Chapter 6

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# Chapter 6 - PowerPoint PPT Presentation

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

Prof. Huei-Wen Ferng

### Preliminary

Technology Tutorials

Prof. Huei-Wen Ferng

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

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

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

• Four main effects produced by physical conditions:
• Attenuation that increases with distance
• 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
• The signal strength decreases with distance according to the relationship:

Prof. Huei-Wen Ferng

• 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 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
• 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
• 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
• Slow frequency hopping
• Antenna diversity

Prof. Huei-Wen Ferng

Spectrum Efficiency

Prof. Huei-Wen Ferng

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’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
• 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

• 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 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 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 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
• 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

Example:

V1 = R1

V2 = R1 R2R3

V3 = R1 R3

Prof. Huei-Wen Ferng

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

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

• 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

• 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

• 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

The first generation cellular phone system

Prof. Huei-Wen Ferng

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
• 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
• 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

• 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
• 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
• 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
• 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 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
• 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

• 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
• There are 3 ways to increase the capacity
• Operate with smaller cells
• 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
• 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
• D.J. Goodman, “Wireless Personal Communications Systems”, Ch9 and Ch3.
• Ch9: Preliminary
• Ch3: AMPS system

Prof. Huei-Wen Ferng