Implementation Issues for Channel Estimation and Detection Algorithms for W-CDMA Sridhar Rajagopal and Joseph Cavallaro ECE Dept.

Contents Introduction W-CDMA Channel Estimation and Detection DSP Implementation ASIC Implementation Other Current Projects Future Work

The CDMA Research Group We cover the entire (spread) spectrum! – Algorithms – Implementation issues D M

Implementation Issues Important because –Real-time –Low Power –Mobility /Size DSPs –Signal Processing Communications ASICs / FPGAs –Speed / Size

W-CDMA CDMA : Code Division Multiple Access W-CDMA : Wideband CDMA (5 MHz) –Next Generation Communication Systems –Integrating Multimedia Capabilities –QoS /Multi-rate Services –Higher Data Rates 2048,384,144 Kbps

Uplink - Async, Multiuser Uplink - Async, Multiuser Direct Path Reflected Paths Base Station Noise + MAI User 1 User 2

Downlink - Sync, Single User Direct Path Reflected Paths Base Station Noise + MAI User 1 User 2 User 1

Determining the Channel Channel Estimation –Need to know the Channel for proper detection Delays and Amplitudes : Multiuser/path –Send sequence of known bits (Pilot / Preamble) –2 types Code Multiplexed with Data Time Multiplexed with Data Detection –Use knowledge of channel for detection of Data bits

W-CDMA Standards Not fixed yet…... Uplink –Channel Estimation - Time Multiplexed –Multiuser Detection Downlink –Channel Estimation - Common Pilot –Detection : Rake Receivers/ Equalizers

Channel Estimation Uplink –Time Multiplexed Maximum Likelihood Subspace Downlink –Continuous LMS Based Adaptive

Multiuser Detection Optimal Sub-optimal Linear Interference Cancellation Neural Network MAI Whitening Decorrelating MMSE Serial SIC Parallel PIC MLSE (Viterbi)

Base-Station Receiver Channel Estimator Multiuser Detector Demux Decoder Data Pilot Estimated Amplitudes & Delays Demodulator Antenna

CDMA Uplink System Channel Encoder Channel Encoder Channel Encoder Spreading AWGN Matched Filter Matched Filter Channel Estimator Matched Filter Multi- User Detector Channel Decoder + User 1 d 1 User 2 d 2 User K d K R(t) User 1 d 1 ' User 2 d 2 ' User K d K ' y1y1 y2y2 yKyK Demux

Maximum Likelihood Channel Estimation Send a time-multiplexed Preamble (Pilot). Channel properties extracted Compare with known pilot and estimate. Keep estimate for remaining data bits (static). Repeat preamble every frame, if no tracking.

The Maximum Likelihood Algorithm Compute the correlation matrices Compute the channel estimate Calculate the noise covariance matrix K. Calculate the channel impulse response vector z. Extract the ampitudes and delays using least squares fit.

The ML Algorithm Complexity Complex-Real Dot Product. Complex-Real Matrix Product. Complex -Real Product. Real Square roots. –Solving quadratic equation for least squares fit. Critical code : Matrix-vector / Dot Product Assuming Unity Noise Covariance Offline

Differencing Multistage Multiuser Detection Based on the principle of Parallel Interference Cancellation (PIC) Cross-correlation information used to remove interference of other users Repeated iterations for convergence Differencing techniques to improve performance

The Differencing Multistage Detector Split the cross correlation matrix into lower, upper and the diagonal matrix. Calculate impulse response x is called the differencing vector.

Multistage Detector Complexity Matrix Multiplication: –Computed only once for one frame Dot Product: –Computed iteratively Critical code: Dot Product

TI Tools Used Evaluation Modules (EVM) for C6201 and C6701 fixed and floating point DSPs –64 KB each internal program & data memory –256 KB SBSRAM, 8 MB SDRAM (external) C Compiler ver 3.0 from Code Generation Tools Code Composer ver 4.02 for profiling

DSP Implementation: Channel Estimation Floating point implementation found more feasible due to matrix inversions and square-roots. Code optimized for the DSP Use of Specialized approximate instructions –Approximate reciprocal square roots –Approximate reciprocals Use of Assembly Code for critical part. –TI's C67 floating point benchmarks for Matrix-Vector Multiplication & Dot Product Data Memory requirements for Channel Estimation

Approximate Instructions & Assembly L = 150, P =3, N= 31, SNR = 5dB, SINR = -10 dB Number of users --> Execution time(in milliseconds) --> Use of specialized instructions and assembly code on C6701 DSP C6701: Original C6701: with Intrinsics C6701: with Assembly 10% improvement 100% improvement

Data Memory Requirements Data to be placed in External memory 130 6

DSP Implementation: Multistage Detection 16-bit Fixed Point C Code Code optimized for the DSP Use of Assembly Code for critical part –TI's C62 fixed point assembly benchmarks for Dot Product Data memory requirements for Multistage Detection

Data Memory Requirements Data can be placed completely in Internal memory

x 10 4 Total Number of Iterations Number of Flops Users:K=15 SNR=6dB Conventional Method Differencing Method Flops Count conventional differencing 2X speedup for a three-stage detector

Real-Time Requirements Real-Time capability by C6201 DSP NUMBER OF USERS MAX BIT RATE PER USER (kb/s) SNR=10dB Window Size=12 Conventional Method Differencing Method 12users 150kb/s

Trends in Recent DSPs More internal memory and higher clock speeds –C6203 : 512 KB data, 384 KB program, 250 MHz –useful for uplink channel estimation algorithms. Specialized Blocks in the DSP Core. –Viterbi decoding in C54. Lower Voltage operation –1.2 V in C5402, useful for saving power consumption in the mobile.

ASIC Implementation MOSIS Tiny-Chip (40-pin DIP) –8 synchronous users –12-bit fixed point implementation –6000 transistors –1.2  m CMOS technology –190kb/s for each user – 3-stage cascade delay < 15  s

Advantages of ASICs Highly paralleled instructions: 4 RISC IPC (instructions per cycle) –accumulating while shifting, loading and storing –recoding while loading Application specific architecture –faster I/O –smaller on chip memory –smaller ALU

Chip (Single Stage) Architecture SHIFT ALUALU RECODER REG (L+L’)A Control Logic Internal signals External signals

Chip Layout 12-bit ALU Soft Decisions Cross- Correlation Recoding logic 2.0 mm

The Actual Chip Photograph

3-stage Cascade Mode Sin Hin Fin Load CLK Sout Hout Fout 1/2 Sin Hin Fin Load CLK Sout Hout Fout 1/2 Sin Hin Fin Load CLK Sout Hout Fout 1/2 Matche d Filter Output Detector Output Hand Shakin g Load R Clock Output Valid

System Timing Load R 1st Stage2nd Stage 3rd Stage Final Output

Interference Cancellation

Scalable ASIC Design Xilinx FPGA XC4000: 500k gates, 96MHz

DSP-ASIC Comparison TI’s ‘C54xx: General purpose DSP core + ASIC

Other Current Projects Simulation Testbed –Entire Chain of Algorithms Simulink - RTW Rapid Prototyping Matlab to DSP Copper Contest –Implementation of Multistage Detector using 0.15 micron Copper Technology

Wireless LAN Project Home Area Wireless LAN High Speed Office Wireless LAN Outdoor CDMA Cellular Network

Future Work Fixed Point Implementations on DSPs/ASICs – Uplink & Downlink Algorithms Approximations using Linear Algebra Support Long Codes and Fading Multistage Detector –Execution time Predictability – Increase Efficiency GPP Comparisons : Praful, Partha, Dr.Adve Effect of DMA and Caches