Brief Overview of Residue Number System (RNS) VLSI Signal Processing 台灣大學電機系 吳安宇

Outline History Why RNS is needed ? Fundamental concepts in RNS. Conversion between Decimal and Residue. Conventional complex RNS Difficult arithmetic operations Applications

History The ancient study of the residue numbering system begins with a verse from a third-centry book, Suan-Ching, by Sun Tzu. We have things of which we do not know the number, If we count them by three, the remainder is 2, If we count them by fives, the remainder is 3, If we count them by sevens, the remainder is 2, How many things are there?

History We commemorate this contribution as the Chinese Remainder Theorem, or CRT. This theorem, as well as RNS, was set forth in the 19th by Carl Friedrich Gauss in his celebrated Disquisitiones Arithmetical.

Why RNS? It is a “carry-free” system that performs addition, substraction, and multiplication as parallel operations. FA FA FA FA HA

Definition The RNS is defined in terms of a set of relatively prime moduli. If P denotes the moduli set, then The dynamic range M is

Definition Any integer in the residue class ZM has a unique L-tuple representation given by where Xi=(X mod pi) and is called the ith residue.

Example X1 1 2 X2 3 4 → X 5 6 7 8 9 10 11 12 13 14

Example 4 5 6 7 8 9 10 11 1 2 3 X → X1 X2 X3 16 17 18 19 20 21 22 23 12 13 14 15 28 29 30 31 32 33 34 35 24 25 26 27 40 41 42 43 44 45 46 47 36 37 38 39 52 53 54 55 56 57 58 59 48 49 50 51

Sign Representation For signed RNS, any integer in (-M/2, M/2], has a unique RNS L-tuple representation where xi=(X mod pi) if X>0, and (M-|X|) mod pi otherwise. X1 1 2 X2 3 → X -6 -5 -4 -3 -2 -1 4 5

Operation

Operation (Example) 7(1, 3, 2 ) +3(0, 3, 3 ) 7(1, 3, 2 ) +3(0, 3, 3 ) 10(1 mod 3, 6 mod 4, 5 mod 5) = (1,2,0) 7(1, 3, 2 ) - 3(0, 3, 3 ) 4(1 mod 3, 0 mod 4, -1 mod 5) = (1,0,4) 7(1, 3, 2 ) *3(0, 3, 3 ) 21(0 mod 3, 9 mod 4, 6 mod 5) = (0,1,1)

Conversion Efficient and rapid implementation of the operation (Xi o Yi) mod pi must be found. Only for the case where pi=2n can be easily implemented in conventional system. Xi (n bits) RAM or ROM Table Lookup Zi = (Xi o Yi) mod pi Zi (n bits) Yi (n bits)

Decimal-to-residue Conversion To be competitive system, the speed of data acquisition and the accompanying decimal-to-residue conversion must be equally fast.

Decimal-to-residue Conversion 2j mod pi j=4 i=1 i=2 j=3 j=2 j=1 j=0 1 3 4 2 pi-(25mod pi)

Residue-to-decimal Conversion Generally speaking, the speed limitation of residue-to-decimal conversion is a problem in residue number system. The R/D problem can be solved in one of the following two ways: Mixed radix conversion (MRC) Chinese Remainder Theorem (CRT)

Multiplicative Inverse The multiplicative inverse of g of modulo pi is denoted as gi-1 and satisfies Example: Pi=5 g gi-1  1 2 3 4 g gi-1 Pi=6  1 2 3 4 5 g gi-1 Pi=7  1 2 4 3 5 6

Mixed Radix Conversion RNS MRC The MRC representation is given by where v1=1, v2=p1, v3=p1p2, v4=p1p2p3, …

Mixed Radix Conversion

Mixed Radix Conversion Use nesting subtractions and multiplicative inverse to make sequential conversion.

Block Diagram of MRC M M M M M M p12-1 p13-1 p23-1 p1L-1 p2L-1 pL-1,L-1 M

where si=M/pi and si-1 is the multiplicative Chinese Remainder Theorem where si=M/pi and si-1 is the multiplicative inverse of si mod pi, so that

Chinese Remainder Theorem X1 s1-1 mod p1 s1 X2 s2-1 mod p2 s2 mod M XL sL-1 mod pL sL

Chinese Remainder Theorem 1*2 mod 3 20 0*3 mod 4 15 mod 60 4*3 mod 5 12

Conventional Complex RNS A complex number Z is defined to be

Difficult Arithmetic Operations Magnitude comparison / Sign detection Division Base extension Scaling / Rounding / Truncation Overflow detection

Magnitude Comparison Sign Detection Not a weighted number system. Every digits are equally important. The problem makes the RNS-based signal processors inefficient.

Division Blend of nested subtractions and magnitude comparisons, so it is more difficult in the RNS. RNS is not closed under division, since the RNS is an integer system, it is. Slow, high-overhead operation and should be avoided in RNS.

Base Extension Increase the dynamic range Increase the resolution. R/D and D/R conversions are needed. R/D conversion L - tuple RNS D/R conversion K - tuple RNS Moduli extension

Scaling / Rounding / Truncation Prevent dynamic-range overflow. A special form of division. The scaling operation is easily to implement when scaling factor is: power of two (2n) in 2’complement. product of one or more of the moduli in RNS.

Overflow Detection Take the form of magnitude comparison. To be avoided by using scaling operation during run-time.

Applications Filtering (FIR, IIR) Adaptive system Linear transformations