2001/12/14Prof. Huei-Wen Ferng 1 Chapter 6 Advanced Mobile Phone System (AMPS)

2001/12/14Prof. Huei-Wen Ferng 2 Preliminary Technology Tutorials

2001/12/14 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

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2001/12/14 Prof. Huei-Wen Ferng 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: => ( ) –B: => ( ) –C: => ( ) –D: => ( )

2001/12/14 Prof. Huei-Wen Ferng 8 CDMA A · B = ( ) = 0 B · C = ( ) = 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 = ( ). E can convert the signal S to S · C = ( )/8 = 1

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2001/12/14 Prof. Huei-Wen Ferng 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.

2001/12/14 Prof. Huei-Wen Ferng 11 Attenuation Due to Distance The signal strength decreases with distance according to the relationship:

2001/12/14 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 P receive and S (signal strength in dBm), i.e., S=10log 10 (1000P) dBm. –The log-normal of P implies that S is normal distributed.

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2001/12/14 Prof. Huei-Wen Ferng 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

2001/12/14 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

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2001/12/14 Prof. Huei-Wen Ferng 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

2001/12/14 Prof. Huei-Wen Ferng 19 Spectrum Efficiency

2001/12/14 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

2001/12/14 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.

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2001/12/14 Prof. Huei-Wen Ferng 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

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2001/12/14 Prof. Huei-Wen Ferng 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

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2001/12/14 Prof. Huei-Wen Ferng 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 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 (FEC)

2001/12/14 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.

2001/12/14 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

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2001/12/14 Prof. Huei-Wen Ferng 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

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2001/12/14 Prof. Huei-Wen Ferng 34 V 1 = R 1 V 2 = R 1  R 2  R 3 V 3 = R 1  R 3 Example:

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2001/12/14 Prof. Huei-Wen Ferng 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.

2001/12/14 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 ) –WHOT I XELL YOU THREE TIMEB IS VRUE

2001/12/14 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

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2001/12/14 Prof. Huei-Wen Ferng 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; d min ) = (64, 6; 32) –In up-link transmissions, the matrix serve as a digital carrier due to its orthogonal property

2001/12/14 Prof. Huei-Wen Ferng 42 Walsh Hadamard Matrix W 1 = | 0 | W 2 = W 4 =

2001/12/14Prof. Huei-Wen Ferng 43 AMPS System The first generation cellular phone system

2001/12/14 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)

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2001/12/14 Prof. Huei-Wen Ferng 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

2001/12/14 Prof. Huei-Wen Ferng 47 Frequency Bands and Physical Channels The band for forward transmissions, from cell site to mobile station, is 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.

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2001/12/14 Prof. Huei-Wen Ferng 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

2001/12/14 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 The notch (at 6KHz) removes signal energy at the frequencies associated with the 3 SAT of the AMPS system.

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2001/12/14 Prof. Huei-Wen Ferng 54 SAT and ST The SAT (Supervisory Audio Tone) transmitted with user information serves to identify the base station assigned to a call. Each base station has its own SAT- at 5970 Hz, 6000 Hz, or 6030 Hz. An analog signals from AMPS terminals can also contain a 10 KHz sine wave referred to as a ST (Supervisory Tone). –On-hook and Off-hook indications signaling The channel reuse principles (Section 9.3.2)

2001/12/14 Prof. Huei-Wen Ferng 55 Digital Signals AMPS also transmits important network control information in digital form. AMPS digital signal are sine waves either 8 KHz above or 8 KHz below the carrier. The signal format is Manchester coded binary frequency shift keying at a rate of 10 Kbps

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2001/12/14 Prof. Huei-Wen Ferng 57 Spectrum Efficiency Frequency modulation in 30 KHz physical channels Signal-to-Interference ratio (SIR) –SIR >= (SIR)req = 18 dB –Reuse factor N = 7 (Figure 9.9) Spectrum efficiency –E=395 /7*25 = 2.26 conversations/cell/MHz –395 traffic channels, 25 MHz/system, 7 cells in a cluster

2001/12/14 Prof. Huei-Wen Ferng 58 Logical Channel Categories FOCC: Forward (Downlink) Control Channel –Carries the same information from one base station to all of the mobile terminals (Broadcast) RECC: Reverse (Uplink) Control Channel –Carries information from many mobile terminals that do not have voice channel (Random access) FVC: Forward Voice Channel (Dedicated) RVC: Reverse Voice channel (Dedicated) Forward and reverse traffic channel –User information (Dedicated)

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2001/12/14 Prof. Huei-Wen Ferng 67 Tasks Performed by Terminals Initialization mode –The terminal turns the power on –A conversation ends –Loses contact with the current base station Idle mode Access mode (from Idle mode) –The terminal presses the SEND button –An incoming call request detected (MIN) –A registration event stimulated Conversation mode

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2001/12/14 Prof. Huei-Wen Ferng 72 Capacity There are 3 ways to increase the capacity –Operate with smaller cells –Obtain additional spectrum allocations –Improve spectrum efficiency NAMPS (Narrowband-AMPS) –Messages similar to AMPS –Synchronization sequences –Digital versions of the SAT and ST

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2001/12/14 Prof. Huei-Wen Ferng 74 Review Exercises What is the purpose of the busy/ idle bits in the FOCC? Why are they not used in the other control channel formats? Explain how the AMPS system users supervisory audio tones (SAT) and a digital color code (DCC). Why are both required? Explain why it is sometimes desirable for the AMPS system to set up a call through a base station that is not the nearest base station to the terminal. How does the AMPS system achieve this effect?

2001/12/14 Prof. Huei-Wen Ferng 75 References D.J. Goodman, “ Wireless Personal Communications Systems ”, Ch9 and Ch3. Ch9: Preliminary Ch3: AMPS system