Efficient packet recovery in wireless networks

Wireless medium access control (MAC) protocols usually provide reliability in the presence of packet errors. The efficiency of these reliability mechanisms is generally low since they require the entire packet to be retransmitted even though only parts of it may have been corrupted. To address this issue, this paper presents an efficient mechanism for the recovery of packets corrupted due to both channel errors and collisions. The proposed mechanism first determines the cause of the errors. Next, the symbols with errors are isolated by using the error vector magnitude (EVM) of received symbols as the feature for detection. Using explicit feedback about which blocks of symbols have errors, only the erroneous blocks are then retransmitted. Our results show that the proposed mechanism increases the efficiency of the MAC protocol by providing higher throughput. I. INTRODUCTION Wireless transmissions are susceptible to two sources of errors: (i) poor channel conditions due to fading, path loss and noise; (ii) collisions due to concurrent transmissions from other nodes. To provide reliability, most wireless MAC proto- cols include mechanisms to detect errors, and retransmission strategies to recover the corrupted packet. The key limitation of existing mechanisms is that when a packet is corrupted during transmission, the entire packet is retransmitted. This leads to unnecessary and costly overhead since most channel errors and collisions affect only part of the packet. To address this issue, this paper presents an efficient packet recovery mechanism for wireless networks such as IEEE 802.11 which use orthogonal frequency division multiplexing (OFDM) as the physical layer. The most common method to detect corrupted packets is through error detection codes. Mechanisms such as automatic repeat request (ARQ) are then used to retransmit the corrupted packet (1). Most ARQ schemes provide reliability by retrans- mitting the entire corrupted packet. A scheme for marking bits that are likely in error is presented in (2) and it is shown that the throughput may be increased by retransmitting only these bits. However, this approach requires specialized hardware, and has a high overhead feedback strategy. A packet recovery mechanism that works by combining multiple copies of a packet from different access points is proposed in (3). The overhead and delay associated with combining agents limits the application of this scheme. Moreover, only AWGN channels are considered in (3) and its effectiveness in more practical channels has not been established. In (4), a software- only solution to packet recovery based on harnessing partial packets is presented. This scheme relies on incremental redun- dancy and coding schemes, which can become a bottleneck at high data rates. In contrast, the proposed packet recovery mechanism does not require any customized hardware, has lower overhead, and provides real time operation. The main contribution of this paper is a methodology that detects the symbols that were corrupted during transmission. The transmitter can then use precise retransmissions of only the corrupted symbols. The proposed methodology to localize the errors is based on calculating the EVM of the received symbol. Each received packet with errors is first classified as either a packet corrupted due to a collision or due to channel errors. Based on the cause of corruption, two slightly different methodologies are proposed to isolate the symbols with errors. The receiver then conveys the information about the corrupted symbols to the transmitter using the acknowledgment (ACK) packet. The transmitter retransmits only the corrupted sym- bols, which are then used by the receiver to reconstitute the packet. Extensive simulation results are used to evaluate the performance of the proposed packet recovery mechanism. The rest of the paper is organized as follows. Section II presents the proposed packet recovery mechanism. Sections III and IV present mechanisms for detecting the cause of packet errors and localizing the errors, respectively. Section V analyses the proposed mechanism, Section VI presents simulation results, and Section VII concludes the paper. II. EFFICIENT PACKET RECOVERY MECHANISM