Chapter 6

1. Chapter 6 Advanced Mobile Phone System (AMPS) Prof. Huei-Wen Ferng

2. Preliminary Technology Tutorials Prof. Huei-Wen Ferng

3. 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 Prof. Huei-Wen Ferng

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7. CDMA • Each station has its own unique chip sequence (CS) • All CS are pair-wise orthogonal • For example :(codes A, B, C and D are pair-wise 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) Prof. Huei-Wen Ferng

8. CDMA • A·B = (1+1-1-1+1-1+1-1) = 0 • B·C = (1-1-1-1+1+1-1+1) = 0 • Example: 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 Prof. Huei-Wen Ferng

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10. Mobile Radio Signals • Four main effects produced by physical conditions: • Attenuation that increases with distance • Random variation due to environmental features, i.e., shadow fading. • Signal fluctuations due to the motion of a terminal, i.e., Rayleigh fading. • Distortion due to that the signal travels along different paths, i.e., multi-path fading. Prof. Huei-Wen Ferng

11. Attenuation Due to Distance • The signal strength decreases with distance according to the relationship: Prof. Huei-Wen Ferng

12. 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. • Let P (received power) be a log-normal distributed random variable with mean Preceive and S (signal strength in dBm), i.e., S=10log10(1000P) dBm. • The log-normal of P implies that S is normal distributed. Prof. Huei-Wen Ferng

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14. 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 Prof. Huei-Wen Ferng

15. Multi-path 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 inter-symbol interference (see Fig. 9.6) • Path delay = the maximum delay difference between all the paths Prof. Huei-Wen Ferng

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18. 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 • Interleaving • Equalization • PAKE receivers • Slow frequency hopping • Antenna diversity Prof. Huei-Wen Ferng

19. Spectrum Efficiency Prof. Huei-Wen Ferng

20. Spectrum Efficiency (Cont’d) • 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 Prof. Huei-Wen Ferng

21. Spectrum Efficiency (Cont’d) • Channel 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. Prof. Huei-Wen Ferng

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24. 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 Prof. Huei-Wen Ferng

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26. 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 Prof. Huei-Wen Ferng

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28. 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 andthe 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 (FEC) Prof. Huei-Wen Ferng

29. 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) • Code rate r=k/n. Prof. Huei-Wen Ferng

30. 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 Prof. Huei-Wen Ferng

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32. 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 Prof. Huei-Wen Ferng

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34. Example: V1 = R1 V2 = R1 R2R3 V3 = R1 R3 Prof. Huei-Wen Ferng

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36. Interleaving • Most error-correcting codes are effective only when transmission error occurs 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. Prof. Huei-Wen Ferng

37. 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 Prof. Huei-Wen Ferng

38. 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 Prof. Huei-Wen Ferng

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41. 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 Prof. Huei-Wen Ferng

42. Walsh Hadamard Matrix • W1 = | 0 | 0 0 0 1 W2 = 0 0 0 1 0 0 0 1 W4 = 0 0 0 1 1 1 1 0 Prof. Huei-Wen Ferng

43. AMPS System The first generation cellular phone system Prof. Huei-Wen Ferng

44. Network Elements • The AMPS specification refers to terminals as mobile stations and to base station as land stations. • The common terminology for an AMPS switch is mobile telephone switching office (small and large MTSO). • The communication links between the base stations and switch are labeled land lines (copper wires, optical fibers or microwave systems) Prof. Huei-Wen Ferng

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46. AMPS Identification Codes • Mobile Identification Number (MIN) • Area code (3 digits), Exchange number (3 digits) and subscriber number (4 digits) • Electronic Serial Number (ESN) • System Identifier (SID) • Station Class Mark (SCM) • Indicates capabilities of a mobile station • Supervisory Audio Tone (SAT) • Digital Color Code (DCC) • Help mobile stations distinguish neighboring base stations from one another Prof. Huei-Wen Ferng

47. Frequency Bands and Physical Channels • The band for forward transmissions, from cell site to mobile station, is 870-890 MHz. • The reverse band, for transmissions by mobiles, is 45 MHz lower. • An AMPS physical channel occupies two 30 KHz frequency bands, one for each direction. Prof. Huei-Wen Ferng

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49. Radiated Power • An AMPS terminal is capable of radiating signals at 6 or 8 different power levels (6 mW to 4W). • 10 log 4000 = 36 dBm • The radiated power at a a base station is typically 25 W. • Discontinuous transmission (DTX) • Speech activity detector • ON-OFF state • Power saving and Interference reducing Prof. Huei-Wen Ferng

50. Analog Signal Processing • Compression and pre-emphasis are established techniques for audio signal transmission. • An amplitude limiter confines the maximum excursions of the frequency modulated signal to 12 KHz. • Low pass filter Attenuates signal components at frequencies above 3 KHz, refer to Fig. 3.5. • The notch (at 6KHz) removes signal energy at the frequencies associated with the 3 SAT of the AMPS system. Prof. Huei-Wen Ferng