Cooperative communication fully leverages the broadcast nature of wireless channels and exploits time/spatial diversity in a distributed manner, thereby achieving significant improvements in system capacity and transmission reliability. Cooperative diversity has been well studied from the physical layer perspective. Thereafter, cooperative MAC design has also drawn much attention recently. However, very little work has addressed cooperation at the routing layer. In this paper, we propose a simple yet efficient scheme for cooperative routing by using cooperative metrics including packet delivery ratio, throughput, and energy consumption efficiency. To make a routing decision based on our scheme, a node needs to first determine whether cooperation on each link is necessary or not, and if necessary, select the optimal cooperative scheme as well as the optimal relay. To do so, we calculate and compare cooperative routing metric values for each potential relay for each different cooperative MAC scheme (C-ARQ and CoopMAC in this study), and further choose the best value and compare it with the noncooperative link metric. Using the final optimal metric value instead of the traditional metric value at the routing layer, new optimal paths are set up in multihop ad hoc networks, by taking into account the cooperative benefits from the MAC layer. The network performance of the cooperative routing solution is demonstrated using a simple network topology.
Multihop wireless networks in forms of ad hoc networks, mesh networks and sensor networks have become active research topics in recent years in both academia and industry. Different types of nodes are deployed pervasively in various environments such as office buildings, wildlife reserves, battle fields, and metropolitan area networks. However, lots of challenging tasks still remain for building multihop ad hoc networks, despite significant progress achieved so far.
Traditional techniques conceived for wired networking provide inefficient performance when applied in wireless ad hoc networks. Efforts are being made to improve the existing techniques and protocols with new features suitable for the wireless paradigms. For example, different from wired transmission, broadcast is an inherent feature in wireless communications, that is, information transmitted from a source node can be overheard by not only the destination node, but also neighboring nodes surrounding the source. In traditional wireless networks, signals received by the neighboring nodes are treated as interference and many techniques have been developed to alleviate its effect. However, such signals actually contain useful information for the destination node. In fact, if the information can be properly forwarded by the surrounding node(s), the reception performance at the destination can be improved. This fact motivates the application of a new technology, known as
The theory behind cooperation communication has been studied in depth [
Although significant efforts have been made on the physical layer and MAC layer issues of cooperative communications, there has been very little work so far on the cross-layer design of cooperative systems, especially on how to combine cooperation with routing. While some of the studies focused on the theoretical analysis on routing and cooperative diversity [
Apart from designing a brand new cross-layer cooperative routing protocol, an alternative way to extend cooperative communications to routing layer is to design routing metrics that reflect potential cooperation gain and find optimal paths using the new cooperative metrics. Reference [
As a contribution to this direction, this paper proposes a cross-layer cooperative scheme by using various cooperative metrics instead of traditional routing metrics to exploit cooperative diversity at the routing layer. To perform the scheme, a new cooperative metric is calculated for each potential relay and every individual cooperative transmission scheme for each link beforehand. Then, by comparing the obtained best cooperative link metric with the traditional noncooperative link metric, a node decides whether cooperative retransmission on each link needs to be initiated or not. By choosing the best among all these calculated metrics, the optimal relay node as well as the optimal MAC scheme are selected for each link. Finally, the optimal path from source to destination is established through routing algorithms using the new optimal link metrics. In this way, the potential cooperative benefit from the MAC layer is exploited at the routing layer.
The studied metrics include packet delivery ratio (PDR), throughput, and energy efficiency, considering different requirements in various network scenarios. The proposed routing algorithm is implemented and evaluated using a simple topology to demonstrate the performance improvement by the proposed cooperative routing scheme.
The rest of the paper is organized as follows. The cross-layer cooperative network is introduced in Section
The cross-layer cooperative networking system considered in our study involves the physical layer, the MAC layer as well as the routing layer. In this section, the network model is introduced first, and then the average packet error rate of the data transmission is derived. In the second subsection, we summarize the principles of the two
We start our description from an introduction of power consumption of a communication system in different modes. A transmitting node consumes
Thus, the signal received at a transmitter, which is
The average signal-to-noise ratio (SNR) of the received signal,
Rayleigh fading is assumed in our channel model, but our analysis can be extended to other fading channels such as Rician or Nakagami. In order to simplify higher layer implementation, we obtain the average packet error rate (PER) performance through analysis. The instantaneous SNR at the receiver through a Rayleigh fading channel has an exponential distribution as:
We rely on the following expression to approximate PER over additive white gaussian noise (AWGN) [
Given an average SNR value, the PER performance averaged over Rayleigh fading is given as:
In the case of adaptive coding and modulation (ACM), the MCS scheme at the physical layer is determined according to the given channel condition. For instance, the channel condition between the transmitter and the receiver can be represented by the SNR value of the received signal. By checking a threshold value, an appropriate data rate is selected [
Based on the above information, the PER between each transmission pair can be calculated. After that, the PDR values, that is, the percentage numbers of packet successfully delivered among all the packets at the MAC layer, can easily be obtained using
Note that the packet delivery ratio on each link can also be obtained at the MAC layer by counting the percentage of packets that are acknowledged by ACK messages from the receiver. However, for simplification and feasibility reasons, the PDR values in this study are calculated according to the physical layer abstraction procedure described above.
As mentioned earlier, there exist two typical categories of cooperative MAC in the literature, namely,
In
If the relay node is chosen in CoopMAC, the data packet is first sent to the relay at
CoopMAC (virtual-hop relay scheme).
As the first step in
The message sequences when the cooperative retransmission is executed successfully are illustrated in Figure
C-ARQ (cooperative retransmission scheme).
To operate a multihop ad hoc network, cooperative MAC mechanism itself is not sufficient. A smart routing protocol is needed for path establishment from source to destination.
Different from traditional routing decision, the best route selected by our routing scheme needs to take cooperative gains which are obtained from the underlying MAC layer into consideration. Cooperative routing is enabled when potential cooperation gain exists in comparison with traditional routing.
A very simple topology in Figure
Topology to illustrate cooperative routing.
When cooperative communications are introduced into this network,
First, the cooperative link metric needs to be calculated for each cooperative MAC scheme for each relay candidate. With two different MAC (
In summary, with cooperative routing, not only the best path for data transmission is selected, but also the best cooperative MAC scheme as well as the best relay candidate are chosen. Different paths with different cooperative schemes and relays will be selected according to network requirements through different metrics from the MAC layer to the routing layer. Note that within a one-hop transmission link (i.e., a pair of source and destination nodes together with their neighboring nodes), the source node decides the transmission scheme and which relay to cooperate with. Thereafter, the neighboring nodes can be notified with this decision through the MAC header of the packet sent from the source node.
As mentioned in the preceding section, various metrics will be used for routing decision making in multihop networks. In this section, we will explain how to calculate these link metrics with different underlying cooperative MAC mechanisms.
The study is carried out considering different network performance parameters, such as packet delivery ratio, throughput, and energy efficiency. Denote
The data rates used on the links from
Packet delivery ratio at the MAC layer is used as an indicator for link reliability in this study. The PDR of the one-hop
In
For
Comparing
The PDR performance for the whole path
As shown in (
For traditional
For
Based on the same principle, the throughput of
Comparing
For the simplicity and feasibility of routing algorithms, we define the end-to-end effective throughput to be the geometric mean of the throughput on each link along the path. The effective throughput performance for the whole path
Based on the above information, the path with the maximal value of
As mentioned earlier, the energy consumed by nodes in the idle mode is neglected in this study. Therefore, the
For
For
Similar to the effective throughput metric, the energy efficiency for the whole path is shown as follows, and the path with the maximal value of
After introducing all the different routing metrics in Section
In our network, the relays are selected from the neighboring nodes of a transmitter-receiver pair, and these relays generate their own traffic as well. Path establishment starts from a traditional routing procedure first, and the nodes send packets and obtain information from each other, for example, channel information (SNR in our case) for each link. Based on the gathered information, the cooperative metrics are updated. Thereafter, new routes will be set up with the updated cooperative metrics. The routing algorithm is summarized as Algorithm
The proposed routing algorithm follows the same principle as the original Dijkstra’s shortest path algorithm, but updated with small modifications for analogous operations. The pseudocode of the implementation is listed as Algorithm
In our algorithm, different metrics have different operations to calculate link cost. With regard to routing based on packet delivery ratio, the link cost is the probability of unsuccessful transmission through the link. The data packets are delivered successfully to the destination along the path only when the transmission on each link is successful. Therefore, in the modified Dijkstra’s algorithm, we use
With respect to possible real-life implementation of our algorithm, it is highly feasible to integrate the above cooperative routing scheme with popular routing protocols such as the optimized link state routing protocol (OLSR) protocol. OLSR is a proactive link-state routing protocol designed for mobile ad hoc networks, which uses HELLO messages for neighbor discovering and then topology control (TC) messages for disseminating link state information throughout the whole network [
To evaluate the performance of the proposed cooperative routing scheme, we have implemented the DCF, CoopMAC, and C-ARQ mechanisms and the modified Dijkstra’s routing algorithm described in the previous section in MATLAB.
A simple network topology with 6 nodes is configured in our simulations, as shown in Figure
Simulation parameters.
14 bytes | 20 MHz | ||
500 bytes | 15 | ||
1400 mW | DIFS | 34 | |
900 mW | SIFS | 16 | |
Basic datarate | 6 Mbps | Slottime | 9 |
RF efficiency | 0.5 | Path loss exponent | 4.0 |
Network topology for cooperative routing simulations.
The transmission rate of the data packet is determined according to the average SNR value of the received signal at the receiver. The required channel conditions are assumed to be obtained beforehand and the overhead for channel estimation is ignored in this study. The MCS set and their corresponding parameters are listed in Table
Modulation and coding scheme set.
MCS Index | 0 | 1 | 2 | 3 | 4 |
---|---|---|---|---|---|
Modulation | BPSK | QPSK | 16QAM | 16QAM | 64QAM |
Code rate | 1/2 | 3/4 | 1/2 | 3/4 | 3/4 |
Data Rate (Mbps) | 6 | 18 | 24 | 36 | 54 |
−1.0 | 3.0 | 4.0 | 6.0 | 9.0 | |
Scope (dB) | <12 | 12~16.8 | 16.8~17.5 | 17.5~18 | >18 |
0.2 | 2.3 × 103 | 2.6 × 104 | 1.1 × 105 | 1.2 × 106 | |
2.8 | 2.5 | 2.4 | 1.9 | 1.5 |
Firstly, we set
The optimal path from source (Node S) to destination (Node D) is shown in Figure
Route for highest packet delivery ratio.
Figure
Routing for highest throughput.
The optimal paths with regard to energy efficiency are shown in Figure
Routing for highest energy efficiency.
Secondly, we investigate the performance of cooperative routing in comparison with traditional routing under different channel conditions.
Figure
Traditional routing versus cooperative routing (PDR).
Furthermore, the throughput performance comparison under different channel conditions is shown in Figure
Traditional routing versus cooperative routing (throughput).
Finally, the numerical results based on energy efficiency for end-to-end data transmission are shown in Figure
Traditional routing versus cooperative routing (energy efficiency).
The research effort on how to apply cooperative communication into routing decisions in multihop wireless networks is still in its infant stage. In this paper, we propose a metric-based routing scheme that integrates both cooperative MAC mechanism selection and relay selection into routing decision making. Various routing metrics are proposed considering link reliability, throughput, and energy consumption.
The network performance with cooperative routing is evaluated using a simple network topology. The obtained numerical results demonstrate that cooperative communication is effective for PDR performance enhancement but less effective for throughput enhancement. Furthermore, cooperative communication has basically no advantage over traditional transmission with regard to energy efficiency for the scenarios studied in this work.
The simulations in our work were done by implementing the routing algorithm and applying it to a simple network topology. Certainly, it could be more convincing with results provided from real-life network testbeds. Integrating a complete routing protocol with the proposed cooperative routing metrics is left for future work. In addition, our focus is only on static networks such as wireless mesh networks in this study therefore node mobility is not taken into consideration. Although the proposed routing algorithm may apply to mobile ad hoc networks in principle, how effective cooperative communications are as well as extra protocol overhead introduced due to mobility are left for further investigation.
The PER estimation of the modulation with convolutional code schemes in 802.11 g is illustrated here. The parameters of
Parameters tuning for PER estimation.