Cross-layer performance analysis

Traditional simulation methodologies independently model the network and physical layer, based on the notion of layering and information hiding protocol design paradigm proposed by the open systems interconnect (OSI) model. When multiple layer simulations are required, an abstraction of one layer is inserted into the other to provide the multiple layer simulation. However, recent advances in wireless communication technologies such as adaptive modulation and adaptive antenna algorithms demand a cross-layer perspective to this problem. It is hence, often necessary to examine the overall network architecture across multiple layers of the network hierarchy. This is particularly true for a network that consists of a variety of heterogeneous components. In such a scenario the design of an intelligent overall architecture and protocols, seeking to improve performance on an end-to-end basis is a challenging task.

In the line of ongoing research efforts, the benefits and possible parametric characterization issues arising due to the cross-layer integration of lower physical and higher network layers has been investigated at MPRG.

Physical Layer Description:

A complete physical layer simulator based on the IEEE 802.11a standard was used in our cross-layer simulations. OFDM was selected as the modulation of choice to combat frequency selective fading and to randomize the burst errors caused by a wideband fading channel. The key parameters of the OFDM standard used in our physical layer simulation are illustrated in Table 1.

Network Layer Description:

The system consists of a transmitter (base station (B.S)) and arbitrary number of receivers (mobile station (M.S)). Specifically, a cellular type of system is developed simulated with information transfer taking place at the downlink. A number of uncoordinated users contend for the available bandwidth. In response to which, the base-station (BS) implements a greedy scheduling algorithm that makes channel assignments based on the CSI.

Table 1: Key Parameters for the PHY OFDM

Figure 1: An adaptive modulation scheme

• Baseline System – I (Fixed rate channel encoder)

• Full PHY-MAC layer simulation, System - II (adaptive channel encoder with MAC interaction)

In the Baseline System – I, a dynamic time division multiple access (DTDMA) based MAC layer is implemented with a fixed rate channel encoder (BPSK modulation with R½ convolutional code). The schematic diagram of such a model is shown in Figure 2. The entire system model will be described in more detail in the later part of this report.

System – II with a variable rate channel encoder and MAC interaction is shown in Figure 3. This system implements a DTDMA based MAC layer along with a variable rate channel encoder which is responsible for switching between one of three available transmission modes depending on the channel condition, or more specifically, the estimated BER value. As expected, the novel design of System – II would outperform the above discussed Baseline system and is hence an obvious choice for cross-layer design.

Figure 2: Baseline System–I: Fixed Rate Channel Encoder

Figure 3: CSI Dependent MAC System–II: Adaptive Channel Encoder with MAC Interaction

Figure 4: Snapshot of a network simulation

Figure 4. shows the snapshot of a network simulation. The developed simulation tool facilitates cross-layer optimization.

• Beamforming and/or adaptive nulling algorithms are processed in Matlab

• Ad-hoc network node mobility and packet/burst communication simulated in OPNET

Figure 5: Snapshot of smart antenna system – OPNET/Matlab co-simulation