Sarmad Sohaib

Modeling and analysis of the spectral response for AlGaAs/GaAs HPTs for short wavelength optical communication

Detailed spectral response (SR) modeling for heterojunction bipolar phototransistors (HPTs) is presented in this work. All the related physical parameters are taken into account for the resolution of photogenerated excess minority carrier continuity equations in the active layers of the HPT. The layer dependence of the optical flux absorption profile at near-bandgap wavelengths is also investigated and its generalization as a single-exponential has been refuted for HPTs based on GaAs material systems (InGaP-GaAs/AlGaAs-GaAs). The variation in the responsivity of the device with changing base width is analyzed at various wavelengths and a detailed experimental setup for optical characterization of HPTs is also provided. The measured results at 635, 780, 808, and 850 nm show good agreement to the modeled data, validating the newly developed theoretical model.

Asynchronous Cooperative Relaying for Vehicle-to-Vehicle Communications

Cooperative diversity exploits the broadcast nature of wireless channels and uses relays to improve link reliability. Most of the cooperative communication protocols are assumed to be synchronous in nature, which is not always possible in vehicle-to-vehicle (V2V) communication due to fast moving nature of the nodes. Also the relay nodes are assumed to be half duplex which in turn reduces the spectral efficiency. In this paper, we propose an asynchronous cooperative communication protocol exploiting polarization diversity, which does not require synchronization at the relay node. Dual polarized antennas are employed at the relay node to achieve full duplex amplify-and-forward (ANF) communication. Hence the transmission duration is reduced which results into an increased throughput rate. Capacity analysis of the proposed scheme ascertains the high data rate as compared to conventional ANF. Bit error rate (BER) simulation also shows that the proposed scheme significantly outperforms both the non-cooperative single-input single-output and the conventional ANF schemes. Considering channel path loss, the proposed scheme consumes less total transmission energy as compared to the conventional ANF and non-cooperative scheme. Thus the proposed scheme is suitable for high rate and energy efficient relay-enabled communication.

Asynchronous Polarized Cooperative MIMO Communication

In cooperative wireless network, the users exploit spatial diversity by cooperating with each other. This alleviates the detrimental effects of fading and offers reliable data transfer. In this paper, we present a novel asynchronous cooperative multi-input multi-output (MIMO) communication scheme in the presence of polarization diversity which does not require synchronization at the relay node. Utilizing dual-polarized antennas, the relay node achieves full duplex amplify-and-forward (ANF) communication. Hence the transmission duration is significantly reduced which in turn results into an increased throughput rate. Capacity analysis of the proposed system ascertains the high data rate as compared to the conventional ANF protocol. Bit error rate simulation also shows that the proposed scheme significantly outperforms both the non-cooperative single-input single-output and the conventional ANF schemes.

Energy allocation for green multiple relay cooperative communication

Cooperative communication achieves diversity through spatially separated cooperating nodes, which are battery powered in most applications. Therefore the energy consumption must be minimized without compromising the quality of service. In this context, we present a novel energy allocation scheme for multiple relay nodes that results in efficient cooperative multiple-input multiple-output (MIMO) communication. Considering channel path loss, the total transmission energy is distributed between the source and the relay nodes. The energy distribution ratio between the relay and direct link is optimized such that the quality of received signal is maintained with minimum total transmission energy consumption. We calculate the energy distribution ratio analytically and verified it through computer simulation. With the new energy allocation scheme, the system also obtains an increased channel capacity as compared to the cooperative scheme with conventional equal energy allocation and the non-cooperative scheme. Optimal relay positioning with the proposed energy allocation scheme is also explored to maximize the capacity.

Energy-efficient delay tolerant space time codes for asynchronous cooperative communications

Cooperative communication protocols are generally assumed to be synchronous in nature, which is not always possible in distributed wireless networks. Lack of synchronisation may cause space time codes to lose their property of being full rank. The solution proposed, thus far, increase the size of the codes in the temporal that in turn increases the energy consumption and complexity of the receiver. In addition, the channel is required to be static over the length of codes, which is a hard condition when the size of the ST code increases. In order to mitigate these issues, we have proposed delay tolerant codes in this paper that do not increase receiver complexity and require less circuit energy. The simulation results for various delay profiles show that the proposed codes require less total energy per bit when relays are within a small range. Therefore, the proposed scheme is better suited to short-range applications such as sensor networks. Copyright

Power allocation for efficient cooperative communication

Cooperative communication achieves diversity through spatially separated cooperating nodes, which are battery powered in most applications. Therefore the energy consumption must be minimized without compromising the transmission quality (bit error rate). In this context, we present a novel power allocation scheme that results in efficient cooperative multiple-input multiple-output (MIMO) communication. Considering channel path loss, the total transmission energy is distributed between the source and the relay nodes. The energy distribution ratio between the relay and direct link is optimized such that the quality of received signal is maintained with minimum total transmission energy consumption. We calculate the energy distribution ratio analytically and verified it through computer simulation. With the new power allocation scheme, the system also obtains an increased channel capacity as compared to cooperative scheme with conventional equal power allocation and non-cooperative scheme. Optimal relay positioning with proposed energy allocation scheme is explored to maximize the capacity.

Detailed Analysis on the Spectral Response of InP/InGaAs HPTs for Optoelectronic Applications

We analyze an analytical spectral-response model for heterojunction phototransistors (HPTs) in order to understand the behavior of lattice-matched InPZIn0.47Ga0.53As HPTs with changing device and material parameters. The preliminary modeling of the spectral response lead to a good agreement between theoretical and experimental results for incident wavelength radiations at 980, 1310, and 1550 nm. We then performed several simulations in order to determine the individual influences of several parameters, such as the base-layer thickness and the surface-recombination velocity on the responsivity of the device. A decreasing trend for the surface-recombination parameter with increasing wavelengths was observed, and it is attributed to the greater recombination rate for high-energy photons generated near the surface.

Power Allocation for Multi-Relay Amplify-And-Forward Cooperative Networks

Nodes in most cooperative networks are powered by batteries and some of which are even non-rechargeable. Therefore, power allocation schemes must be developed to save the transmit power and improve the life-time of the system. In this context, we present a novel power allocation scheme for multiple relay nodes that results in efficient cooperative communication. Considering channel path loss, the total transmission energy is distributed between the source and the relay nodes. The energy distribution ratio between the relay and direct link is optimized such that the quality of received signal is maintained with minimum total transmission energy consumption. We calculate the energy distribution ratio analytically and verified it through computer simulation. With the new power allocation scheme, the system also obtains an increased channel capacity as compared to cooperative scheme with conventional equal power allocation. Optimal relay positioning with proposed energy allocation scheme is also explored to maximize the capacity.

Energy analysis of asynchronous polarized cooperative MIMO protocol

Cooperative communication exploits the broadcast nature of wireless channel and uses relay nodes to provide better reliability and higher data rates without increasing power and bandwidth. In this paper, the energy analysis of the asynchronous polarized cooperative (APC) scheme is performed. APC employs multiple antennas at the relay and destination nodes to achieve full duplex amplify-and-forward (ANF) communication. Hence the transmission duration is reduced which results into an increased spectral efficiency. Considering channel path loss, the APC scheme consumes less total transmission energy as compared to ANF and non-cooperative scheme over more practical distance between the nodes. Thus the APC scheme is both spectral and energy efficient, and is suitable for the cooperative communication systems.

Centralized dynamic frequency allocation for cell-edge demand satisfaction in fractional frequency reuse networks

Fractional frequency reuse (FFR) has emerged as a well-suited remedy for inter-cell interference reduction in the next-generation networks by allocating frequency reuse factor (FRF) of unity for the cell-center (CC) and higher FRF for the cell-edge (CE) users. However, this strict FFR comes at a cost of equal partitioning of frequency resources to the CE which most likely has varying demands in current networks. In order to mitigate this, we propose a centralized dynamic resource allocation scheme which allocates demand-dependent resources to CE users. The proposed scheme therefore outperforms the fixed allocation scheme of strict FFR for both CC and CE users. Complexity analysis provides a fair means of analyzing the suitability of proposed algorithm. We have also compared the proposed methodology with a reference dynamic fractional frequency reuse (DFFR) scheme. Results show maximum performance gain of up to 30% for 3 reference cells employing Rayleigh fading—through normalized area spectral efficiency (ASE) analysis for both fixed allocation and DFFR. Spectral efficiency analysis also indicates per-cell performance gain for both CC and CE users. Further, detailed three-dimensional ASE plots give insights into the affects to other cells. Due to dynamic nature of traffic loads, the proposed scheme is a candidate solution for satisfying the demands of individual cells.