | 0 0 0 1 W2 = 0 0 0 1 0 0 0 1 W3 = 1 1 1 0 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Review Exercises How does the code rate r of a channel code influence compression efficiency C and tolerance of interference (S/I)req in personal communications systems? How can soft capacity benefit a personal communications system? Is it possible for TDMA or FDMA system to operate with soft capacity? 2018/9/16 Peter Yeh