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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

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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)

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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

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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.

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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.

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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

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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

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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.

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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

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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

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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)

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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.

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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

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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

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Example:

V1 = R1

V2 = R1 R2R3

V3 = R1 R3

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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.

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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

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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

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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

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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

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Amps system

AMPS System

The first generation cellular phone system

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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)

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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

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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.

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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

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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.

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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

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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

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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)

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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

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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

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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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