1. Advanced Mobile Phone System (AMPS)
Chapter 6
Advanced Mobile Phone System (AMPS)
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2. Preliminary
Technology Tutorials
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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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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: => ( )
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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 = ( ). E can convert the signal S to S·C = ( )/8 = 1
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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.
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11. Attenuation Due to Distance
The signal strength decreases with distance according to the relationship:
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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.
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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
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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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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
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19. Spectrum Efficiency
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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
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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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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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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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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)
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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.
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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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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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34. Example: V1 = R1 V2 = R1  R2  R3 V3 = R1  R3 2019/1/15
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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.
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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
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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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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
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42. Walsh Hadamard Matrix W1 = | 0 | 0 0 0 1 W2 = 0 0 0 1 0 0 0 1 W4 = 0 0
1 1
1 0
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43. The first generation cellular phone system
AMPS System
The first generation cellular phone system
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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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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
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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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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
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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.
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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)
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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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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
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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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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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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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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?
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75. References
D.J. Goodman, “Wireless Personal Communications Systems”, Ch9 and Ch3.
Ch9: Preliminary
Ch3: AMPS system
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