A novel construction of lattices is proposed. This construction can be thought of as Construction A with codes that can be represented as the Cartesian product of L linear codes over F p1 , . . . , F pL , respectively; hence, is referred to as the product construction. The existence of a sequence of such lattices that are good for quantization and Poltyrev-good under multistage decoding is shown. This family of lattices is then used to generate a sequence of nested lattice codes which allows one to achieve the same computation rate of Nazer and Gastpar for computeand-forward under multistage decoding, which is referred to as lattice-based multistage compute-and-forward.Motivated by the proposed lattice codes, two families of signal constellations are then proposed for the separation-based compute-and-forward framework proposed by Tunali et al. together with a multilevel coding/multistage decoding scheme tailored specifically for these constellations. This scheme is termed separation-based multistage compute-and-forward and is shown having a complexity of the channel coding dominated by the greatest common divisor of the constellation size (may not be a prime number) instead of the constellation size itself.
I. INTRODUCTIONCompute-and-forward is a novel information forwarding paradigm in wireless communications in which relays in a network decode functions of signals transmitted from multiple transmitters and forward them to a central destination. If these functions are chosen as linear integer combinations, lattice codes are one of the most natural ways to implement a compute-and-forward scheme since a lattice is closed under addition. If the channel state information is not available at the transmitters, compute-and-forward can be implemented effectively by allowing the relay to choose integer coefficients depending on the channel coefficients and signal-to-noise ratio (SNR). Such a scheme which uses lattices over integers has been analyzed by Nazer and Gastpar for AWGN networks in [1] where achievable computation rates were derived. Based on this approach, in [2], Tunali et al. considered the use of lattices over Eisenstein integers for the compute-and-forward paradigm and successfully extended the result on achievable rates in [1] to lattices over Eisenstein integers.The lattice codes adopted in [1] are based on those generated by Construction A [3] [4] whose decoding complexity typically depend on decoding of the underlying linear codes. One main drawback of the Construction A lattices is that in order to be Poltyrev-good, the underlying linear codes have to be implemented over very large prime fields which in turn result in high decoding complexity. To alleviate this drawback, in the first part of the paper, inspired by Theorem 2 in [5], we propose a novel lattice construction called product construction that can be thought of as Construction A [3] with codes which can be represented as the Cartesian product of L linear codes over F p 1 , . . . , F p L , respectively. This construction is shown to b...