 Complementary sequences

 For complementary sequences in biology, see complementarity (molecular biology).
In applied mathematics, complementary sequences (CS) are pairs of sequences with the useful property that their outofphase aperiodic autocorrelation coefficients sum to zero. Binary complementary sequences were first introduced by Marcel J. E. Golay in 1949. In 1961–1962 Golay gave several methods for constructing sequences of length 2^{N} and gave examples of complementary sequences of lengths 10 and 26. In 1974 R. J. Turyn gave a method for constructing sequences of length mn from sequences of lengths m and n which allows the construction of sequences of any length of the form 2^{N}10^{K}26^{M}.
Later the theory of complementary sequences was generalized by other authors to polyphase complementary sequences, multilevel complementary sequences, and arbitrary complex complementary sequences. Complementary sets have also been considered; these can contain more than two sequences.
Contents
Definition
Let (a_{0}, a_{1}, ..., a_{N − 1}) and (b_{0}, b_{1}, ..., b_{N − 1}) be a pair of bipolar sequences, meaning that a(k) and b(k) have values +1 or −1. Let the aperiodic autocorrelation function of the sequence x be defined by
Then the pair of sequences a and b is complementary if:
for k = 1, ..., N − 1.
Or using Kronecker delta we can write:
where C is a constant.
So we can say that the sum of autocorrelation functions of complementary sequences is a delta function which is an ideal autocorrelations for many applications like radar pulse compression and spread spectrum telecommunications.
Examples
 As the simplest example we have sequences of length 2: (+1, +1) and (+1, −1). Their autocorrelation functions are (2, 1) and (2, −1), which add up to (4, 0).
 As the next example (sequences of length 4), we have (+1, +1, +1, −1) and (+1, +1, −1, +1). Their autocorrelation functions are (4, 1, 0, −1) and (4, −1, 0, 1), which add up to (8, 0, 0, 0).
 One example of length 8 is (+1, +1, +1, −1, +1, +1, −1, +1) and (+1, +1, +1, −1, −1, −1, +1, −1). Their autocorrelation functions are (8, −1, 0, 3, 0, 1, 0, 1) and (8, 1, 0, −3, 0, −1, 0, −1).
 An example of length 10 given by Golay is (+1, +1, −1, +1, −1, +1, −1, −1, +1, +1) and (+1, +1, −1, +1, +1, +1, +1, +1, −1, −1). Their autocorrelation functions are (10, 1, 2, 3, −2, −1, 0, −1, 0, 1) and (10, −1, −2, −3, 2, 1, 0, 1, 0, −1).
Properties of complementary pairs of sequences
 Complementary sequences have complementary spectra. As the autocorrelation function and the power spectra form a Fourier pair complementary sequences also have complementary spectra. But as the Fourier transform of a delta function is a constant we can write
 where C_{S} is a constant.
 S_{a} and S_{b} are defined as a squared magnitude of the Fourier transform of the sequences. The Fourier transform can be a direct DFT of the sequences, it can be a DFT of zero padded sequences or it can be a continuous Fourier transform of the sequences which is equivalent to the Z transform for Z = e^{jω}.
 CS spectra is upper bounded. As S_{a} and S_{b} are nonnegative values we can write
 also
 If any of the sequences of the CS pair is inverted (multiplied by −1) they remain complementary. More generally if any of the sequences is multiplied by e^{jφ} they remain complementary;
 If any of the sequences is reverted (inverted in time) they remain complemantary;
 If any of the sequences is delayed they remain complementary;
 If the sequences are interchanged they remain complementary;
 If both sequences are multiplied by the same constant (real or complex) they remain complementary;
 If both sequences are decimated in time by K they remain complementary. More precisely if from a complementary pair ((a(k), b(k)) we form a new pair (a(Nk), b(N*k)) with zero samples in between then the new sequences are complementary.
 If alternating bits of both sequences are inverted they remain complementary. In general for arbitrary complex sequences if both sequences are multiplied by e^{jπkn/N} (where k is a constant and n is the time index) they remain complementary
 A new pair of complementary sequences can be formed as [a b] and [a −b] where [..] denotes concatenation and a and b are a pair of CS;
 A new pair of sequences can be formed as {a b} and {a −b} where {..} denotes interleaving of sequences.
 A new pair of sequences can be formed as a + b and a − b.
Golay pair
A complementary pair a, b may be encoded as polynomials A(z) = a(0) + a(1)z + ... + a(N − 1)z^{N−1} and similarly for B(z). The complementarity property of the sequences is equivalent to the condition
for all z on the unit circle, that is, z = 1. If so, A and B form a Golay pair of polynomials. Examples include the Shapiro polynomials, which give rise to complementary sequences of length a power of 2.
Applications of complementary sequences
 Multislit spectrometry
 Ultrasound measurements
 Acoustic measurements
 radar pulse compression
 WiFi networks,
 3G CDMA wireless networks
 OFDM communication systems
 Train wheel detection systems
 Nondestructive tests (NDT)
 Communications
See also
 Pseudorandom binary sequences (also called maximum length sequences or Msequences)
 Gold sequences
 Kasami sequences
 Walsh–Hadamard sequences
 Binary Golay code (Errorcorrecting code)
 Ternary Golay code (Errorcorrecting code)
References
 M.J.E. Golay (1949). "Multislit spectroscopy". J. Opt. Soc. Amer. 39: 437–444.
 M.J.E. Golay (April 1961). "Complementary series". IRE Trans. Inform. Theory IT7: 82–87.
 M.J.E. Golay (1962). "Note on “Complementary series″". Proc. IRE 50: 84.
 R.J. Turyn (1974). "Hadamard matrices, BaumertHall units, foursymbol sequences, pulse compression, and surface wave encodings". J. Combin. Theory (A) 16 (3): 313–333. doi:10.1016/00973165(74)900569.
 Peter Borwein (2002). Computational excursions in probability and number theory. CMS Books in Mathematics. SpringerVerlag. pp. 110–119. ISBN 0387954449.
 P.G. Donato; J. Ureña; M. Mazo; C. De Marziani; A. Ochoa (2006). "Design and signal processing of a magnetic sensor array for train wheel detection". Sensors and actuators. A, Physical 132 (2): 516–525. doi:10.1016/j.sna.2006.02.043.
Categories: Sequences and series
 Signal processing
 Pseudorandom number generators
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