Christian Schlegel

Multiple Access Demodulation in the Lifted Signal Graph With Spatial Coupling

Demodulation in a random multiple access channel is considered where the signals are chosen uniformly randomly with unit energy. It is shown that by lifting (replicating) the graph of this system and randomizing the graph connections, a simple iterative cancellation demodulator achieves the same performance as an optimal symbol-by-symbol detector of the original system. The iterative detector has a complexity that is linear in the number of users, while the direct optimal approach is known to be NP-hard. However, the maximal system load of this lifted graph is limited to

Irregular repetition slotted ALOHA with multiuser detection

\alpha , even for large signal-to-noise ratios (SNRs)—the system is interference limited. Spatial coupling between subsequent lifted graphs is introduced, and anchoring the initial graphs, the achievable system load

Controlling the Error Floor in LDPC Decoding

\alpha

Thresholds of spatially coupled systems via Lyapunov's method

can go to infinity as the SNR goes to infinity. Our results apply to several well-documented system proposals, such as interleave-division multiple access, partitioned spreading, and certain forms of multiple-input multiple-output communications.

Spectrally-efficient differential turbo-coded modulation for multi-gigabit satellite links

Irregular repetition slotted ALOHA is a random access scheme where users transmit multiple copies of a packet to the receiver to provide time-domain diversity; then, the receiver attempts to iteratively decode packets and cancel their interference contribution in other time slots, increasing the chance of resolving collisions and decoding more packets. This scheme was shown to support larger system loads compared to conventional slotted ALOHA. In the original scheme, collision resolution between l colliding packets is possible only when l - 1 of those packets are decoded in prior iterations, which is probable because of the other copies of each packet transmitted in other slots. However, although asymptotic analysis promises high efficiency of this scheme, the analysis relies on the large number of slots participating in the iterative collision resolution, which requires large receiver complexity and introduces delay. In this paper, we study this scheme for receivers capable of decoding multiple colliding packets jointly, which increases the chance of decoding more packets under large system loads. Asymptotic analysis for this generalized model is provided and it is shown by simulations that a multiuser detector supports larger loads for low number of slots jointly processed.

Spatially coupled streaming modulation

The error floor of LDPC is revisited as an effect of dynamic message behavior in the so-called absorbing sets of the code. It is shown that if the signal growth in the absorbing sets is properly balanced by the growth of set-external messages, the error floor can be lowered to essentially arbitrarily low levels. Importance sampling techniques are discussed and used to verify the analysis, as well as to discuss the impact of iterations and message quantization on the code performance in the ultra-low BER (error floor) regime.

Additive-Increase Multiplicative-Decrease MAC Protocol with Multi-Packet Reception

The threshold, or saturation phenomenon of spatially coupled systems is revisited in the light of Lyapunovs theory of dynamical systems. It is shown that an application of Lyapunovs direct method can be used to quantitatively describe the threshold phenomenon, prove convergence, and compute threshold values. This provides a general proof methodology for the various systems recently studied.

Iterative Demodulation and Channel Estimation for Asynchronous Joint Multiple Access Reception

Efficient exploitation of the wide bandwidth available in the EHF (extremely high frequency) domain will be a main pillar for the development of future-generation terabit satellite networks. State-of-the-art systems use spectrally-efficient coded modulations, which are based on coherent demodulation that requires the use of complex and expensive analog PLL circuitry, which are vulnerable to high Doppler shifts and phase noise, the latter, being a significant impairment in the W-band. The latest trends in digital communications are to use fully digital receivers. Therefore, we consider a novel modulation method based on differential turbo-coded modulation and A-Posteriori-Probability (APP) channel estimation for application in multi-gigabit W-band satellite links. The proposed scheme utilizes the combination of an outer 2/3 binary parity channel code and differential 8-PSK modulation, similar to a binary repeat-accumulate serial turbo coding. The turbo-demodulator uses a double-spread interleaver and Log-MAP decoding performed on the 8-PSK trellis. Counteracting channel impairments and frequency drifts is primarily accomplished by APP channel estimation which is integrated into the differential demodulator, and consists of a simple smoothing filter. Preliminary results have shown a robust behavior of the system, achieving high link availability.

Iterative demodulation and channel estimation for joint random access satellite communications

A novel modulation format based on coupling for streaming data transmissions is proposed. The construction utilizes an infinite parity-check matrix of a low-density parity-check convolutional code which is used for data encoding with real-domain addition instead of modulo-two addition. The demodulation and decoding is accomplished using iterative bit estimation and interference cancellation. We demonstrate that the threshold saturation effect of spatial graph coupling holds for the proposed format and the achievable communication rate approaches the additive white Gaussian noise channel capacity and exceeds the capacities of traditional PAM modulations.

Random access with multipacket reception and adaptive filtering

A distributed medium-access control (MAC) protocol for the time-slotted channel with multi-packet reception (MPR) capability is proposed. In contrast to the commonly assumed collision channel, in the MPR channel with channel capacity K, it is possible to decode up to K packets whose transmissions overlap in time. In order to exploit the MPR capability, the additive-increase multiplicative-decrease MAC (AIMD-MAC) protocol is designed to adaptively adjust the access probabilities of the independent nodes based on their local transmission histories. The examined performance metrics for evaluating this protocol include aggregate throughput, average packet delay and system fairness. Extensive simulations show that the performance of AIMD-MAC is superior to that of S-Aloha* under light traffic loads and achieves the optimum level after the system is saturated. With a suitable parameter set, AIMD-MAC can be adequately applied in a dynamic wireless environment.