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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

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

- The signal strength decreases with distance according to the relationship:

Prof. Huei-Wen Ferng

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 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
- PAKE receivers
- 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

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 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

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

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

- 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

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

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

- 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

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

- 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

- 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

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