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Adaptive WCDMA: Theory And Practice. Savo G. Glisic Copyright ¶ 2003 John Wiley & Sons, Ltd. ISBN: 0-470-84825-1 16 Standards In this chapter we discuss the basic Code Division Multiple Access (CDMA) standards and 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) of each standard. At this stage we use what we have learnt so far in this book to discuss the motivations behind the solutions. We believe that at the end of the book the reader should have the required knowledge to follow the closing discussion on the various choices for the different 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 reverse channel data path is shown in Figure 16.2. Data and chip rates in different points in the system are indicated on the picture. The voice source is encoded by using variable rate codec with four possible rates of 1.2 to 9.6 kbps. This is the way to exploit voice activity factor. Prior to modulation, convolutional encoder and block interleaver are used. Modulation for the reverse CDMA channel is 64-ary orthogonal signaling. One of the possible modulation symbols will be transmitted for each six-code symbol. Modulation symbol number = c0 + 2c1 + 4c2 + 8c3 + 16c4 + 32c5 (16.1) c5 shall represent the last or the most recent and c0 the first or the oldest binary valued (0 and 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 chip rate 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 566 STANDARDS Reverse CDMA channels (1.23-MHz channel received by base station) Access Access Ch 1 • • • Ch n Traffic Ch 1 Traffic • • • • • • • • • • • • • • • • • • • • Ch 55 User address long code PNs Figure 16.1 Example of logical reverse CDMA channels received at a base station. Zero-shift plot PN sequence I-channel Long code mask Long code generator Convolutional encoder and repetition r = 1/3 K = 9 Code symbol 28.8 ksps FIR filter 1/2 PN chip delay = 406.9 ns Q 64-ary orthogonal modulator Block interleaver I D Figure 16.2 I 1.2288 MHz Code symbol Information bit 9.6 kbps 4.8 kbps 2.4 kbps 1.2 kbps 1.2288 MHz PN chip FIR filter 28.8 ksps I(t ) D/A and filtering Q(t ) D/A and filtering PN chip Walsh chip 307.2 kHz Q Zero-shift plot PN sequence Q-channel A cos(2pft ) + A sin(2pft ) Reverse CDMA channel data path example. equal to 208.333 µs. 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 different pseudonoise (PN) sequences. Data and PN sequence combination involves modulo-2 addition of the encoded, interleaved data stream with two PN code streams, each operating at 1.2288 MHz. The first sequence is referred to as the long code sequence. This sequence shall be a time shift of a sequence of length 242 –1 chips and shall be generated by a 567 IS 95 STANDARD Table 16.1 Reverse traffic channel numerology Data Rate (bps) Parameter PN chip rate Code rate TX duty cycle Code symbol rate Modulation 9600 1.2288 1/3 100.0 28 800 6 4800 1.2288 1/3 50.0 28 800 6 2400 1.2288 1/3 25.0 28 800 6 1200 1.2288 1/3 12.5 28 800 6 Walsh symbol rate Walsh chip rate Walsh symbol PN chips/code symbol PN chips/Walsh symbol PN chips/Walsh chip 4800 307.20 208.33 42.67 4800 307.20 208.33 42.67 4800 307.20 208.33 42.67 4800 307.20 208.33 42.67 256 256 256 256 4 4 4 4 1 1 3 2 x1 x2 4 x3 5 x4 6 x5 7 x6 8 x7 9 x8 10 x9 39 x 10 Units Mcps Bits/code sym % Sps Code sym/ Walsh sym Sps kcps µs PN chip/ code sym PN chip/ Walsh sym PN chip/ Walsh sym 40 41 x 39 x 40 42 x 41 x 42 Modulo-2 addition lsb Long code sequence msb 42-Bit logic code matrix Figure 16.3 Long code generation and masking. linear generator using the following polynomial: p(x) = x 42 + x 35 + x 33 + x 31 + x 27 + x 26 + x 25 + x 22 + x 21 + x 19 + x 18 + x 17 + x 16 + x 10 + x 7 + x 6 + x 5 + x 3 + x 2 + x 1 + 1. The long code will be generated by masking the 42-bit state variables of the generator with a 42-bit mask. The actual PN sequence is generated by the modulo-2 addition of all 42 masked output bits of the sequence generator as shown in Figure 16.3. 16.1.3 Code mask The structure of the code mask is shown in Figure 16.4. The mask, used for the PN spreading, will vary depending on the channel type on which the mobile station is communicating. Access channel M24 through M41 shall be set to ‘1’. M19 through M23 shall 568 STANDARDS Access channel long code mask 24 23 41 1 1 1 ••• 19 18 ACN PCN 16 15 9 8 REG_ZONE 0 PILOT_PN 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 32 31 41 0 0 0 ••• 41 40 39 0 1 0 ESN 0 Private long code mask 0 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 to the 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 serial number (ESN) and a private long code unique for each mobile identification number (MIN). The public long code shall be as follows: M32 through M41 shall 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 access channel and the reverse traffic channel shall be offset quadrature phase shift keying (OQPSK) spread prior to actual transmission. This offset quadrature spreading on the reverse 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 the zero-time offset codes. The generating functions for the I and Q short PN codes shall be as follows: PI (x) = x 15 + x 13 + x 9 + x 8 + x 7 + x 5 + 1 PQ (x) = x 15 + x 12 + x 11 + x 10 + x 6 + x 5 + x 4 + x 3 + 1 16.1.4 Data burst randomizer algorithm The data burst randomizer generates a masking stream of 0 s and 1 s that randomly mask out the redundant data generated by the code repetition. The masking stream pattern is determined by the frame data rate and by the block of 14 bits taken from the long code sequence. These mask bits are synchronized with the data flow and the data is selectively masked by these bits through the operation of the digital filter. The 1.2288-MHz-long 569 IS 95 STANDARD 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 power control group (1.25 ms) before each reverse traffic channel frame boundary. b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 b12 b13 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 b13 shall represent the last (or most recent) bit to enter the sift register. Each 20-ms reverse traffic channel frame shall be divided into 16 equal length (i.e. 1.25 ms) power control groups numbered from 0 to 15. The data burst randomizer 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 b0 if (b8 = 0 and b12 = 0), 2 + b1 if (b8 = 1 and b12 = 0) 4 + b2 if (b9 = 0 and b12 = 1), 6 + b3 if (b9 = 1 and b12 = 1) 8 + b4 if (b10 = 0 and b13 = 0), 10 + b5 if (b10 = 1 and b13 = 0) 12 + b6 if (b11 = 0 and b13 = 1), 14 + b7 if (b11 = 1 and b13 = 1) An example is shown in Figure 16.5. 570 STANDARDS 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 Full rate 12 13 14 15 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Code symbols transmitted: Power control group number 1 33 65 97 … 481 513 545 2 34 66 98 … 452 514 546 Previous frame 12 13 14 15 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Code symbols transmitted: 1 17 33 49 … 241 257 273 2 18 34 50 … 242 258 274 Previous frame 12 13 14 15 0 Previous frame 12 13 14 15 0 b 0 1/2 rate 1/4 rate 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Code symbols transmitted: 1 9 17 25 … 121 129 137 2 10 18 26 … 122 130 138 1/8 rate 3 4 5 6 7 8 9 10 11 12 13 14 15 Code symbols transmitted: 1 5 9 13 … 61 65 69 2 6 10 14 … 62 66 70 Sample masking streams shown PN bits used are for the 14-bit PN sequence: for scrambling (b0, b1, …, b13) = 0 0 1 0 1 1 0 1 1 0 0 1 0 0 b b b b b b b b b b b b b 1 2 3 4 5 6 7 8 9 1 1 1 1 0 1 2 3 PCG 15 PCG 14 1 2 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 default multiplex option’s 9600-bps and 4800-bps transmission rates, the frame quality indicator shall be a cyclic redundancy check (CRC). For the 9600-bps and 4800-bps rates, the frame quality indicator (CRC) shall be calculated on all bits within the frame, except the frame quality indicator (CRC) itself and the encoder tail bits. The 9600-bps transmission rate shall use a 12-bit frame quality indicator (CRC), which shall be transmitted within the 192-bit long frame. The generator polynomial for the 9600-bps rate g(x) = x 12 + x 11 + x 10 + x 9 + x 8 + x 4 + x + 1 The 4800-bps transmission rate shall use a 8-bit CRC, which shall be transmitted within the 96-bit long frame. The generator polynomial for the 4800-bps rate g(x) = x 8 + x 7 + x 4 + x 3 + x + 1 16.1.6 The CRCs procedure The circuit block diagrams for 9600 and 4800 bps are shown in Figures 16.6 and 16.7, respectively. Initially, all shift register elements shall be set to logical one and the switches shall be set in the up position. The register shall be clocked 172 times (for 192-bit frame) or 80 times (for 96-bit frame) with the traffic or the signaling bits and mode/format indicators 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). 571 IS 95 STANDARD Input 0 X0 X1 X3 X4 X7 X9 X8 X 10 X 11 Output 0 Denotes one-bit storage element Up for first 172 bits Down for last 12 bits Denotes modulo-2 addition Figure 16.6 Reverse traffic channel frame quality indicator calculation at 9600-bps rate for the default multiplex option(1). Input 0 X0 X1 X2 X3 X4 X5 X6 X7 Output 0 Denotes one-bit storage element Denotes modulo-2 addition Up for first 80 bits Down for last 8 bits 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 in the order calculated. 16.1.7 Base station Transmitter Each BS within a given system shall use the same CDMA frequency assignments for each 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 rates are supported: 9600, 4800, 2400 and 1200 bps. The data rate shall be selectable on a frame-by-frame (i.e. 20-ms) basis without consideration for the rate in the previous or subsequent frames. Although the data rate may vary on a 20-ms basis, the modulation 572 STANDARDS Forward CDMA channel (1.23-MHz channel transmitted by base station) Pilot chan W0 Sync chan W32 Paging Ch 1 W1 Paging Traffic Ch 7 Ch 1 Up to W7 W = Walsh symbol number Figure 16.8 Traffic Ch N W8 Traffic data Up to Traffic Traffic Ch 24 Ch 25 W31 Traffic Ch 55 Up W33 to W63 Mobile power control subchannel 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 transmitted using lower energy, as shown in Table 16.2. Note that all the symbols in the interleaver block are from the same frame. Thus they are 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 CDMA channel. It is an unmodulated spread spectrum signal that is used by a mobile station operating within the geographic coverage area of the base station. It is used by the mobile station to acquire synchronization with the pilot PN sequence, to provide a phase reference 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. Code for the pilot channel shall be a quadrature sequence of length 215 (i.e. 32768 PN chips in length). Code polynomial PI (x) = x 15 + x 13 + x 9 + x 8 + x 7 + x 5 + 1 PQ (x) = x 15 + x 12 + x 11 + x 10 + x 6 + x 5 + x 4 + x 3 + 1 for 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 , a binary 1 is inserted in the sequence generator output after the contiguous succession of 14 binary 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. 573 IS 95 STANDARD I-channel pilot PN sequence W0 + + Pilot channel: all O´s + W32 1.2288 Sync channel Convolutional data encoder and repetition 1200 bps Paging channel Convolutional data encoder and 9.6 kbps repetition 4.8 kbps 2.4 kbps Paging channel p Long code mask Forward traffic channel data Convolutional 9.6 kbps encoder and 4.8 kbps repetition 2.4 kbps 1.2 kbps User i long code mask Forward traffic Convolutional channel data 9.6 kbps encoder and 4.8 kbps repetition 2.4 kbps 1.2 kbps User j long code mask + 4800 sps Repeat Block four interleaver times Block interleaver 19.2 ksps + MHz + Symbol cover Wp 1.2288 MHz 19.2 ksps + + + + + Symbol cover Long generator Block interleaver WI Power control 19.2 bit ksps + 1.2288 M u x + + MHz Symbol cover Long generator Block interleaver Long code generator Symbol scrambling Power control 19.2 bit ksps + M u x + WJ 1.2288 MHz + + Symbol cover + Symbol scrambling Modulo-2 addition Q-channel pilot PN sequence 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 CDMA channel. Time offsets may be reused within a CDMA cellular system, so long as the coverage area of the BS emitting a given pilot PN sequence time offset does not overlap the coverage area of another BS using the same pilot PN sequence time offset. Distinct pilot channels shall be designated by an index identifying an offset value from a zero offset pilot PN sequence (in increments of 64 PN chips). The zero offset pilot PN sequence 574 STANDARDS Table 16.2 Transmitted symbol energy versus data rate Data rate (bps) 9600 4800 2400 1200 Energy per modulation symbol ES ES ES ES = Eb /2 = Eb /4 = Eb /8 = Eb /16 shall be such that the start of the sequence shall be output at the beginning of every even second in time, referenced to system time. The start of the zero offset pilot PN sequence for either the I or the Q sequence shall be defined as the state of the sequence generator for which the previous 15 outputs were ‘0’. Five hundred and eleven unique values shall 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 PN sequence 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 chip period (= 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 781.1 µs after the start of every even second of the system time. The same pilot PN sequence offset shall be used on all CDMA frequency assignments for a given BS. The sync channel shall be an encoded, interleaved, modulated direct-sequence spread spectrum signal that is used by mobile stations operating within the geographic coverage area of that BS (a cell or a sector within a cell) to acquire synchronization to the long code sequence and to acquire system timing. Sync channel acquisition is the second step that 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 addition operation prior to transmission. This data scrambling shall be performed on the data output from the block interleaver at the 19 200-cps rate. The data scrambling shall be accomplished by performing the modulo-2 addition of the interleaver output symbol with the binary value of the PN chip that is valid at the start of the transmission period for that symbol as shown in Figure 16.10. This sequence generator shall operate at 1.2288MHz 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 decimated version of the sequence used by the mobile station for direct-sequence spreading of the reverse traffic channel (either the public long code or the private long code).