Chapter 6 North American Cellular System Based on Code Division Multiple Access 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

CDMA ID Codes CDMA terminals and BSs employ an extensive set of ID codes 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 = 1228800/9600 = 128 ch/b 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 1.2288Mch/s 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Walsh Hadamard Matrix W 1 = | 0 | 0 0 0 1 W2 = 0 0 0 1 0 0 0 1 W3 = 1 1 1 0 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 1228800ch/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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Comparisons of AMPS,TDMA and CDMA 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

Pilot Channel The pilot channel transmit a sequence of 0s at a rate of 1.2288Mch/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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

Paging Channel There are maximum 2048 paging channel slots The total duration is 80 ms x 2048 = 163.84 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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

=16 =15 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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) 2018/9/16 Peter Yeh

Message Structure Order: Half of the CDMA messages are order message type 00000111 on paging channel specifies an order On the traffic channel, the message type for an order is 00000001 2018/9/16 Peter Yeh

Acknowledgement Messages Ack. Message In addition to layer 2 ack. and retransmission, IS-95 provides for signaling ack at higher protocol layer 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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-email Slotted page message: carries more than one paging notification in each 80 ms time slot (384bits or 768 bits) of a paging channel 2018/9/16 Peter Yeh

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 … 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

Soft Handoff The system decides whether to begin a handoff or complete a soft handoff by receiving the pilot strength measurement message Fig. 6.18 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 2018/9/16 Peter Yeh

2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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 2018/9/16 Peter Yeh

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? 2018/9/16 Peter Yeh