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

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

3. 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?

4. 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.

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

6. 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

7. 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.

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

9. 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

10. 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

11. Operation

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

13. 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)

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

15. 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)

16. 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)

17. 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

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

19. Mixed Radix Conversion

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

21. 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

22. 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

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

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

25. Conventional Complex RNS
A complex number Z is defined to be

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

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

28. 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.

29. 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

30. 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.

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

32. Applications Filtering (FIR, IIR) Adaptive system
Linear transformations