基於CRC的輕量化OFDMA網路高可靠度資料遞送服務

Abstract

近年來，正交分頻多工(Orthogonal Frequency Division Multiplexing Access； OFDMA) 調變編碼技術已經被廣泛的應用在許多不同類型的無線通訊系統，例如：Wi-Fi、WiMAX和LTE等。為確保傳輸品質及效率，傳統上在Data link Layer，若干個Frame會被集合封裝成一Burst，並會附加循環冗餘碼(Cyclic Redundancy Check; CRC)編碼，接收端可以經由檢查訊框檢查序列（Frame Check Sequence; FCS）得知Burst傳送過程中是否發生錯誤，在經由整合式自動重傳機制(Hybrid Automatic Retransmission Request; HARQ) 通知發送端重送錯誤資料。雖然CRC具備運算速度快的特性，但由於僅能達成錯誤檢測的功能，在錯誤產生時，僅能以重送全部資料的方式校正錯誤。 由於在封裝時OFDMA 封包必然會產生一些留白(padding)，因此本次論文提出一個以CRC為基礎的錯誤碼偵測方法，此方法利用的padding空間的大小為基準，動態計算部份封包的FCS值，並將部份封包的FCS值padding在封包內。由於利用的是padding的空間，所以不會增加傳送時所需頻寬。當網路環境不佳時，封包產生錯誤比例升高，可能每次傳送都會發生錯誤，此時可以利用額外padding的資訊，判斷錯誤發生的區段， 就算下次重送的封包還是錯誤，此方法可利用比對前後兩次封包資訊的方法，還原成正確的封包，因此本方法可有效在網路環境不佳時，有效下降封包需要重送的次數。 本研究以前述提出的偵錯及校錯機制，嘗試套用在至少三種主要的OFDMA網路應用情境中，如多跳中繼(Multihop Relaying)、群播服務(MBS)、高速移動通訊(Telematics)等，這些情境都具備無線節點可能接收到多次相同訊號的特性，符合實現本方法的條件。由模擬結果中發現，本研究提出的方法將有效運用於極端惡劣無線信道環境中，以及對於在高速移動環境中所導致的都普勒(Doppler)效應，由改善其高誤碼率(BER)，提升OFDMA網路的通訊可靠度及頻譜效率。

OFDMA-based wireless transmission technologies such as WiMAX are now widely deployed, and the FCS scheme is used to enhance the reliability of OFDMA-based systems. As the padding overhead cannot be completely avoided in such systems when variable-length MAC frames are encapsulated, we propose a novel CRC-based error correction scheme that utilizes the frame padding overhead to carry as much extra segmented FCS information as possible; compared with the legacy FCS scheme, our approach greatly enhance the error detection and correction upon the retransmission. In a multi-hop relay environment, a mobile station might receive a multicast frame more than once, so the error bit correction scheme could be triggered in the first transmission. In the original CRC scheme is optional and only has limited error detection capability in upper MAC layer, but in our work, CRC is compulsory and all sub-FCSs are carried by padding space without using any extra bit. The proposed approach achieves a significant performance improvement over the legacy FCS scheme in terms of error detection and correction of multicast frames in a transparent mode multi-hop relay network. Additionally, in some specific applications, the proposed scheme also achieves a significant performance improvement over the legacy FCS scheme in terms of error detection and correction. For example, the emergency service over Multicast/Broadcast Service (MBS) is a promising technology, because it allows simultaneous delivery of emergency messages to large-scale user communities in a cost-effective manner. In high-mobility environment, mobile station might receive an emergency MBS frame more than once, so the proposed scheme could be cross referenced by duplicated frames received in a WiMAX Mobile Multihop Relay (MMR) network operating in transparent mode. The simulation results show that the mobile stations get significant improvement of goodput and utilization in an error-prone WiMAX network.