Adaptive demodulation using rateless erasure codes

Abstract

Abstract-We introduce a rate-adaptive system in which the receiver demodulates only those bits that have a high probability of being correct, treating nondemodulated bits as erasures. Several sets of decision regions, derived using composite hypothesis testing, are proposed for 16-QAM and 16-phase-shift keying, which allow for the simple implementation of this demodulation strategy. We demonstrate that pre-encoding the data with a Raptor code allows for simple reconstruction of the message, regardless of the erasure pattern introduced from the nondemodulated bits. We prove the optimality of the proposed decision regions in selecting the most likely subset of bits from any received symbol in moderate-to-high signal-to-noise ratios, and we analyze the performance of demodulating with these decision regions over an additive white Gaussian noise channel. Also demonstrated is the strong performance of 16-QAM for this application, compared with other power-efficient constellations and the near-optimality of using Gray mapping, even under the proposed alternate sets of decision regions.

Adaptive Demodulation with Differentially Coherent Detection

“…This is explained by the relatively poorer performance of the β-DSs for 16-DAPSK. Since the ADM system always operates in conjunction with a rateless erasure code, the additional coding gain would be the standard gain observed in [1] or [13] (approximately 3dB gain in [13] using a Raptor code).…”

“…For ∆ β,1 < r d ' ≤ 1 or r d ' < ∆ β,4 , b 0 is sufficiently reliable that it is always kept; for ∆ β,3 < r d ' ≤ ∆ β,2 , b 0 is sufficiently unreliable that it is always discarded; for all other values of r d ', the transition region is used. A method is presented in [11] to find ∆ β,1 , ∆ β,2 , ∆ β,3 , and ∆ β,4 using (11b) and (11c). For example, to find ∆ β,1 , we would use (11b) and (11c) to determine the exact value of r d ' such that the optimum decision scheme would have transitioned halfway between the b 0 reliable region and our "transition region".…”

“…Adaptive Demodulation (ADM) is a new rate adaptive technique, first proposed in [1], which retains the advantages of traditional rate adaptive systems such as adaptive modulation and incremental redundancy, while avoiding some of their shortcomings. For communication over a timevarying channel, rate adaptive systems offer an advantage over fixed rate systems since they can operate efficiently as the channel conditions change while maintaining a target bit error rate (BER) instead of being designed for the "worst case" channel conditions.…”

“…The ADM system in [1] avoids the need for CSI at the transmitter, has a fixed buffer size (roughly equal to the size of the message), and requires the receiver to perform a decoding operation only once. In an ADM system, the transmitter sends data at a fixed rate using a standard constellation and the receiver demodulates the data a nonfixed rate by demodulating only those bits with high reliability and treating the unreliable bits as erasures.…”

“…In [1], the ADM receiver uses sets of specially designed decision regions to demodulate some or all of the bits in a transmitted symbol (depending upon the desired level of reliability), and assumes a coherent phase reference is available at the receiver. In this paper, we consider how to implement the ADM system using differentially coherent detection: an attractive alternative when a reliable phase reference is not available.…”