Tài liệu Adaptive WCDMA (P16) pdf

Figure 16.1 Example of logical reverse CDMA channels received at a base station.PN chip + I I Q Long code mask Long code generator Zero-shift plot PN sequence I-channel 1.2288 MHz PN c

Standards

In this chapter we discuss the basic Code Division Multiple Access (CDMA) standardsand give a brief history of the standard proposals We present the main system parameters,which are essential for the understanding of the system concept (common air interface) ofeach standard At this stage we use what we have learnt so far in this book to discuss themotivations behind the solutions We believe that at the end of the book the reader shouldhave the required knowledge to follow the closing discussion on the various choices for thedifferent system parameters, and all the advantages and the drawbacks of these choices

16.1 IS 95 STANDARD

16.1.1 Reverse link

Available channels in the uplink are shown in Figure 16.1 A block diagram of a reversechannel data path is shown in Figure 16.2 Data and chip rates in different points inthe system are indicated on the picture The voice source is encoded by using variablerate codec with four possible rates of 1.2 to 9.6 kbps This is the way to exploit voiceactivity factor

Prior to modulation, convolutional encoder and block interleaver are used

Modulation for the reverse CDMA channel is 64-ary orthogonal signaling One of thepossible modulation symbols will be transmitted for each six-code symbol

Modulation symbol number= c0+ 2c1+ 4c2+ 8c3+ 16c4+ 32c5 ( 16.1)

c5shall represent the last or the most recent and c0 the first or the oldest binary valued (0and 1) code symbol of each group of six code symbols that form a modulation symbol.One out of 64 Walsh symbols is transmitted for each different value of equation (16.1).Construction rules for Walsh functions are described in Chapter 2 On the basis of the chiprate 1.22881447 Mchips (indicated in Figure 16.2), the period of time required to transmit

a single modulation symbol, referred to as a Walsh symbol interval, will be approximately

Adaptive WCDMA: Theory And Practice.

Savo G Glisic Copyright ¶ 2003 John Wiley & Sons, Ltd.

ISBN: 0-470-84825-1

Figure 16.1 Example of logical reverse CDMA channels received at a base station.

PN chip

+ I

I

Q

Long code mask Long code generator

Zero-shift plot PN sequence I-channel 1.2288 MHz

PN chip

1.2288 MHz Code symbol

Code symbol Information

64-ary orthogonal modulator Walsh chip

307.2 kHz Zero-shift plot PN sequence Q-channel

1/2 PN chip

delay = 406.9 ns

FIR filter

FIR filter

D/A and filtering

D/A and filtering

I(t )

Q( t )

A cos(2pft )

A sin(2pft)

Figure 16.2 Reverse CDMA channel data path example.

equal to 208.333µs The period of time associated with 164th of the modulation symbol

is referred to as a Walsh chip and will be approximately equal to 3.2552083333 .µs.The reverse traffic channel numerology is shown in Table 16.1

16.1.2 Direct-sequence spreading

The reverse traffic channel and the access channel will be combined with three ent pseudonoise (PN) sequences Data and PN sequence combination involves modulo-2addition of the encoded, interleaved data stream with two PN code streams, each operating

differ-at 1.2288 MHz The first sequence is referred to as the long code sequence This sequenceshall be a time shift of a sequence of length 242–1 chips and shall be generated by a

Walsh sym

4

Modulo-2 addition

42-Bit logic code matrix

Long code sequence

x 1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9 x 10 x 39 x 40 x 41 x 42

Figure 16.3 Long code generation and masking.

linear generator using the following polynomial:

PCN 1

Access channel long code mask

PCN — Paging channel number

ACN — Access channel number

REG_ZONE — Registration zone for the forward CDMA channel

PILOT_PN — PN offset for the forward CDMA channel

Public long code mask

ESN Private long code mask

Private long code

Figure 16.4 Long code mask format.

be set to the access channel number chosen randomly M16 through M18 shall be set tothe code channel for the associated paging channel (i.e the range shall be 1 through 7)

M9 through M15 shall be set to the REG ZONE for the current base station (BS) M0

through M8 shall be set to the PILOT PN value for the CDMA channel

In the reverse traffic channel the mobile station shall use one of two long codes unique

to that mobile station: a public long code unique to the mobile station’s electronic serialnumber (ESN) and a private long code unique for each mobile identification number(MIN) The public long code shall be as follows: M32through M41shall be set to ‘0’ and

M0 through M31 shall be set to the mobile station’s ESN value

The second and third PN sequences are the I and Q ‘short codes’ The reverse accesschannel and the reverse traffic channel shall be offset quadrature phase shift keying(OQPSK) spread prior to actual transmission This offset quadrature spreading on thereverse channel shall use the same I and Q PN codes as the forward I and Q PN codes.These codes are of length 215 The reverse CDMA channel I and Q codes shall be thezero-time offset codes The generating functions for the I and Q short PN codes shall be

as follows:

PI(x) = x15+ x13+ x9+ x8+ x7+ x5+ 1

PQ(x) = x15+ x12+ x11+ x10+ x6+ x5+ x4+ x3+ 1

16.1.4 Data burst randomizer algorithm

The data burst randomizer generates a masking stream of 0 s and 1 s that randomly maskout the redundant data generated by the code repetition The masking stream pattern isdetermined by the frame data rate and by the block of 14 bits taken from the long codesequence These mask bits are synchronized with the data flow and the data is selectivelymasked by these bits through the operation of the digital filter The 1.2288-MHz-long

IS 95 STANDARD 569

code sequence shall be input to a 14-bits shift register, which is shifted at 1.2288 MHz.The contents of this shift register shall be loaded into a 14-bit latch exactly one powercontrol group (1.25 ms) before each reverse traffic channel frame boundary

b0b1b2b3b4b5b6b7b8b9b10b11b12b13

The binary (0 and 1) contents of this latch shall be denoted as where b0 shall represent

the first bit to enter the shift register and b13shall represent the last (or most recent) bit toenter the sift register Each 20-ms reverse traffic channel frame shall be divided into 16equal length (i.e 1.25 ms) power control groups numbered from 0 to 15 The data burstrandomizer algorithm shall be as follows:

Data rate selected: 9600 bps

Frame transmission shall occur on power control groups numbered

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15

Data rate selected: 4800 bps

Frame transmission shall occur on power control groups numbered

b0,2+ b1,4+ b2,6+ b3,8+ b4,10+ b5,12+ b6

Data rate selected: 2400 bps

Frame transmission shall occur on power control groups numbered

b0 if b8= 0, 2+ b1 if b8 = 1

4+ b2 if b9= 0, 6+ b3 if b9 = 1

8+ b4 if b10= 0, 10+ b5 if b10= 1

12+ b6 if b11= 0, 14+ b7 if b11= 1

Data rate selected: 1200 bps

Frame transmission shall occur on power control groups numbered

20 ms = 193 bits = 576 code symbols

= 96 walsh symbols = 16 power control groups 1.25 ms = 12 bits = 36 code symbols

= 6 Walsh symbols = 1 power control group Code symbols transmitted:

Code symbols transmitted:

Code symbols transmitted:

Code symbols transmitted:

Previous frame

Previous frame

Previous frame

PN bits used for scrambling

Sample masking streams shown are for the 14-bit PN sequence:

(b0, 1, …, b13) = 0 0 1 0 1 1 0 1 1 0 0 1 0 0

PCG 15 PCG 14

Power control group number

Figure 16.5 Reverse CDMA channel variable data rate transmission example.

16.1.5 Reverse traffic channel frame quality indicator

Each frame of the traffic channel shall include a frame quality indicator For the defaultmultiplex option’s 9600-bps and 4800-bps transmission rates, the frame quality indicatorshall be a cyclic redundancy check (CRC) For the 9600-bps and 4800-bps rates, theframe quality indicator (CRC) shall be calculated on all bits within the frame, except theframe quality indicator (CRC) itself and the encoder tail bits The 9600-bps transmissionrate shall use a 12-bit frame quality indicator (CRC), which shall be transmitted withinthe 192-bit long frame The generator polynomial for the 9600-bps rate

or 80 times (for 96-bit frame) with the traffic or the signaling bits and mode/formatindicators as input The switches shall be set in the down position, and the register shall

be clocked an additional 12 times (for 192-bit frame) or 8 times (for 96-bit frame)

IS 95 STANDARD 571

Denotes modulo-2 addition

Up for first 172 bits Down for last 12 bits

Figure 16.6 Reverse traffic channel frame quality indicator calculation at 9600-bps rate for the

default multiplex option(1).

Denotes modulo-2 addition

Output

0

0

Denotes one-bit storage element

Up for first 80 bits Down for last 8 bits

Input

Figure 16.7 Reverse traffic channel frame quality indicator calculation at 4800-bps rate for the

default multiplex option(1).

The 12 or 8 additional output bits shall be the check bits The bits shall be transmitted inthe order calculated

16.1.7 Base station

Transmitter

Each BS within a given system shall use the same CDMA frequency assignments foreach of the CDMA channels The channel structure is shown in Figures 16.8 and 16.9

Variable data rate transmission

The forward traffic channel shall support variable data rate operation Four data ratesare supported: 9600, 4800, 2400 and 1200 bps The data rate shall be selectable on aframe-by-frame (i.e 20-ms) basis without consideration for the rate in the previous orsubsequent frames Although the data rate may vary on a 20-ms basis, the modulation

W1 W33 Up to

Forward CDMA channel (1.23-MHz channel transmitted by base station)

Pilot

chan

Sync chan

Up

W = Walsh symbol number

Traffic data

Mobile power control subchannel

Figure 16.8 Example of a forward CDMA channel transmitted by a base station.

symbol rate is kept constant by code repetition at 19.2 kilo-symbols per second (ksps).The modulation symbols that are transmitted at the lower data rates shall be transmittedusing lower energy, as shown in Table 16.2

Note that all the symbols in the interleaver block are from the same frame Thus theyare all transmitted at the same energy Power control bits are always transmitted with

energy Eb

Pilot channel

The pilot channel is transmitted at all times by the BS on each active forward CDMAchannel It is an unmodulated spread spectrum signal that is used by a mobile stationoperating within the geographic coverage area of the base station It is used by themobile station to acquire synchronization with the pilot PN sequence, to provide a phasereference and to provide sync channel frame timing

The acquisition of the pilot channel pilot PN sequence is the first step in the process

of the mobile station acquiring the system timing or reacquiring the system timing Codefor the pilot channel shall be a quadrature sequence of length 215 (i.e 32768 PN chips

in length)

Code polynomial

PI(x) = x15+ x13+ x9+ x8+ x7+ x5+ 1

PQ(x) = x15+ x12+ x11+ x10+ x6+ x5+ x4+ x3+ 1for the in-phase (I) sequence and for the quadrature (Q) phase sequence is used The length

of these sequences is 215–1 In order to generate a pilot PN sequence of length 215, abinary 1 is inserted in the sequence generator output after the contiguous succession of 14binary 0 outputs (that occurs only once per period of the sequence) The chip rate for the

pilot PN sequence shall be 1.2288 Mcps The pilot PN sequence period is 26.666 ms.

Exactly 75 pilot PN sequence repetitions occur every 2 s

IS 95 STANDARD 573

bit

MHz

MHz Symbol

W32 1.2288

scrambling

MHz +

ksps

1.2288

1.2288

1.2288 control

Pilot channel : all O´s

Sync channel

data

1200 bps

Convolutional encoder and repetition

Convolutional encoder and repetition

Convolutional encoder and repetition

Convolutional encoder and repetition

Block interleaver

Block interleaver

Block interleaver

Block interleaver

4800 sps Repeat four times

19.2 ksps

19.2 ksps

Long generator

Long generator

Long code generator

Symbol scrambling

Symbol cover

Q-channel pilot PN sequence

M u x

M u x

Symbol cover

Power control bit

Symbol cover

Paging channel p

Long code mask

User i long code mask

User j long code mask

Figure 16.9 Forward CDMA channel structure.

Pilot channel index

Each BS shall use a time offset of the pilot PN sequence to identify its forward CDMAchannel Time offsets may be reused within a CDMA cellular system, so long as thecoverage area of the BS emitting a given pilot PN sequence time offset does not overlapthe coverage area of another BS using the same pilot PN sequence time offset Distinctpilot channels shall be designated by an index identifying an offset value from a zerooffset pilot PN sequence (in increments of 64 PN chips) The zero offset pilot PN sequence

Table 16.2 Transmitted

symbol energy versus data rate Data rate

(bps)

Energy per modulation symbol

be possible for the pilot PN sequence offset (the offset index of ‘111 111 111’ binary shall

be reserved) The pilot PN sequence offset shall be denoted as a 9-bit binary pilot PNsequence index for a given BS The timing offset for a given pilot PN sequence shall

be equal to the offset index value multiplied by 64 multiplied by the pilot channel chipperiod (= 813.802 ns) For example, if the pilot PN sequence offset index is 15 (decimal),

the pilot PN sequence offset will be 15× 64 × 813.802 ns = 781.1 µs.

In this case the pilot PN sequence will start 781.1µs after the start of every evensecond of the system time The same pilot PN sequence offset shall be used on allCDMA frequency assignments for a given BS

The sync channel shall be an encoded, interleaved, modulated direct-sequence spreadspectrum signal that is used by mobile stations operating within the geographic coveragearea of that BS (a cell or a sector within a cell) to acquire synchronization to the longcode sequence and to acquire system timing Sync channel acquisition is the second stepthat the mobile station takes in acquiring the system

Forward traffic channel data scrambler

The forward traffic channel data shall be scrambled by an additional modulo-2 additionoperation prior to transmission This data scrambling shall be performed on the dataoutput from the block interleaver at the 19 200-cps rate The data scrambling shall beaccomplished by performing the modulo-2 addition of the interleaver output symbol withthe binary value of the PN chip that is valid at the start of the transmission period forthat symbol as shown in Figure 16.10 This sequence generator shall operate at 1.2288-MHz clock rate although only one output of 64 shall be used for data scrambling (i.e

at a 19 200-cps rate) The PN sequence used for data scrambling shall be the decimatedversion of the sequence used by the mobile station for direct-sequence spreading of thereverse traffic channel (either the public long code or the private long code)

CDMA2000 575

1

4 Long

code generator

Convolutional encoder and code repetition

Block interleaver

19.2 kbps

19.2 kHz

1.2288 Mcps

Decimator

52.0833 µs = one code symbol

64 PN chips per code symbol

PN chip used for scrambling (input to x or gate)

800 Hz MUX

MUX timing control

Power control bit

Scrambled code symbol

or Power control bit

Figure 16.10 Data scrambler timing.

16.2 IS-95B CDMA

IS-95B specifies the high-speed data operation using up to eight parallel codes, resulting in

a maximum bit rate of 115.2 kbps The summary of IS-95A and IS-95B system parameters

is given in Table 16.3 The system block diagrams and numerology for rate set 1 and rateset 2 are shown in Figures 16.11 and 16.12, respectively

16.3 CDMA2000

The goal has been to provide data rates that meet the IMT-2000 performance requirements

of at least 144 kbps in a vehicular environment, 384 kbps in a pedestrian environment and

2048 kbps in an indoor office environment The main focus of standardization has been

to provide 144 and 384 kbps with approximately 5-MHz bandwidth The main parameters

of CDMA2000 are listed in Table 16.4

The two main alternatives for the downlink are multicarrier and direct spread options(see Figure 16.13) Transmission on the multicarrier downlink (nominal 5-MHz band) isachieved by using three consecutive IS 95B carriers where each carrier has a chip rate of1.2288 Mcps For the direct spread option, transmission on the downlink is achieved byusing a nominal chip rate of 3.6864 Mcps

16.3.1 Physical channels

Uplink physical channels

In the uplink, there are four different dedicated channels The fundamental and the plemental channels carry user data

sup-Table 16.3 IS-95 interface parameters

EVRC 8 kbps ACELP 13 kbps

Downlink: Slow quality loop

800 bps

A A

channel code channel

information bits for

User m with rate set 1

r = 1/2, K = 9

Convolutional encoder

r = 1/2, K = 9

Convolutional encoder

r = 1/2, K = 9

Code symbol

Code symbol

Add frame quality indicators

Long code generator

1.2288 mMcps

Decimator

Decimator Decimator

Walsh Function m Modulation

symbol Power control bit

Block interleaving

Symbol repetition

Symbol repetition

Symbol repetition 19.2 ksps 9.6 ksps Long code mask for paging Channel p Add 8-bit encoder tail per frame

Modulation symbol Modulation

symbol

Modulation symbol

Modulation symbol

Modulation symbol

Block interleaving

Block interleaving Long code generator 1.2288 Mcps 19.2 ksps

4.8 ksps

9.6 kbps 4.8 kbps 2.4 kbps 1.2 kbps

19.2 ksps

19.2 ksps

19.2 kbps 9.6 kbps 4.8 kbps 2.4 kbps

MUX*

19.2 ksps 19.2 ksps

Figure 16.11 System block diagram for rate set 1.