1. Chapter 6 North American Cellular System Based on Code Division Multiple Access
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2. Background and Goals
CDMA is a form of spread spectrum (SS) communication
SS technology dates from 1940s and original applications were in military
The earliest proposals to apply SS to cellular systems appeared in the late 1970s and were developed in the late 1980s
CDMA was more spectrum efficiency than AMPS (FDMA)
Capacity increases on the order of 5:1 to 10:1
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3. Architecture Interim Standard 95 (IS-95) is dual-mode operation
IS-95 prescribes SS signals with a bandwidth of 1.23 MHz in each direction
Corresponds to the BW of 41 AMPS channels
Innovation- Soft handoff
transmits coded speech signals to two BSs simultaneously
Both BSs send their demodulated signal to the switch
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4. Architecture
The switch estimates the quality of the two signals and send one of them to a speech decoder
In the forward direction, switch sends coded speech signals to the base stations
The terminal combined the signals received from the two BSs and demodulate the result
CDMA soft handoff requires BSs to operate in synchronism with one another
GPS is used to synchronize two BSs
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6. CDMA ID Codes
CDMA terminals and BSs employ an extensive set of ID codes
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8. Radio Transmission
CDMA radio transmission differs significantly from the techniques employed in the other systems
CDMA employ different transmission techniques in the forward and reverse directions
There are two stages of modulation in CDMA
CDMA employs variable-bit-rate traffic channels
CDMA has a reuse factor of one
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9. Modulations Single-stage modulation Two stages modulation
an information signal modulates a radio-frequency carrier
Two stages modulation
an information signal modulates digital carrier and radio-frequency carrier
In IS-95, digital carrier is a binary signal with a switching rate of W = 1,228,800 chips per second
Original source information rate of IS-95 is 9600b/s
The processing gain is G = /9600 = 128 ch/b
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12. Demodulation
The radio demodulator produces a lowpass signal with a bandwidth related to the chip rate W
The digital demodulator multiplies the signal by the digital carrier signal
For each transmitted bit, we obtain G products corresponding to the G chips in the digital carrier
Adds these G products and make a binary decision about the transmitted bit
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15. Demodulation Synchronization is a major task of a SS receiver
Difficulty: multi-path propagation
Solution: Multiple correlator in each receiver
Each correlator operates with a digital carrier synchronized to one propagation path
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17. Reverse-Direction Radio Transmission
Physical Channels
A reverse CDMA physical channel corresponds to a binary code assigned to a terminal
The binary code, labeled channel identifier, is part of the digital carrier
The digital carrier in the reverse channel is a logical combination of a 42-bits channel identifier and the output of a “long code” produced by a binary pseudorandom number generator operating at Mch/s
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20. Reverse-Direction Radio Transmission
The channel identifier contains the electronic serial number (ESN) of the terminal
The repeat rate of the RNG is 41.4 days
Variable-bit-rate from 1200 b/s to 9600 b/s
Interleaver permutes the order of the code bits
Walsh modulator is a (64,6;32) block code for reverse direction
In the forward direction, each row of the Walsh matrix serves as a digital carrier
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22. Walsh Hadamard Matrix W 1 = | 0 | 0 0 0 1 W2 = 0 0 0 1 0 0 0 1 W3 =
1 1
1 0
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23. Radio Modulation In reverse Channel:
Produces a form of four-level modulation referred to as offset quadrature phase shift keying (OQPSK)
Each chip in the SS signal modulates in-phase and quadrature components of the carrier-frequency sine wave
Each symbol corresponds to two bits
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25. Radiated Power
The Aim of the power control is to make all reverse-direction signals in a cell arrive at the BS with the same strength
Closed-loop power control
The terminal performs power adjustments 800 times per second under the control of the BS
Open-loop power control
Terminal to adjust its transmitter power as a function of the power it measures in the received forward-direction signal
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27. Forward Link Radio Transmission
The digital carrier of a forward-direction channel is a row of a 64 x 64 Walsh Hadamard matrix
All digital carriers in the forward direction are mutually orthogonal
To control co-channel interference from different BSs, the modulator in each BS offset signal in time at surrounding BSs
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28. Forward Link Radio Transmission
Interference from signal on other physical channel within its own cell and from terminals in other cells
CDMA BS transmits information in the four logical channel formats:
pilot channels, sync channels, paging channels, and traffic channels
The channel data rate can range from 1200 b/s to 9600 b/s
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31. Forward Link Radio Transmission
Pilot Channel:
Input a sequence of 0s
The digital carrier for the pilot channel is Walsh(0)
The output of the pilot channel consists of ch/s, all of which are 0s
Sync Channel:
Input is a 1200 b/s information signal
The digital carrier is Walsh(32), the 32 0s followed by 32 1s
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33. Radio Modulation
The CDMA forward link employs the four level modulation technique similar to reverse link
One important difference is a time offset in the in-phase and quadrature signal paths
Each BS has its own time offset, transmitted in control messages
There are 512 possible time offsets with offset i corresponding to delay of 64i chips
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35. Comparisons of AMPS,TDMA and CDMA
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36. Logical Channels IS-95 specifies four logical channels
Pilot, Sync, Paging, and traffic channels
There are 64 physical channels available for CDMA BS transmission
One of these channel is active as pilot channel and another one serves as a sync channel
The remaining channels are paging and traffic channels
There are at most seven paging channels
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38. Pilot Channel
The pilot channel transmit a sequence of 0s at a rate of Mch/s
Two purposes for Pilot Channel:
By tuning to the pilot channel, terminals communicating with the BS acquire carrier phase and timing reference
Terminal in other cells tune to the pilot channel obtain signal strength for handoff procedure
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39. Sync Channel The sync channel transmission rate is 1200b/s
The sync channel repeatedly transmits one message including system time and the time delay
The sync channel also transmits system ID and Network ID
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41. Paging Channel
The digital carriers of paging channels are Walsh 1 to Walsh 7
The information rate on a paging channel is either 4800 b/s or 9600b/s
BSs transmit paging message in time slots of duration 80 ms (384 bits or 768 bits)
The maximum length of a paging message is 1184 bits
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42. Paging Channel There are maximum 2048 paging channel slots
The total duration is 80 ms x 2048 = s
Each terminal can examine a fraction of the slots
EX: The terminal can examine one slot every 16 slots and the terminal is idle 15 slots out of every 16 slots
The monitor slots of a terminal are determined by the terminal’s directory number and ESN
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43. Access Channel
A terminal without a call in progress uses an access channel to send message to a BS for three principle purposes:
to originate a call
to respond to a paging message
to register its location
Each BS operates with up to 32 access channels
The transmission rate of each access channel is 4800 b/s with information conveyed in 20 ms
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44. Access Channel
Each frame consists of 88 information bits and 8 tail bits
A preamble consisting of 1-16 frames, each containing 96 0s
The length of an access message body is: 3 ~ 10 frames
Thus a transmission on an access channel is: 4 ~ 26 frames
To send a message on an access channel, a terminal chooses at random an access channel number from 0 to acc_chan
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45. Access Channel
After transmitting a message, the terminal waits for an Ack to arrive on the paging channel
If the Ack is not received within a timeout period, the terminal pause for a random time interval and transmits again, at increased power
Each transmission referred to as a probe, and 16 probes is an attempt
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46. =16
=15
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47. CDMA Traffic Channels
On a reverse traffic channel, the 42-bit channel ID corresponding to a CDMA physical channel is a combination of 32 bits SEN and a fixed 10 bits
The forward traffic channel is the 64 bit carrier, a row of the 64 x 64 Walsh matrix
Two characteristics of system IS-95
Carry variable bit rates ranging from 9600 b/s to 1200 b/s
Carry a dynamic mixture of user information and user control information
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48. Variable-Bit-Rate Speech Transmission
The system performs encoding and decoding operations on 20 ms speech frames
In each 20 ms interval, the encoder process 160 samples of speech, obtained from an A-to-D converter operating at 8000 samples/s
The coder represents speech at four bit rates: 8000 b/s, 4000 b/s, 2000 b/s, and 800 b/s, producing 160, 80, 40, or 16 bits per 20 ms frame
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49. Signaling on CDMA Traffic Channels
To exchange network control information, CDMA BS and terminals interrupt or reduce the flow of speech information and insert messages into traffic channels
There are 5 modes of operations
blank-and-burst: control message completely replace the speech
three dim-and-burst modes: mixture of speech information and control information in each 20 ms
Speech only
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51. Signaling on CDMA Traffic Channels
When control messages are present, the traffic channel always operate at 9600 b/s
The four possible rate of signal information transfer are:
168 / 0.02 = 8400 b/s,
152 / 0.02 = 7600 b/s,
128 / 0.02 = 6400 b/s,
and 88 / 0.02 = 4400 b/s
The corresponding speech rates are, 0, 1200 b/s, 2400 b/s, and 4800 b/s
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52. Power Control Subchannel
BSs monitor the received power and transmits power control commands at a data rate 800 b/s
The BS inserts 16 power control bits into every frame (20 ms) on a forward traffic channel
This multiplexing take place after the convolution coding, repetition, and interleaving illustrated in Fig. 6.10
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54. Power Control Sub-channel
A power control bit replaces 2 traffic channel code bits in every 1.25 ms
A 0 bit indicates to increase the mean output power level 1 dB
Whereas 1 indicates to decrease the mean output level 1 dB
The effect of the replacing is to weaken the error-correcting capability of the rate1/2 convolution code
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55. Messages
IS-95 paging and access channels correspond to the AMPS forward and reverse control channels
Corresponding to the forward and reverse voice channels in AMPS, IS-95 transmits signaling messages on forward and reverse traffic channels
Terminals in nearby cells measure the strength of the pilot as part of CDMA handoff procedures
The sync channel broadcast one message that conveys system information to all terminals in a cell
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56. Message Structure
With the exception of the Broadcast messages and the Sync channel message, the CDMA system need Layer 2 Acknowledgments
This goal is achieved by means of Ack and selective-repeat retransmission of messages containing errors
It uses access channel to ack paging channel messages and the reverse channel to ack forward traffic channel messages vice versa
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57. Message Structure CDMA message format
Begins with an 8-bit (octet base) message-length field
8 bits message type
7 bits ack.
3-bit message sequence field (selective retransmission)
3-bit ack. Sequence
1 ack. required bit
Message contents: 1 – 2008 bits
CRC field 30 bits (sync, paging, access channel) 16bits (traffic channels)
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58. Message Structure Order: Half of the CDMA messages are order
message type on paging channel
specifies an order
On the traffic channel, the message type for
an order is
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59. Acknowledgement Messages
Ack. Message
In addition to layer 2 ack. and retransmission, IS-95 provides for signaling ack at higher protocol layer
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60. Message Content Sync Channel
The PN_OFFSET, long code state, and system time fields contain the information required by the terminal to synchronize its random number generator
The PN_OFFSET control the timing of the signals transmitted by each base station
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62. Message Content Paging Channels:
System parameter message carries information on mobility management to be used
Access parameters: indicates the number of access channels in use in the current cell
CDMA channel list: Broadcasts the radio-frequency carriers
Neighbor list: broadcasts to all terminals with nearby BSs and plays an important role in soft handoff
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63. Message Content
Lock until power cycled instructs a terminal to turnoff its transmitter until receives an unlock order (CDMA is power sensitive)
Abbreviated alert order: cause the terminal to emit an audible tone before assigning a voice channel
Feature notification message: can deliver special services such as caller ID or voice-
Slotted page message: carries more than one paging notification in each 80 ms time slot (384bits or 768 bits) of a paging channel
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64. Message Content Traffic Channels
Long code transition: allow a terminal and a BS to move between a public long code and a private code
Message Encryption Mode order …
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65. Power Control Power control at mobile terminals Power control at BS
In order to confine the received signal powers to a narrow range, IS-95 performs a combination of open-loop and closed-loop operations
Power control at BS
The BS sends a Power control parameter message to request power quality from terminal
The terminal responds with a power measurement report
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66. Soft Handoff The most novel IS-95 network control operation
As a terminal moves from one cell to another, it communicates simultaneously with the BSs in both cells
In common with NA-TDMA and GSM, CDMA handoff is mobile assisted, with each terminal that influence handoff decision
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67. Soft Handoff
Terminal receives the pilot signal from a BS to determine the signal quality sent by the BS
To measure the strength of the pilot signal from a BS, the terminal programs the searching correlator with the PN_OFFSET of that BS
The PN_OFFSET identifies each BS in a system
According to the strength of pilot signal, BSs can be classified into four categories: active list, candidate list, neighbor list, and the remaining list
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68. Soft Handoff
The system decides whether to begin a handoff or complete a soft handoff by receiving the pilot strength measurement message
Fig shows the soft handoff procedure
Power control under soft handoff:
If all of BSs command a 1 dB power increase, the terminal obeys this command
If at least one BS commands a 1 dB decrease, the terminal decreases its power by 1 dB
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70. Soft Handoff Other Types of Handoff
Transferring a call from a CDMA traffic channel to an analog voice channel
Hard handoff: Change from a CDMA frequency band to another or move into cells controlled by different switch
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71. Mobility Management
Terminal send REGISTRATION message to the system in order to facilitate efficient paging
Registration message contains terminal ID and the paging time slots monitored by the terminal operating in sleep mode in order to conserve battery power
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72. Mobility Management Main registration strategies
Timer-based registration
Distance-based registration
Five types of registration event:
Power up, power down, elapsed time > threshold, moving distance > threshold, and enter a new registration zone
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73. Authentication and Privacy
Each user can operate in a private long code mask
After a call is set up, either the system or terminal can initiate operation with a private long code mask by transmitting a LONG CODE TRANSMISSION order
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74. Status of CDMA Began commercial service in 1996
Date rate: 9600 b/s
Operation in 1900 MHz PCS bands
CDMA is the selected approach for the 3 G systems such as CDMA 2000 and WCDMA
The data transmission rates ranging from 144 k bps for mobile users to 2 M bps for stationary users
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75. Review Exercises
How do CDMA base stations use signals received from the global positioning system? Why is it important that all base stations operate in synchronism?
Why does the CDMA system employ closed-loop power control in reverse-direction transmissions?
How does variable-bit-rate speech transmission help the performance of the CDMA system?
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