CH 4. Air Interface of the IS-95A CDMA System

Transcription

1 CH 4. Air Interface of the IS-95A CDMA System 1 Contents Summary of IS-95A Physical Layer Parameters Forward Link Structure Pilot, Sync, Paging, and Traffic Channels Channel Coding, Interleaving, Data Scrambling, and Modulation Power Control Sub-channel, Spreading, and Pulse-Shaping Reverse Link Structure Access and Traffic Channels Channel Coding, Interleaving, and Modulation Burst Transmission, Direct and Quadrature Spreading 2

2 Summary of of IS-95A Physical Layer Parameters Chip Rate BW / Carrier Spacing Spreading Codes Frame Length Forward Orthogonal Code Modulation / Spreading Channel Coding Voice Coding Power Control Diversity 1.23 MHz / 1.25 MHz Forward : I/Q short PN codes(2 15 = chips : ms) Reverse : I/Q short PN codes(2 15 = chips : ms) and Long PN code(2 42-1) 20 ms, ms (Sync Ch.) Walsh code Forward : BPSK / QPSK Reverse : 64-ary orthogonal / OQPSK Forward : Convolutional code (r=1/2, k=9) Reverse : Convolutional code (r=1/3, k=9) Variable rate QCELP (8.6 / 4.0 / 2.0 / 0.8 kbps for rate set 1 and / 6.25 / 2.75 / 1.05 kbps for rate set 2) and EVRC Forward : Power Allocation Reverse : Closed loop ( Rate : 800 Hz ) + Open loop + Outer loop Forward : Path + Time + Space (Handover) diversity Reverse : Path + Time + Space (Antenna, Handover) diversity 3 Forward Link Link Structure [1],[2] [1],[2] The IS-95 forward link (base station-to-mobile station direction) consists of pilot, sync, paging, and traffic channels. Among these, pilot and sync channels are called the broadcasting channel. IS-95 base stations may support up to 64 forward link channels per each sector for 1.23MHz band, as shown in Fig. 4.1, 1 pilot channel 1 sync channel up to 7 paging channels up to 55 traffic channels In IS-95 forward link, 64 Walsh codes are used to isolate each channel, along with I/Q short PN codes to reduce the multipath interference and other-cell interference. In IS-95 forward link, BPSK data modulation is employed. In IS-95 forward link, a convolutional coding with rate ½ and constraint length 9 is employed. All forward link channels are summed at base band prior to transmission. All forward link channels should be aligned within 1/8 PN chip errors. 4

3 Forward Link Link Structure (cont.) Fig. 4.1 An example of IS-95 forward link channel assignments. 5 Forward Link Link Structure: Pilot Channel No information data (all zero data): only I/Q short PN codes Used for code and carrier synchronization Used for multi-path searching for rake combining Used for channel estimation for coherent demodulation Used for power measurements for handover, etc. 10% ~ 20% of total transmit power is assigned to the pilot channel Fig. 4.2 Pilot channel modulation. symbol mapping pilot gain 6

4 Forward Link Link Structure: Sync Channel Used to transmit the system time obtained from GPS satellites A sync channel frame is ms in length equivalent to the period of I/Q short PN codes and is aligned with the PN codes. A sync channel super frame is 80 ms in length consisting of three sync channel frame. Messages to be transmitted on the sync channel shall begin only at the start of a sync channel super frame. The sync channel message is transmitted at a rate of 1200 bps. The sync channel message contains System time System and network identification Pilot PN offset of the base station State of the long code shift register Paging channel data rate, etc. 7 Forward Link Link Structure: Sync Channel (cont.) 96 bits Sync Frame #1 Sync Frame #2 Sync Frame #3 80 ms Fig. 4.3 Sync channel super frame. Walsh 32 Sync Channel Data 1.2 kbps Convolutional Encoding (r=1/2, k=9) 2.4 ksps Symbol Repetition 4.8 ksps Block Interleaver 4.8 ksps A Fig. 4.4 Sync channel modulation. 8

5 Forward Link Link Structure: Paging Channel The paging channel is used to transmit control information from the base station to the mobile station for call setup. Up to 7 paging channels can be associated with a single FA (frequency assignment, 1.23MHz). The mobile station always monitors a paging channel and responds to pages through one of access channels associated with that particular paging channel. The paging channel data is transmitted at 4800 or 9600 bps. The paging channel is normally operated in slot mode, where control messages for a particular mobile is sent in a pre-defined time slot. During registration, the mobile is assigned a time slot in which it will receive control messages. 9 Forward Link Link Structure: Paging Channel (cont.) The paging channel message contains Page messages System parameters: PN offset, system ID, network ID, base station ID, search windows, handoff parameters, etc. Access parameters: Number of access channels, number of access probes, authentication data, etc. Neighbor cell list, etc. Fig. 4.5 Paging channel modulation. scrambling 10

6 Forward Link Link Structure: Traffic Channel The forward traffic channel is mainly used to transfer voice and data from the base station to the mobile station. The forward traffic channel is also used for transmission of signaling data. Traffic Channel Data Rates (Variable Data Rates) Rate Set 1: 9600, 4800, 2400, and 1200 bps Rate Set 2: 14400, 7200, 3600, and 1800 bps When signaling data is to be transmitted, data rate is always changed to the full rate (9600 or 14400). There are two options to transmit signaling data: Blank and burst, Dim and burst. Blank and burst: The entire traffic channel frame is used to send only signaling data. Dim and burst: The traffic channel frame is used to send both primary traffic and signaling data. 11 Forward Link Link Structure: Traffic Channel (cont.) Traffic Channel Information Data ADD CRC ADD Tail 8 Bits Convolutional Encoding (r=1/2, k=9) Symbol Repetition Block Interleaver B 8.6 kbps 4.0 kbps 2.0 kbps 0.8 kbps 9.2 kbps 4.4 kbps 2.0 kbps 0.8 kbps 9.6 kbps 4.8 kbps 2.4 kbps 1.2 kbps 19.2 ksps 9.6 ksps 4.8 ksps 2.4 ksps 19.2 ksps 19.2 ksps Long Code mask for user m Long Code B Decimator 19.2 Ksps Power control bit Decimator 800 Hz MUX 800 bps W 64,n Short PN_I FIR FIR cosω c t Σ s(t) Short PN_Q symbol mapping -sinω c t traffic Ch. gain Fig. 4.6 Forward traffic channel modulation for RS1. 12

7 Forward Link Link Structure: Traffic Channel (cont.) Traffic Channel Information Data ADD CRC ADD Tail 8 Bits Convolutional Encoding (r=1/2, k=9) Symbol Repetition Puncturing Block Interleaver B 13.4 kbps 6.3 kbps 2.8 kbps 1.1 kbps 14.0 kbps 6.8 kbps 3.2 kbps 1.4 kbps 14.4 kbps 7.2 kbps 3.6 kbps 1.8 kbps 28.8 ksps 14.4 ksps 7.2 ksps 3.6 ksps 28.8 ksps 19.2 ksps 19.2 ksps Long Code mask for user m Long Code B Decimator 19.2 Ksps Power control bit Decimator 800 Hz MUX 800 bps W 64,n Short PN_I FIR FIR cosω c t Σ s(t) Short PN_Q symbol mapping -sinω c t traffic Ch. gain Fig. 4.7 Forward traffic channel modulation for RS2. 13 Forward Link Link Channel Coding [2] [2] The sync, paging, and forward traffic channels shall be convolutionally encoded prior to transmission. The convolutional code used in the forward link shall be of rate 1/2 with a constraint length of 9. The generator functions of the code shall be g 0 and g 1 that equal 753(octal) and 561(octal), respectively. Fig. 4.8 Convolutional encoder. 14

8 Forward Link Link Block Interleaving [2] [2] All symbols after repetition are block interleaved by using a bit reversal method or modified bit reversal method. For example, the sync channel shall use a block interleaver spanning ms which involves 128 modulation symbols. The 128 input symbols are written into a linear array with addresses viewed by 7-bit binary number a 6 a 5 a 4 a 3 a 2 a 1 a 0. For reading, the mapping of addresses shall be performed as c 0 =>a 6, c 1 =>a 5, c 2 =>a 4, c 3 =>a 3, c 4 =>a 2, c 5 =>a 1, c 6 =>a Forward Link Link Block Interleaving (cont.) Table. 4.1 Write operation for 128 symbols with two time repetition. Address 0 Address

9 Forward Link Link Block Interleaving (cont.) Table. 4.2 Read operation for 128 symbols. 17 Forward Link Link Data Scrambling [2] [2] Fig. 4.9 Data scrambling. 18

10 Forward Link Link Power Control Sub-channel [2] [2] Fig Position of power control bits. 19 Forward Link Link Quadrature Spreading [1],[2] [1],[2] Following Walsh orthogonal spreading, each channel is spread in quadrature. The I and Q channel spreading sequences (also called short PN codes) have a length of 2 15 chips (i.e., chips = ms) due to zero insertion. The I and Q channel spreading is used to mitigate multipath interference and other-cell interference. The characteristic polynomials of the PN sequences are ( ) P x x x x x x x I ( ) = P x x x x x x x x x Q =

11 Forward Link Link Quadrature Spreading (cont.) Fig Forward channel signal constellation and phase transition. 21 Forward Link Link Pulse-Shaping Filter [2] [2] Following the I/Q spreading operation, I and Q impulses are applied to pulse-shaping filters to limit the spectrum of a transmitted signal. The pulse-shaping filter should satisfy the condition that δ 1 =1.5 db (pass band ripple), δ 2 =40 db, f p =590 khz, f s =740 khz. Fig Frequency response specifications of a pulse-shaping filter. 22

12 Forward Link Link Pulse-Shaping Filter (cont.) Table tap coefficients of the sample pulse-shaping filter with four times over-sampling. 23 Forward Link Link Pulse-Shaping Filter (cont.) Fig Comparison with a truncated ideal low-pass filter. 24

13 Forward Link Link Pulse-Shaping Filter (cont.) Fig Frequency response of the sample pulse-shaping filter. 25 Forward Link Link Pulse-Shaping Filter (cont.) Fig Signal waveform after pulse-shaping. 26

14 Reverse Link Link Structure [1],[2] [1],[2] The IS-95 reverse link is composed of access channels and reverse traffic channels. Each channel in the reverse link is identified by the long PN code with the period of T c. Each traffic channel is identified by a private user long code. Each access channel is identified by a public long code. In IS-95 reverse link, the quadrature spreading by I/Q short PN codes is employed, along with the direct spreading by long PN code. The I/Q short PN codes are the same as those used in the forward link. The Q channel PN sequence is delayed by half a PN chip to reduce the signal fluctuation due to zero crossing (OQPSK). In IS-95 reverse link, noncoherent 64-ary orthogonal modulation scheme is employed. In IS-95 reverse link, a convolutional coding with rate 1/3 and constraint length 9 is employed. 27 Reverse Link Link Structure (cont.) [1],[2] [1],[2] Fig An example of IS-95 reverse link channels. 28

15 Reverse Link Link Structure: Access Channel Used for call origination by a mobile, response to paging, and registration. Up to 32 access channels are associated with a single paging channel. The data rate on the access channel is 4800 bps. Each access probe (or access slot ) consists of an access preamble and message capsule as shown in Fig The access preamble is used for a base station to obtain a synchronization to a mobile. The maximum sizes of access preamble and message capsule are all 16 frames and the minimum sizes are 1 and 3 frames, respectively. After transmitting an access probe, the mobile waits a specified period for an acknowledgement from the base station. If an acknowledgement is received, the access attempt is completed. Otherwise, the next access probe is transmitted at a power level higher than the previous one after a pseudo-randomly generated delay. The entire process to send an access probe and receive an acknowledgement is called an access attempt, which is depicted in Fig Reverse Link Link Structure: Access Channel (cont.) Fig Access probe structure. 30

16 Reverse Link Link Structure: Access Channel (cont.) Fig Access probe sequence (ALOHA). 31 Reverse Link Link Structure: Access Channel (cont.) Public Long Code Mask Long Code Short PN_I FIR cosω c t A Σ s(t) D FIR Short PN_Q 1/2 T c symbol mapping -sinω c t Ch. gain Fig Access channel modulation. 32

17 Reverse Link Link Structure: Traffic Channel Transmits user information such as voice and data. Transmits also signaling data. Each traffic channel is identified by a private user long code. Reverse Traffic Channel Data Rate Rate Set 1: 9600, 4800, 2400, and 1200 bps Rate Set 2: 14400, 7200, 3600, and 1800 bps The reverse traffic channel data is transmitted in burst mode for variable rate transmission, which is due to closed-loop power control in reverse link. When a signaling data is to be transmitted, the data rate is changed to the full rate. 33 Reverse Link Link Structure: Traffic Channel User Long Code Mask Long Code Short PN_I cosω c t FIR A Data Burst Randomizer Σ s(t) D FIR Frame Data Rate Short PN_Q 1/2 T c symbol mapping -sinω c t Ch. gain Fig Reverse traffic channel modulation for RS1. 34

18 Reverse Link Link Channel Coding [1],[2] [1],[2] The access channel and reverse traffic channel shall be convolutionally encoded prior to transmission. The convolutional encoder shall be of rate 1/3 with a constraint length of 9. The generator functions of the code shall be g 0 equals 557(octal) and g 1 equals 663(octal), and g 2 equals 711(octal). Fig k=9, rate 1/3 convolutional encoder. 35 Reverse Link Link Block Interleaving [2] [2] The mobile station shall interleave all coded symbols on the reverse traffic channel and access channel prior to modulation and transmission. The interleaver shall be an array with 32 rows and 18 columns (576 cells), spanning 20 ms. Coded symbols shall be written into the interleaver by columns filling the complete 32 x 18 matrix. Reverse Traffic channel coded symbols shall be output from the interleaver by rows in the following order. At 9600 bps: At 4800 bps : At 2400 bps : At 1200 bps :

19 Reverse Link Link Modulation [1],[2] [1],[2] Modulation for the reverse link channel shall be 64-ary orthogonal modulation. After interleaving, every six consecutive symbols are grouped to form a Walsh symbol, which is then mapped to one of Walsh functions. The modulation symbols shall be selected according to the following rule: Modulation Symbol Index = c c1 + 4c2 + 8c3 + 16c4 32 c 5 where c 5 represents the latest (or most recent) and c 0 the first (or oldest) binary valued code symbol of each Walsh symbol. The 64 by 64 Walsh matrix is used to generate Walsh functions by means of the following recursive procedure: H H H H H = H = N N N 64 HN H N H32 H 32 The period of a Walsh symbol shall be 64 Walsh chips, which correspond to 256 PN chips ( μs). 37 Reverse Link Link Burst Transmission [2] [2] Prior to transmission, the Walsh symbol stream is gated with an ON-OFF filter that allows transmission of certain power control groups and deletion of others, as shown in Fig The gated-on and gated-off groups are determined by the data rate of the frame and by a block of 14 bits taken from the long PN code in the data burst randomizer. For 4800 bps, transmission shall occur on power control groups numbered: b, + b b1, 4 + b2, 6 + b3, 8 + b4, 10 + b5, 12 + b6, 14 7 For 2400 bps, transmission shall occur on power control groups numbered: b if b = 0, or 2+ b if b = 1, b if b = 0, or 6+ b if b = 1, b if b = 0, or 10+ b if b = 1, b6 b11 = if 0, or 14+ b if b = 1. 38

20 Reverse Link Link Burst Transmission (cont.) Fig Reverse Traffic Channel variable rate transmission. 39 Reverse Link Link Direct Sequence Spreading [1],[2] [1],[2] Prior to transmission, the reverse traffic channel and the access channel shall be spread by either a private user long code or a public long code. The long code shall be periodic with period

21 Reverse Link Link Quadrature Spreading [1],[2] [1],[2] Following the direct sequence spreading, the reverse traffic channel and access channel are spread in quadrature. The sequences used for this spreading shall be the same as those used on the forward link channel. The characteristic polynomials of the PN sequences are ( ) ( ) P x x x x x x x I = P x x x x x x x x x Q = The data spread by the Q channel PN sequence shall be delayed by half a PN chip and a resulting signal constellation is that of OQPSK, as shown in Fig Reverse Link Link Quadrature Spreading (cont.) Fig Reverse CDMA channel signal constellation. 42

22 References 1. Samuel C. Yang, CDMA RF System Engineering, Artech House, Qualcomm, CDMA System Training Handbook-vol. 1,