Signal Transmission with Unequal Error Protection in Wireless Relay Networks

Abstract

This paper studies a relaying technique in cooperative networks using hierarchical modulation. Hierarchical modulation is useful in applications that require different protection classes of the information. In particular, a cooperative network with one source, one relay, and one destination is considered. Two different protection classes are modulated by a hierarchical 2/4-amplitude shift keying (ASK) constellation at the source. Based on the instantaneous received signal-to-noise ratio (SNR) at the relay, the relay decides to retransmit both classes by using a hierarchical 2/4-ASK constellation, or the more protection class by using a 2-ASK constellation, or remains silent. Optimal thresholds are chosen to minimize the bit-error-rate (BER) of the less protection class while the BER of the more protection class meets a given requirement. Numerical and simulation results are provided to verify the analysis. The results show that the optimal thresholds improve the performance significantly.This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.

Optimized receiver design for decode-and-forward relays using hierarchical modulation

“…We note that the combining method in [2] does not perform as well as the other methods, since it only uses the QPSK received signal to detect the BL, and thus results in just the QPSK BER performance (with diversity order 1). Our suboptimal combining method of minimizing an upper-bound BER significantly outperforms the combining method used in [1]. We can observe approximately 1dB gain in the medium and high SNR region, i.e., say, the channel SN R ≥ 8dB.…”

“…We also present a suboptimal method to find the weight for the BL by minimizing an upper-bound to the BER, which has a closed-form solution and performs very close to the optimal. Both the optimal and suboptimal methods for the BL significantly outperform the combining methods in [1], [2]. A suboptimal combining method for the EL in the second step is also presented, which performs well compared to the optimal.…”

“…We use ρ ∈ (0.5, 1] to denote the normalized power allocated to the MSB signal b l , andρ 1−ρ to denote the normalized power allocated to the LSB signal e l . 1 In the following, we just consider the received signals at the destination. In the second time slot, depending on the number of successfully decoded layers at the relays, the destination can receive different signals in each sub-band.…”

“…One is always sending the BL using QPSK, and the other is always sending the BL/EL using hierarchical 16-QAM. For the BL, we plot the uncoded BER performances for the combining methods in [1], [2], and our optimal and suboptimal methods. For the EL, we plot our simple combining method, the locally suboptimal method, and the lower bound.…”

“…We will see later that, because of different modulation schemes used at the relays, classical maximal-ratio combining (MRC) cannot straightforwardly be applied at the destination. In [1] and [2], simple combining methods are used, however, both are suboptimal and perform significantly worse than the optimal.…”