Terry Ferrett

Noncoherent Physical-Layer Network Coding with FSK Modulation: Relay Receiver Design Issues

A channel-coded physical-layer network coding strategy is refined for practical operation. The system uses frequency-shift keying (FSK) modulation and operates noncoherently, providing advantages over coherent operation: there are no requirements for perfect power control, phase synchronism, or estimates of carrier-phase offset. In contrast with analog network coding, which relays received analog signals plus noise, the system relays digital network codewords, obtained by digital demodulation and channel decoding at the relay. The emphasis of this paper is on the relay receiver formulation. Closed-form expressions are derived that provide bitwise log-likelihood ratios, which may be passed through a standard error-correction decoder. The role of fading-amplitude estimates is investigated, and an effective fading-amplitude estimator is developed. Simulation results are presented for a Rayleigh block-fading channel, and the influence of block length is explored. An example realization of the proposed system demonstrates a 32.4% throughput improvement compared to a similar system that performs network coding at the link layer. By properly selecting the rates of the channel codes, this benefit may be achieved without requiring an increase in transmit power.

Noncoherent physical-layer network coding using binary CPFSK modulation

Physical-layer network coding is a high-throughput technique for communicating over the two-way relay channel, which consists of two terminals that communicate exclusively via an intermediate relay. An exchange of messages begins with both terminals transmitting binary data sequences simultaneously to the relay. The relay determines the modulo-2 sum of the sequences, which it modulates and broadcasts to the terminals. Since each terminal knows the information it transmitted, it can determine the information transmitted by the other terminal by subtracting its own information from the broadcast signal. Prior work on the topic of physical-layer network coding has assumed that the signals transmitted by the two terminals arrive at the relay with perfectly aligned phases, permitting coherent reception. In this paper, we relax the assumption of aligned phases and consider noncoherent reception of binary continuous-phase frequency-shift keying signals. A derivation of the relay receiver is given for varying amounts of channel state information, and results are provided showing the error performance of the proposed system without an outer error-correcting code and with an outer turbo code.

An iterative noncoherent relay receiver for the two-way relay channel

Digital network coding improves the throughput of the two-way relay channel by allowing multiple sources to transmit simultaneously to the relay. This work considers the development of a relay receiver applying a specific modulation and channel coding technique - turbo-coded noncoherent orthogonal FSK in the two-way relay channel operated with digital network coding. The relay receiver supports any modulation order which is a power of two, and iterative channel decoding with information feedback from decoder to demodulator, using bit interleaved coded modulation with iterative decoding (BICM-ID). The performance of the receiver is investigated in fading channels through error-rate simulations and capacity analysis, and results show an energy efficiency improvement of 0.5-0.9 dB over similar systems which do not utilize BICM-ID.

Receiver design for noncoherent digital network coding

Physical-layer network coding is considered for the two-way relay network with realistic assumptions on the coherence of the channel. In contrast to analog network coding, which relays received analog signals plus noise, our system relays digital network codewords, obtained by digital demodulation and channel decoding. By using binary frequency-shift keying and noncoherent reception, the relay may operate without knowledge of the phases of the signals transmitted simultaneously by the two sources. The channels between the end nodes and the relay are modeled as noncoherent block fading channels, and an outer turbo code is used. A noncoherent receiver is formulated for the relay, which estimates the fading amplitudes but not the phases. Several block sizes are considered, and the effect of block size on error-rate performance is investigated. As a baseline for performance comparison, the system is also simulated using perfect knowledge of the fading amplitudes, and it is observed that the performance lost to channel estimation is negligible for sufficiently large blocks. An example realization of the proposed system demonstrates a 32.4% throughput improvement compared to a similar system that performs network coding at the link layer.

Noncoherent digital network coding using multi-tone CPFSK modulation

Digital network coding is a relaying technique that increases throughput in two-way relay networks. In contrast with analog network coding, which relays received analog signals plus noise, digital network coding relays digital codewords. The digital codewords are created by demodulation, channel decoding, and re-encoding at the relay. By using FSK and noncoherent reception, the relay may operate without knowledge of the phases of the signals transmitted by the two source terminals. In this paper, previous work on binary FSK is extended to multi-tone FSK, where the number of tones may be any power of 2. The relay receiver is formulated for any number of tones that is a power of two. Binary FSK is compared against quaternary FSK, which requires no expansion of bandwidth compared with binary FSK. The comparison is made using two metrics: the simulated bit-error rate (both with and without an outer turbo code), and the binary information rate between the sources and relay. The results illustrate that the energy-efficiency advantage of quaternary FSK on a point-to-point link is magnified when it is applied to digital network coding.

LDPC code design for noncoherent physical layer network coding

This work considers optimizing LDPC codes in the physical-layer network coded two-way relay channel using noncoherent FSK modulation. The error-rate performance of channel decoding at the relay node during the multiple-access phase was improved through EXIT-based optimization of Tanner graph variable node degree distributions. Codes drawn from the DVB-S2 and WiMAX standards were used as a basis for design and performance comparison. The computational complexity characteristics of the standard codes were preserved in the optimized codes by maintaining the extended irregular repeat-accumulate (eIRA). The relay receiver performance was optimized considering two modulation orders M = {4; 8} using iterative decoding in which the decoder and demodulator refine channel estimates by exchanging information. The code optimization procedure yielded unique optimized codes for each case of modulation order and available channel state information. Performance of the standard and optimized codes were measured using Monte Carlo simulation in the flat Rayleigh fading channel, and error rate improvements up to 1:2 dB are demonstrated depending on system parameters.

Physical-Layer Network Coding Using FSK Modulation under Frequency Offset

Physical-layer network coding is a protocol capable of increasing throughput over conventional relaying in the two- way relay channel, but is sensitive to phase and frequency offsets among transmitted signals. Modulation techniques which require no phase synchronization such as noncoherent FSK can compensate for phase offset, however, the relay receiver must still compensate for frequency offset. In this work, a soft- output noncoherent detector for the relay is derived, under the assumption that the source oscillators generating FSK tones lack frequency synchronization. The derived detector is shown through simulation to improve error rate performance over a conventional detector which does not model offset, for offset values on the order of a few hundredths of a fraction of FSK tone spacing.

Noncoherent analog network coding using LDPC-coded FSK

Analog network coding (ANC) is a throughput increasing technique for the two-way relay channel (TWRC) whereby two end nodes transmit simultaneously to a relay at the same time and band, followed by the relay broadcasting the received sum of signals to the end nodes. Coherent reception under ANC is challenging due to requiring oscillator synchronization for all nodes, a problem further exacerbated by Doppler shift. This work develops a noncoherent M-ary frequency-shift keyed (FSK) demodulator implementing ANC. The demodulator produces soft outputs suitable for use with capacity-approaching channel codes and supports information feedback from the channel decoder. A unique aspect of the formulation is the presence of an infinite summation in the received symbol probability density function. Detection and channel decoding succeed when the truncated summation contains a sufficient number of terms. Bit error rate performance is investigated by Monte Carlo simulation, considering modulation orders two, four and eight, channel coded and uncoded operation, and with and without information feedback from decoder to demodulator. The channel code considered for simulation is the LDPC code defined by the DVB-S2 standard. To our knowledge this work is the first to develop a noncoherent soft-output demodulator for ANC.

Reduced complexity detection for network-coded slotted ALOHA using sphere decoding

Network-coded slotted ALOHA (NCSA) is a refinement to the classic slotted ALOHA protocol which improves throughput by enabling multiple source transmissions per ALOHA slot using physical-layer network coding (PNC). The receiver detects the network-coded combination of bits during every slot and recovers information bits by solving a system of linear equations. This work develops a receiver capable of detecting the network-coded combination of bits during a slot considering an arbitrary number of sources, orthogonal modulation, and a block fading channel. Maximum-likelihood detection of the network-coded symbol at the receiver becomes complex as the number of sources is increased. To reduce this complexity, sphere decoding is applied at the receiver to limit the number of constellation symbols the receiver must consider for detection. The system is simulated for two modulation orders and two through five sources, and error-rate performance results are provided.