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Chapter 6. Advanced Mobile Phone System (AMPS). Preliminary. Technology Tutorials. 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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chapter 6

Chapter 6

Advanced Mobile Phone System (AMPS)

Prof. Huei-Wen Ferng

preliminary

Preliminary

Technology Tutorials

Prof. Huei-Wen Ferng

multiple access
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

slide7
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

slide8
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

mobile radio signals
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

attenuation due to distance
Attenuation Due to Distance
  • The signal strength decreases with distance according to the relationship:

Prof. Huei-Wen Ferng

slow shadow fading
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

fast rayleigh fading
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

multi path propagation
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

technology implications
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

spectrum efficiency
Spectrum Efficiency

Prof. Huei-Wen Ferng

spectrum efficiency cont d
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

spectrum efficiency cont d1
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

slow frequency hopping
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

rake receiver
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

channel coding
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

block codes
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

block codes1
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

convolutional codes
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

slide34

Example:

V1 = R1

V2 = R1 R2R3

V3 = R1 R3

Prof. Huei-Wen Ferng

interleaving
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

interleaving1
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

adaptive equalization
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

walsh hadamard matrix
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

walsh hadamard matrix1
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

amps system

AMPS System

The first generation cellular phone system

Prof. Huei-Wen Ferng

network elements
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

amps identification codes
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

frequency bands and physical channels
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

radiated power
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

analog signal processing
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

sat and st
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)

Prof. Huei-Wen Ferng

digital signals
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

Prof. Huei-Wen Ferng

spectrum efficiency1
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

Prof. Huei-Wen Ferng

logical channel categories
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)

Prof. Huei-Wen Ferng

tasks performed by terminals
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

Prof. Huei-Wen Ferng

capacity
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

Prof. Huei-Wen Ferng

review exercises
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?

Prof. Huei-Wen Ferng

references
References
  • D.J. Goodman, “Wireless Personal Communications Systems”, Ch9 and Ch3.
  • Ch9: Preliminary
  • Ch3: AMPS system

Prof. Huei-Wen Ferng

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