Vahid Pourahmadi

Multilevel coding strategy for two-hop single-user networks

In this paper, a two-hop network in which the information is transmitted from a source via a relay to a destination is considered. It is assumed that both channels are quasi-static fading and all nodes are equipped with a single antenna. The knowledge of the channel for each transmission hop is only available at the corresponding receiver. The relay is assumed to be simple, i.e., not capable of data buffering over multiple coding blocks or rescheduling tasks. Considering a continuum of multilevel codes at both of the source and the relay, in conjunction with decode and forward strategy, we present a scheme to optimally allocate the available source and relay powers to different levels of their multilevel codes. Assuming Rayleigh fading, the performance of this scheme is also evaluated and compared with the previously known strategies.

Relay Placement in Wireless Networks: A Study of the Underlying Tradeoffs

It is known that the achievable data rate per user can be increased when relays are deployed in wireless networks. However, the drawback of this solution is that some of the networks resources should be allocated to the relays. In this paper, we consider a two-tier network in which all users send or receive data in two hops. By applying vector quantization, we compute the relays locations to improve networks average transmission rate. These locations are also computed analytically when the number of relays is less than six. Having determined the relays locations, the networks average transmission rate is evaluated. Subsequently, we define the neutrality-surface such that the performance of any relay network operating below this surface is inferior to that of the same network without relays. Finally, we study the relative relaying gain for different network configurations.

Multilayer Coding Over Multihop Single-User Networks

This paper considers a two-hop network in which information is transmitted from a source via a relay to a destination. It is assumed that channels are quasi-static fading with additive white Gaussian noise and that all nodes are equipped with a single antenna. The channel state information (CSI) of each hop is available only at the corresponding receiver and relay is not capable of data buffering over multiple coding blocks. One commonly used design criterion in such configurations is the maximization of the average received rate at the destination. Considering infinite-layer coding at both the source and the relay, in conjunction with decode and forward strategy at the relay, we present a procedure to optimally distribute the available source and relay powers to different layers of their corresponding codes. Next, we demonstrate how this transmission technique can be generalized to a multihop setting. Assuming Rayleigh fading, the performance of the proposed coding scheme is evaluated for a two-hop network and compared with the performance of previously known strategies.

Degrees of Freedom of MIMO-MAC with Random Access

A distributed random access network with K users and one Access Point (AP) is considered. It is assumed that users and the AP are equipped with M and N antennas, respectively. Each user independently decides whether to transmit in a time slot or not. We initially focus on two-user random access networks and characterize the network average Degrees of Freedom (DoF)1. For the K-user networks, an upper-bound on the network average DoF is first proposed. Then, it is shown that the proposed upper-bound can be achieved using single stream data transmission for many network configurations. Finally, we show through a few examples that there exist some network configurations where multi-stream data transmission in conjunction with interference alignment is necessary in order to achieve the upper-bound.

Degrees of freedom of two-user MIMO networks with random medium access control mechanism

This paper studies a multiple access network with two users and one Access Point (AP). It is assumed that the users and the AP are equipped with M and N antennas, respectively. To access the network, each user independently decides whether to transmit in a time slot or not (no coordination between users). Focusing on the high SNR behavior of the system, this paper presents the optimal value of the network average Degrees of Freedom (DoF) in different network settings. To this end, after finding an upper-bound for the network average DoF, we propose a transmission scheme (based on interference alignment) which achieves this upper-bound. Some illustrative examples are also presented in the paper.

Infinite-layer codes for single-user slowly fading MIMO channels

Given a slowly fading channel, the performance of multi-layer coding is studied for single-user scenarios. Both the source and destination are equipped with multiple antennas. The channel state information is perfectly known at the destination but not at the source. The objective is to maximize the average data rate received at the destination when the destination is able to perform successive decoding. This paper, first, proposes a design rule for constructing an infinite-layer code for Multiple Input Multiple Output (MIMO) channels. Furthermore, we present a procedure describing how the introduced design rule is applied to optimally determine the multi-layer code parameters. The achievable rate of the multi-layer coding and successive decoding for the Rayleigh fading 2×2 MIMO channel is also evaluated.

On the optimal design of two-tier wireless relay networks

It is known that the achievable data rate per user can be increased when relays are deployed in wireless networks. However, the drawback with this solution is that some of the network resources should be allocated to the relays. In this paper, we consider a two-tier network where all users should send/receive data in two hops (via a relay). Applying vector quantization, we approximately find the location of the relays. These approximate relays locations are also computed analytically when the number of relays is less than six. Having the relays locations, the network average transmission rate is evaluated in terms of a set of network parameters. Then, in the multi-dimensional space of these network parameters, we introduce the concept of neutrality-surface. The neutrality-surface is defined such that the performance of any relay network operating below this surface is inferior to that of a simple no-relay network with the same parameters. Finally, we study the relative and differential relaying gain for different network configurations.

Multilayer Codes for Broadcasting over Quasi-Static Fading MIMO Networks

A number of recent source coding techniques compress the source signal into multiple layers such that a destination is able to reconstruct the original signal (with some distortion) even if it has not received all the layers. Implementation of such source coding techniques in wireless networks requires the application of coding mechanisms (such as multilayer coding) which allow unequal error protection for different layers of the transmitted data. In this paper, we study the performance of multilayer coding for quasi-static fading channels where the source and the destination are equipped with multiple antennas and the Channel-State-Information (CSI) is only known at the destination. We limit the study to the scenarios where the destination is only able to perform successive-decoding (joint-decoding is not possible) and the objective is to find the design of a multilayer code which maximizes the average data rate received at the destination. To this end, we first propose a design rule for constructing a proper multilayer code for Multiple-Input-Multiple-Output (MIMO) networks. Furthermore, the paper presents a procedure which uses the proposed design rule to determine the parameters of the multilayer code. The performance of the designed multilayer coding scheme is then studied for different network setups.

On the Accuracy of Channel Modeling Based on the Kronecker Product

We investigate how accurate Kronecker model is compared to Spatial Channel Model (SCM) for MIMO link layer simulations. The simulation results show that Kronecker model is a good approximation for MIMO in link layer simulations compared to SCM.