Reliability Strategies for Data Transmission in Wireless Mesh Network

     Wireless Mesh Network (WMN), also called multi-hop network, is a new-type wireless network. In the WMN, any wireless equipment can act both as an Access Point (AP) and as a router; each node can send and receive signals and directly communicate with one or several peer nodes. In the process of data transmission, if congestion occurs at an AP due to heavy traffic, the data packet, which is originally routed to this AP, will be automatically re-routed to a neighboring node with lighter traffic. In this way, the data packet is routed and transmitted to a next node based on actual network conditions until it reaches the destination.

     The wireless Mesh architecture is no doubt a good solution for wireless networking in wide, open areas either indoor or outdoor. One main characteristic of wireless Mesh network is: It is made up of a group of wireless APs which are distributed in a mesh way, and these APs are interconnected via radio relay links in a peer-to-peer way, thus it can really expand the coverage of wireless “hot spot” of basic Wireless Local Area Network (WLAN) into a large area[1].

     In the future mobile telecommunication systems, Base Stations (BSs) and Relay Stations (RSs) will adopt wireless Mesh architecture for networking, and the communication between User Terminals (UTs) and BSs is likely to be multi-hop. Therefore, there should be necessary link reliability strategies to ensure the data to inerrably arrive at the destination. In 3G and Long Term Evolution (LTE) systems, the reliability of data links is guaranteed with Automatic Repeat Request (ARQ) and Hybrid Automatic Repeat Request (HARQ) schemes. If the WMN nodes evolve from 3rd Generation Partnership Project (3GPP) system nodes, the WMN can achieve reliable data transmission in multi-hop links by means of improved HARQ or ARQ scheme.

1 Transmission Reliability Scheme in 3GPP Systems
In 3GPP systems, the reliability of data transmission in radio links is a result of common work of the HARQ functionality within the Media Access Control (MAC) sublayer and the ARQ functionality within the Radio Link Control (RLC) sublayer[2].

1.1 ARQ
In ARQ, the transmitter adds some error correcting codes to the data packet to be transmitted and sends them together. Upon receiving the packet, the receiver detects the packet, against the attached error correcting codes, for any error, and if any error is found, it will return a No Acknowledgement (NACK) message. In return, the transmitter will retransmit the packet after it receives the NACK message.

1.2 HARQ
HARQ[3] is a link adaptive technique. In HARQ, as the principle of Forward Error Correction (FEC) is followed, the receiver can store the data and ask for retransmission when it fails to decode the received data. Besides, HARQ uses the link layer information as retransmission criterion, so it can automatically adapt to the changes of channel conditions and become insensitive to measurement errors and delay. If integrated with Adaptive Modulation and Coding (AMC) technology, HARQ can get the best result: AMC provides a rough selection for data rate while HARQ can make fine adjustment of the data rate based on the actual channel conditions. The principle of FEC includes incremental redundancy (with the times of retransmission being added, more parity bits are contained) and chase combination (the same data block will be completely retransmitted).

1.3 ARQ/HARQ Interaction
In HARQ assisted ARQ operation, ARQ uses knowledge obtained from the HARQ to learn the transmission/reception status of a Transport Block (TB). If the HARQ transmitter detects a failed delivery of a TB due to maximum retransmission limit being reached, the relevant transmitting ARQ entities are notified and potential retransmissions and re-segmentation can be initiated.

2 Transmission Reliability Mechanisms for WMN
The main difference between wireless Mesh and traditional networks is that the transmission links of WMNs are likely to be multi-hop. As a result, the data transmission reliability issue in WMN can be simply regarded as the transmission reliability problem of multi-hop links. Below, we will discuss how to apply and improve the transmission reliability strategies of single-hop networks to guarantee reliable data transmission in
multi-hop networks.

2.1 HARQ
Roughly, the HARQ schemes[4] in multi-hop networks fall into two groups: hop-by-hop HARQ, where HARQ is implemented in each hop (illustrated in Figure 1), and edge-to-edge HARQ, where HARQ is implemented only at two ends of data transmission (illustrated in Figure 2).

     Hop-by-hop HARQ can be further divided into dynamic HARQ and static HARQ. In dynamic HARQ, resources have to be rescheduled for each HARQ transmission or retransmission. In static HARQ, resource scheduling is conducted only when the data packet is first transmitted. Then the scheduled resource block will be reserved, so no rescheduling is required in retransmission. It will not be released until the data packet is successfully received.

     With regard to edge-to-edge HARQ, its advantage is short transmission delay, and its disadvantage is that any error occurring during the transmission cannot be timely detected, so retransmission requests can only be made by the destination node. In hop-by-hop HARQ, on the contrary, the transmission of each hop is detected for correctness, thus retransmission can be requested timely. Moreover, the hop-by-hop HARQ enables the transmission delay to be further shortened by optimizing the protocols, but the operations involved at the network nodes are quite complex.

     In short, several multi-hop HARQ schemes are available. In selecting the scheme, two factors among others should be considered: one is the overall performance after ARQ is integrated, and the other is actual requirements. Of all requirements, the most important one is edge-to-edge transmission delay. LTE systems require a quite short edge-to-edge transmission delay, i.e. less than 5 ms for the user plane, and the transmission delay in the future International Mobile Telecommunication (i.e. IMT-Advanced) systems should be no more than that in LTE systems. As a result, the multi-hop relay systems are facing a great challenge and need further optimization.

2.2 ARQ
If ARQ is considered only, there are three implementation schemes for multi-hop networks[5]: hop-by-hop,
edge-to-edge, and last-hop. But their implementations are different from each other due to the characteristics of ARQ.

     (1) Hop-by-hop ARQ
     Each RS in the BS-UT path decodes all received RLC packets and checks the receiving sequence of these packets. If the sequence is correct, the RS forwards the packets to a next-hop node.

     (2) Edge-to-edge ARQ
     All RS nodes are not required to decode RLC packets they receive, but after the MAC packets have been decoded, the RS nodes have to process the RLC packets (e.g. cascading) and then directly forward them to next-hop RSs. The ARQ processing of RLC packets is only made at BS and UT.

     (3) Last-hop ARQ
     Except the last-hop RS node, the RS nodes in the BS-UT path are not required to decode received RLC packets, but after the MAC packets have been decoded, these RS nodes process the RLC packets (e.g. cascading) and directly forward them to next-hop RSs. Upon receiving the RLC packets, the last-hop RS node decodes them and checks their receiving sequence. If the sequence is correct, the last-hop RS finally forwards the packets to the terminal.

2.3 HARQ/ARQ Interaction
For the multi-hop scenarios of wireless Mesh networks, HARQ, ARQ or a simple combination of ARQ and HARQ as in LTE systems cannot guarantee the reliability of data transmission or meet the requirements on transmission delay. As a result, new reliability mechanisms have to be worked out for multi-hop scenarios, and these new mechanisms should take the advantages of both ARQ and HARQ. In the sections that follow, we will analyze the interoperability of HARQ and ARQ from the perspective of protocol stack design[6-8].

2.3.1 Layered Cooperative Mechanism
As shown in Figure 3, the layered cooperative mechanism is developed on the basis of LTE protocol stack structure. Its basic principle is to divide Layer 2 into two sublayers: MAC and RLC. HARQ is implemented in MAC sublayer, where a hop-by-hop scheme is used; while ARQ is implemented in RLC sublayer, where an edge-to-edge scheme is used.

     The layered cooperative mechanism is quite flexible. The hop-by-hop HARQ scheme ensures the data can be timely recovered in case of transmission error between neighboring nodes; and the edge-to-edge ARQ is responsible for retransmitting lost data. The selection of hop-by-hop HARQ schemes depends on the hops of links. Besides, in hop-by-hop HARQ, the packet size can be optimized according to actual conditions of each independent link, and one edge-to-edge packet can be included in one or several hop-by-hop frames. A transmission in downlink of an edge-to-edge frame  from the source node to the destination node succeeds until the edge-to-edge protocol transmits an ACK. In this case, one edge-to-edge packet corresponds to one hop-by-hop packet. Figure 4 illustrates the transmission of a downlink edge-to-edge packet.

     The protocol interoperability and parameter configuration in the layered cooperative mechanism are very complex. For example, in designing the retransmission delay threshold of each link node, it is required to avoid simultaneous retransmission of multiple nodes and the waste of radio resources. If the threshold is set too low, the transmitter may not retransmit the packet; but if the threshold is too high, the system efficiency will be degraded. If the data are lost during inter-RS handovers, the utility of each link will be kept quite low before the threshold is reached. Moreover, in the hop-by-hop scheme, the packets are forwarded in certain sequence, so if the received packets are in disorder, the receiving RS has to reorder the packets before forwarding them, leading to a decreased link efficiency.

2.3.2 Relay ARQ Mechanism
This mechanism[9] applies both edge-to-edge scheme and hop-by-hop scheme in one protocol layer, as shown in Figure 5. The entities involved in Relay ARQ protocols are integrated into a process in all links between RSs. The same packet and same sequence number will be transmitted in all links between BS and UT. In this mechanism, three status messages are used: ACK, NACK and Relay-ACK (RACK).

     Let’s take an example of two hops to explain the Relay ARQ mechanism, illustrated in Figure 6. RS saves the packet information from BS and the ACK message from UT. If RS receives a packet from BS but does not receive ACK message from UT, it sends a RACK message to BS, which functions the same as hop-by-hop ACK message in the layered cooperative mechanism. For BS, this RACK message means RS will take charge of the packet. As a result, upon receiving the RACK message, BS will store the packet in the send buffer until it receives ACK message from UT, which is equivalent to the edge-to-edge ACK message in the layered cooperative mechanism. If RS fails to forward the packet to UT, as a result of handover, the previous node will be responsible for retransmitting the packet, and the original transmitter will take the ultimate responsibility of the packet. This mechanism is similar to the layered mechanism. Its advantage lies in its relatively simple protocol architecture, but for the entire system, other protocols have to be adjusted accordingly.

2.3.3 Multi-Hop ARQ Mechanism
In this mechanism, the edge-to-edge ARQ does not terminate at the UT but at the last RS next to UT. And from the last RS to UT, hop-by-hop HARQ will be performed, as shown in Figure 7. As any hop-by-hop schemes can be used, UT can move among different systems. In addition, the edge-to-edge protocol is closely coupled with the hop-by-hop protocol between the last RS and UT.

     Even if the arrival of the packet to the destination node is acknowledged with the downlink edge-to-edge protocol, the last RS will not send hop-by-hop ACK message to UT until it receives ACK message from BS. The implementation process of this mechanism is illustrated in Figure 8. The biggest advantage of this mechanism is that it allows UT to move to other systems. But as the edge-to-edge protocol at the last RS should be closely coupled with the hop-by-hop protocol between the last RS and UT, it has to be adjusted so as to support all hop-by-hop protocols, as well as to guarantee the reliability of the entire link. Compared with other two mechanisms, this mechanism requires less in terms of UT’s calculation complexity and storage. But it has the same disadvantage as the layered cooperative mechanism.

3 Conclusion
Being new network architecture, the wireless Mesh network can greatly improve existing wireless cellular telecommunication  systems. But before it is put into practical application, it is required to study the reliability of its multi-hop links so as to enhance the robustness of the network. This paper discusses several reliability strategies for multi-hop links as well as presents different perspectives of research, for example, how to improve HARQ and ARQ schemes from the perspective of implementation, and how to study Mesh nodes, particularly relay stations, from the perspective of protocol stack design. The independent HARQ or ARQ schemes can be applied in different systems, e.g. 802.16 systems. In terms of transmission delay, the edge-to-edge approaches are better than the hop-by-hop ones. The protocol

     stack-based solutions take consideration of the uniformity in system configuration, while the layered mechanism can offer a reliability strategy with shorter delay than other mechanisms under the same protocol stack structure. In short, all these methods can effectively solve the reliability problem of multi-hop links, but have their own shortages. Therefore, the reliability strategies of data transmission in wireless Mesh networks need further improvement.

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[Abstract] Wireless Mesh Network (WMN) is a new-type wireless network. Its core idea is that any of its wireless equipment can act as both an Access Point (AP) and a router. Each node in the network can send and receive signals as well as directly communicate with one or several peer nodes. One important issue to be considered in wireless Mesh networks is how to secure reliable data transmission in multi-hop links. To solve the problem, the 3GPP system architecture proposes two functionalities: ARQ and HARQ. This paper presents two HARQ schemes, namely hop-by-hop and edge-to-edge, and three ARQ schemes: hop-by-hop, edge-to-edge, and last-hop. Moreover, it proposes three solutions for WMNs from the perspective of protocol stock design: layered cooperative mechanism, relay ARQ mechanism and multi-hop mechanism.