A Zadoff–Chu (ZC) sequence is a complex-valued mathematical sequence which, when applied to a signal, gives rise to a new signal of constant amplitude. When cyclically shifted versions of a Zadoff–Chu sequence are imposed upon a signal the resulting set of signals detected at the receiver are uncorrelated with one another.
Description Zadoff–Chu sequences exhibit the useful property that cyclically shifted versions of themselves are orthogonal to one another. A generated Zadoff–Chu sequence that has not been shifted is known as a root sequence.
The complex value at each position n of each root Zadoff–Chu sequence parameterized by u is given by
x u ( n ) = exp ( − j π u n ( n + c f + 2 q ) N ZC ) , {\displaystyle x_{u}(n)={\text{exp}}\left(-j{\frac {\pi un(n+c_{\text{f}}+2q)}{N_{\text{ZC}}}}\right),\,}
where
0 ≤ n < N ZC {\displaystyle 0\leq n<N_{\text{ZC}}} ,
0 < u < N ZC {\displaystyle 0<u<N_{\text{ZC}}} and gcd ( N ZC , u ) = 1 {\displaystyle {\text{gcd}}(N_{\text{ZC}},u)=1} ,
c f = N ZC mod 2 {\displaystyle c_{\text{f}}=N_{\text{ZC}}\mod 2} ,
q ∈ Z {\displaystyle q\in \mathbb {Z} } ,
N ZC = length of sequence {\displaystyle N_{\text{ZC}}={\text{length of sequence}}} . Zadoff–Chu sequences are CAZAC sequences (constant amplitude zero autocorrelation waveform). Note that the special case q = 0 {\displaystyle q=0} results in a Chu sequence,. Setting q ≠ 0 {\displaystyle q\neq 0} produces a sequence that is equal to the cyclically shifted version of the Chu sequence by q {\displaystyle q} , and multiplied by a complex, modulus 1 number, where by multiplied we mean that each element is multiplied by the same number.
Properties of Zadoff-Chu sequences 1. They are periodic with period N ZC {\displaystyle N_{\text{ZC}}} .
x u ( n + N ZC ) = x u ( n ) {\displaystyle x_{u}(n+N_{\text{ZC}})=x_{u}(n)}
2. If N ZC {\displaystyle N_{\text{ZC}}} is prime, the Discrete Fourier Transform of a Zadoff–Chu sequence is another Zadoff–Chu sequence conjugated, scaled and time scaled.
X u [ k ] = x u ∗ ( u ~ k ) X u [ 0 ] {\displaystyle X_{u}[k]=x_{u}^{*}({\tilde {u}}k)X_{u}[0]} where u ~ {\displaystyle {\tilde {u}}} is the multiplicative inverse of u modulo N ZC {\displaystyle N_{\text{ZC}}} . 3. The auto correlation of a Zadoff–Chu sequence with a cyclically shifted version of itself is zero, i.e., it is non-zero only at one instant which corresponds to the cyclic shift. 4. The cross-correlation between two prime length Zadoff–Chu sequences, i.e. different values of u , u = u 1 , u = u 2 {\displaystyle u,u=u_{1},u=u_{2}} , is constant 1 / N ZC {\displaystyle 1/{\sqrt {N_{\text{ZC}}}}} , provided that u 1 − u 2 {\displaystyle u_{1}-u_{2}} is relatively prime to N ZC {\displaystyle N_{\text{ZC}}} .
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