大規模多元接続ネットワークのためのグラフ構造に基づくランダムアクセス方式

電気通信大学201

逐次干渉除去とZigZag復号に基づくランダムアクセスプロトコルの解析と設計

超多数の端末が散発的に通信するネットワークの実現のため，多元接続方式としてランダムアクセスを適用する研究が多くなされている.ランダムアクセスの一種である非正則繰り返しスロット化 ALOHA(IRSA: Irregular Repetition Slotted ALOHA)は，パケット復調の手段として逐次干渉除去を用いて高いスループット性能を達成するものの，高負荷時における性能の劣化が著しいことが知られている.そこで，本研究では逐次干渉除去に加えてジグザグ復号を組み合わせることで高負荷時におけるスループット性能の劣化を抑圧するジグザグ復号可能な符号化スロット化 ALOHA(ZDCSA: Zigzag Decodable Coded Slotted ALOHA) および拡張 ZDCSA(EZDCSA: Enhanced-ZDCSA) を提案する. 一方で，IRSA と異なり時間フレーム長が固定でないフレームレス ALOHA が提案されており，逐次干渉除去に加えてキャプチャ効果を適用した場合の性能解析がなされている.キャプチャ効果を適用する場合，基地局に到着したパケットの信号対干渉雑音電力(SINR: Signal-to-Interference-plus-Noise Ratio)は基地局に近いユーザが遠いユーザと比較して高くなり復調が容易になると予測される.従来研究では，このような幾何学的配置構造を利用したプロトコルの設計はなされておらず，ユーザ全体で時間スロットあたりのパケットの送信確率を同一としている.本研究ではユーザが自身と基地局との距離を既知であるとし，パケットの送信確率を自律的に決定する距離を考慮した非正則フレームレス ALOHA(DIFA: Distance-aware Irregular Frameless ALOHA)を提案する.本論文では，提案方式である ZDCSA，E-ZDCSA，および DIFAが既存方式と比較して優れた スループット性能を達成することを数値解析より示す.電気通信大学201

Collision Helps - Algebraic Collision Recovery for Wireless Erasure Networks

Current medium access control mechanisms are based on collision avoidance and collided packets are discarded. The recent work on ZigZag decoding departs from this approach by recovering the original packets from multiple collisions. In this paper, we present an algebraic representation of collisions which allows us to view each collision as a linear combination of the original packets. The transmitted, colliding packets may themselves be a coded version of the original packets. We propose a new acknowledgment (ACK) mechanism for collisions based on the idea that if a set of packets collide, the receiver can afford to ACK exactly one of them and still decode all the packets eventually. We analytically compare delay and throughput performance of such collision recovery schemes with other collision avoidance approaches in the context of a single hop wireless erasure network. In the multiple receiver case, the broadcast constraint calls for combining collision recovery methods with network coding across packets at the sender. From the delay perspective, our scheme, without any coordination, outperforms not only a ALOHA-type random access mechanisms, but also centralized scheduling. For the case of streaming arrivals, we propose a priority-based ACK mechanism and show that its stability region coincides with the cut-set bound of the packet erasure network

Reliability-Latency Performance of Frameless ALOHA with and without Feedback

This paper presents a finite length analysis of multislot type frameless ALOHA based on a dynamic programming approach. The analysis is exact, but its evaluation is only feasible for moderate number of users due to the computational complexity. The analysis is then extended to derive continuous approximations of its key parameters, which, apart from providing an insight into the decoding process, make it possible to estimate the packet error rate with very low computational complexity. Finally, a feedback scheme is presented in which the slot access scheme is dynamically adapted according to the approximate analysis in order to minimize the packet error rate. The results indicate that the introduction of feedback can substantially improve the performance of frameless ALOH

Reliability-Latency Performance of Frameless ALOHA with and without Feedback

This paper presents a finite length analysis of multislot type frameless ALOHA based on a dynamic programming approach. The analysis is exact, but its evaluation is only feasible for moderate number of users due to the computational complexity. The analysis is then extended to derive continuous approximations of its key parameters, which, apart from providing an insight into the decoding process, make it possible to estimate the packet error rate with very low computational complexity. Finally, a feedback scheme is presented in which the slot access scheme is dynamically adapted according to the approximate analysis in order to minimize the packet error rate. The results indicate that the introduction of feedback can substantially improve the performance of frameless ALOH

Design of Coded Slotted ALOHA with Interference Cancellation Errors

International audienceCoded Slotted ALOHA (CSA) is a random access scheme based on the application of packet erasure correcting codes to transmitted packets and the use of successive interference cancellation at the receiver. CSA has been widely studied and a common assumption is that interference cancellation can always be applied perfectly. In this paper, we study the design of CSA protocol, accounting for a non-zero probability of error due to imperfect interference cancellation (IC). A classical method to evaluate the performance of such protocols is density evolution, originating from coding theory, and that we adapt to our assumptions. Analyzing the convergence of density evolution in asymptotic conditions, we derive the optimal parameters of CSA, i.e., the set of code selection probabilities of users that maximizes the channel load. A new parameter is introduced to model the packet loss rate of the system, which is non-zero due to potential IC errors. Multi-packet reception (MPR) and the performance of 2-MPR are also studied. We investigate the trade-off between optimal load and packet loss rate, which sheds light on new optimal distributions that outperform known ones. Finally, we show that our asymptotic analytical results are consistent with simulations obtained on a finite number of slots

Modern Random Access for Satellite Communications

The present PhD dissertation focuses on modern random access (RA) techniques. In the first part an slot- and frame-asynchronous RA scheme adopting replicas, successive interference cancellation and combining techniques is presented and its performance analysed. The comparison of both slot-synchronous and asynchronous RA at higher layer, follows. Next, the optimization procedure, for slot-synchronous RA with irregular repetitions, is extended to the Rayleigh block fading channel. Finally, random access with multiple receivers is considered.Comment: PhD Thesis, 196 page