Constellation Rearrangement Strategy for HARQ Schemes

The HARQ scheme with constellation rearrangement strategy is an innovative hybrid automatic repeat request (HARQ) technique that enhances transmission reliability by intelligently repositioning symbol locations on the constellation diagram. Compared to traditional Chase Combining, this method introduces additional degrees of freedom during signal reconstruction. Implementation-wise, this typically involves a constellation mapper function that dynamically reassigns symbol coordinates based on predefined rearrangement patterns or real-time channel state information.

Chase Combining is a classical HARQ approach whose core principle achieves diversity gain through simple retransmission of identical encoded data. The receiver performs maximal ratio combining of signals from multiple transmissions to improve decoding success rates. While straightforward to implement (often requiring just a cyclic buffer for storing received signals), this scheme suffers from relatively low spectral efficiency. In code implementation, Chase Combining can be realized with basic signal stacking algorithms and cyclic redundancy check (CRC) validation loops.

The innovation of constellation rearrangement lies in intelligently repositioning modulation symbols on the constellation diagram during each retransmission. This rearrangement is not random but carefully designed based on channel state information. By altering symbol phase or amplitude positions, the scheme creates superior diversity conditions, enabling the receiver to capture richer signal information. The demodulator must employ specialized joint decoding algorithms—such as iterative soft-decision decoding with rearranged constellation metrics—to properly process these reconfigured symbols. Algorithm implementation often involves lookup tables for rearrangement patterns and modified log-likelihood ratio (LLR) calculations.

Simulation comparisons demonstrate that the constellation rearrangement scheme achieves significant performance gains under high SNR conditions, typically reducing required SNR by 0.5-1 dB compared to Chase Combining. These benefits stem from two primary factors: enhanced coding diversity through rearrangement, and optimized symbol combination effects at the receiver. However, the scheme requires more complex transceiver designs, particularly increasing system overhead for real-time rearrangement computation and pattern storage. Code complexity increases through additional matrix operations for constellation transformations and memory buffers for storing multiple rearrangement patterns.

In practical applications, constellation rearrangement HARQ is particularly suitable for delay-tolerant but high-reliability communication scenarios like satellite or deep-space communications. Chase Combining remains preferable for cost-sensitive systems prioritizing simplicity. The choice between schemes requires comprehensive consideration of performance requirements, implementation complexity, and power budget constraints, with software-defined radio implementations often allowing dynamic switching between the two approaches through configuration parameters.