CoopMACA: a cooperative MAC protocol using packet aggregation

Abstract

In this paper we propose a cooperative MAC protocol for Wireless Local Area Networks (WLAN) that involves the concept of cooperation among nodes to avoid the negative effect caused by multi rate modulation employed in IEEE 802.11 standards. In our proposed protocol a low data rate direct transmission link is replaced by two faster transmission links using a relay node. During transmission, each node selects either direct or indirect transmission (through a helper node) in order to minimize the total transmission time and utilizes the packet aggregation concept to improve the system throughput. The new protocol does not violate the inter frame space specified in IEEE 802.11 and shows compatibility with the standard. We give the mathematical analysis that shows that our proposed protocol increases the system throughput considerably in comparison to the existing ones. The analytical results are supported with the help of simulation. We have shown how this protocol can be implemented in combination with others to improve the system throughput in specific network scenarios.

Appendix 1: Mathematical analysis

The average packet transmission time (T ₁₂) from a sender node in the 12 Mbps region is calculated as follows: The fraction of the average packet transmission time involving a helper node is given by,

$$ \begin{aligned} T_{12H} &= \left( {H_{54,54} + H_{24,54} + H_{54,24} } \right)T_{CoopOH} \\ & + {\frac{{3 \times 8L \times H_{54,54} }}{{54 \times 10^{6} }}} \\ & + \left( {{\frac{8L}{{24 \times 10^{6} }}} + {\frac{16L}{{54 \times 10^{6} }}}} \right)H_{24,54} \\ & + \cdots \left( {{\frac{8L}{{54 \times 10^{6} }}} + {\frac{16L}{{24 \times 10^{6} }}}} \right)H_{54,24} \\ & + \left( {{\frac{{H_{54,54} + H_{24,54} }}{{54 \times 10^{6} }}} + {\frac{{H_{54,24} }}{{24 \times 10^{6} }}}} \right)L_{o\_H\_A} \\ \end{aligned} $$

(16)

$where\, T_{CoopOH} = 2T_{PLCP} + 2T_{MAC} + T_{DIFS} + T_{RTS} + T_{HTS} + T_{ACK} + T_{HS} + 5T_{SIFS} .$

In Eq. 16 $L_{o\_H\_A}$ is additional overhead occurs in CoopMACA by the packet headers used in packet aggregation (Fig. 5). The first term corresponds to the overhead of the packet transmission using a helper node in CoopMACA and the remaining terms are the pay load transmission time in the first and second hop respectively.

$$ T_{12} = T_{12H} + \left( H \right) \times \left( {T_{CoopFH} + {\frac{8L}{{12 \times 10^{6} }}}} \right) + \left( {1 - H} \right) \times \left( {H + {\frac{8L}{{12 \times 10^{6} }}}} \right). $$

(17)

In Eq. 17, the second term corresponds to the contribution from the event involving the collision of HTS frame. Here $\overline{{H_{X,Y} }}$ is the probability of helper node selection failure due to the collision of HTS frame when the helper node is selected from the group (X, Y). The probability $\overline{{H_{X,Y} }}$ is given by,

$$ \overline{{H_{X,Y} }} = H \times P_{ck\_XY} . $$

(18.1)

The term $T_{CoopFH}$ is the over head time during the collision of HTS. $T_{CoopFh}$ is given by

$$ T_{CoopFH} = T_{PLCP} + T_{MAC} + T_{DIFS} + T_{RTS} + T_{HTS} + T_{ACK} + T_{HS} + 4T_{SIFS}.$$

(18.2)

The term is the transmission overhead occurs if there is no helper node exist to assist the transmission are given by,

$$ T = T_{oh} + contention\, slot. $$

(18.3)

The third term represents the effect of the events involving the packet transmission without any helper nodes.

Similarly we can find the average packet transmission time (T ₆) when the sender node is in the 6 Mbps region.

1.1 Average packet transmission time

The average packet transmission time of a packet can be calculated by,

$$ \begin{aligned} T_{S} = & f_{54} T_{54} + f_{24} T_{24} + f_{12} {\frac{{T_{12} }}{{\left( {H_{54,54} + H_{24,54} + H_{54,24} } \right) + 1}}} + \cdots \\ \, f_{6} {\frac{T6}{{\left( {H_{24,54} + H_{24,54} + H_{12,54} + H_{24,24} + H_{54,12} + H_{12,24} } \right) + 1}}} \\ \end{aligned} $$

(19)

$$ {\text{where}} \, f_{54} = {\frac{{r^{2}_{54} }}{{r^{2}_{6} }}},\quad f_{24} = {\frac{{\left( {r_{24} - r_{54} } \right)^{2} }}{{r^{2}_{6} }}} ,\quad f_{12} = {\frac{{\left( {r_{12} - r_{24} } \right)^{2} }}{{r^{2}_{6} }}} {\text{and}} \quad f_{6} = {\frac{{\left( {r_{6} - r_{12} } \right)^{2} }}{{r^{2}_{6} }}}. $$

The first two terms in this expression analyses the effect of packet transmission in a single hop. The third term corresponds to the packet transmission time from the nodes at a data rate of 12 Mbps region that may result in the transmission of two packets (one from 12 Mbps node and the other from the helper node). The denominators of the third term include the probability transmitting an additional packet by aggregation and its effect in the average packet transmission time.