R-3 Repository :: Browsing by Author "Khojastepour, Mohammad"

Browsing by Author "Khojastepour, Mohammad"

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Achievable Rates for Arbitrary Network Topologies with â Cheapâ Nodes
(2003-05-01) Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we derive achievable rates for arbitrary network topologies consisting of â cheapâ nodes. A node is labeled â cheapâ if its radio can only operate in TDD mode when transmitting and receiving in the same frequency band. Two main results are shown. The first result provides an achievable rate for the channel with either continuous alphabet or discrete channel. The second result provides the achievable rate for the Gaussian channel with average power constraint. The two results are applied to the case of Gaussian relay channel and concatenated channel with cheap nodes
Bounds on Achievable Rates for General Multi-terminal Networks with Practical Constraints
(2003-04-20) Khojastepour, Mohammad; Aazhang, Behnaam; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)
We consider the problem of communication in a general multi-terminal network where each node of the network is a potential sender or receiver (or both) but it cannot do both functions together. The motivation for this assumption comes from the fact that current radios in sensor nodes operate in TDD mode when the transmitting and receiving frequencies are the same. We label such a radio as a cheap radio and the corresponding node of the network as a cheap node. We derive bounds on the achievable rates in a general multi-terminal network with finite number of states. The derived bounds coincide with the known cut-set bound of network information theory if the network has just one state. Also, the bounds trivially hold in the network with cheap nodes because such a network operates in a finite number of states when the number of nodes is finite. As an example, application of these bounds in the multi-hop network and the relay channel with cheap nodes is presented. In both of these cases, the bounds are tight enough to provide converses for the coding theorems, and thus their respective capacities are derived.
The Capacity of Average and Peak Power Constrained Fading Channels with Channel Side Information
(2004-03-01) Khojastepour, Mohammad; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We derive the ergodic capacity of discrete-time fading channel with additive Gaussian noise subject to both peak and average power constraint. The average power can be interpreted as the cost that we incur to achieve a certain rate. On the other hand, the motivation of this analysis comes from the fact that there is also a peak power limitation in practical communication system. It is been shown that the optimal power adaption is no longer water-filling or constant power adaption which is the case where there is no limitation on the peak power. The numerical results show that the importance of peak power constraint become negligible for relatively low available average power, while it is limiting the capacity to be finite even as available average power goes to infinity.
Contraction, Smoothness, and Low-Pass Filtering
(2004-05-01) Khojastepour, Mohammad; Aazhang, Behnaam; Baraniuk, Richard G.; Digital Signal Processing (http://dsp.rice.edu/)
We introduce a generalized definition for "low-pass" filters that covers time-varying and nonlinear systems under the same umbrella. We show that the qualitative concept of signal smoothing can be made precise through the concept of contractions in probabilistic metric spaces. For illustration, we consider classical linear time-invariant low-pass filters, nonlinear median filters, and time-varying guaranteed maximum delay schedulers employed in communication systems.
Cut-set Theorems for Multi-state Networks
(2003-10-01) Khojastepour, Mohammad; Aazhang, Behnaam; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)
We derive new cut-set bounds on the achievable rates in a general multi-terminal network with finite number of states. Multiple states are common in communication networks in the form of multiple channel and nodes' states. Our results are broadly applicable and provide much tighter upper bounds than the known single state min-cut max-flow theorem, and hence form an important new tool to bound the performance of multi-node networks. Two examples are presented to illustrate the tightness and the utility of the new bounds. In each of the example applications, the known single-state max-flow min-cut theorem provides a bound strictly looser than the new cut-set bounds. The first illustrative example is single-user compound channels, where both the transmitter and receiver have channel state information. The example of compound channels represents the smallest possible network with only two nodes, but has multiple states due to channel variations. The upper bound derived using the proposed bounds turns out to be the capacity of the compound channel, which implies that the bound is tight in this case. The second example is from a contemporary network problem. Here, we demonstrate the application of new bounds to characterize the limits on rate of information transfer in `cheap' relay networks, where the relay nodes can either transmit or receive, but not both simultaneously. In this case, each constituent channel has a single state but relay nodes can be in one of the two states, transmit or receive mode, giving rise to multiple network states. Here, again, the upper bound coincide with the capacity of the channel if the relay channel is degraded.
Delay-constrained Scheduling: Power Efficiency, Filter Design, and Bounds
(2004-03-01) Khojastepour, Mohammad; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, packet scheduling with maximum delay constraints is considered with the objective to minimize average transmit power over Gaussian channels. The main emphasis is on deriving robust schedulers which do not rely on the knowledge of the source arrival process. Towards that end, we first show that all schedulers (robust or otherwise) which guarantee a maximum queuing delay for each packet are equivalent to a time-varying linear filter. Using the connection between filtering and scheduling, we study the design of optimal power minimizing robust schedulers. Two cases, motivated by filtering connection, are studied in detail. First, a time-invariant robust scheduler is presented and its performance is completely characterized. Second, we present the optimal time-varying robust scheduler, and show that it has a very intuitive time water-filling structure. We also present upper and lower bounds on the performance of power-minimizing schedulers as a function of delay constraints. The new results form an important step towards understanding of the packet time-scale interactions between physical layer metric of power and network layer metric of delay.
Distributed Cooperative Communications in Wireless Networks
(2005-01-01) Khojastepour, Mohammad; Center for Multimedia Communications
The primary challenge in communication over wireless networks, unlike wireline networks, is the existence of interference and channel variations (fading). Having more users at higher data rates means that current point-to-point networks will not scale. To engineer a scalable network, we introduce a new paradigm that exploits different network characteristics. We show that cooperation between users in the net- work, network coding, not only reduces existing (destructive) interferences from other users but it can also generate constructive interference, transforming the destructive interference into useful information. In this thesis, we explore the problem of source and channel coding over wireless networks, ranging from information theoretical analysis to code design and practical implementation issues. We show that significant gains in throughput can be achieved through network coding. Despite the importance of the problem and the work done on wireless networks, little is known about network coding and the effective use of the relaying function and cooperative strategy at the intermediate nodes. A notable example is the lack of an optimal coding scheme over the relay channel, the simplest form of a network, which has remained an outstanding open question for the last three decades. We propose new approaches to network coding that improve upon the best known coding schemes by many decibels. Specifically, we develop two main coding tech- niques, one for the multi-state relay channel and the other for the multiple access with generalized feedback (MAC-GF). We show that by using the new coding tech- niques, higher transmission rates than those previously known are achievable. The first technique achieves the ultimate transmission rate (capacity) for both half-duplex and the original relay channel under certain conditions. These improved capacity re- sults for the relay channel are the only known results since Cover's in 1979 and El Gamal's in 1982. The second coding technique improves the best known achievable transmission rate for the MAC-GF by Willems in 1983. This latter result also im- proves the achievable transmission rate for the Gaussian relay channel over all other known schemes for some channel conditions. We also present a practical code design technique for the relay channel. The design gains more than 4dB over direct transmission and closes the gap to the relay channel Shannon limit to less than 1dB with a code length of only 2 x 10^4 bits. The new coding techniques and transmission strategies developed in this thesis provide important steps toward overcoming the challenges of wireless network coding.
Finite Backlog Effects on Downlink Scheduling
(2003-11-20) Ahmed, Nasir; Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In a multi-user downlink system, the use of channel estimates in the scheduling process can significantly improve the spectral efficiency. Recent work has shown that from a sum rate capacity standpoint, and assuming infinite queue backlogs, the optimal transmission scheme is to select the user with the largest instantaneous channel gain. A more realistic scenario is considered in this work, in which the downlink scheduling problem with finite queue backlogs is analyzed. It is shown that although time division scheduling is optimal for infinite backlogs, this is not the case for finite buffer sizes. In fact, a simple broadcast scheduler is shown to be more power efficient than the maximum rate time division scheduler under such a scenario.
How Quickly Can We Approach Channel Capacity?
(2004-11-01) Baron, Dror; Khojastepour, Mohammad; Baraniuk, Richard G.; Digital Signal Processing (http://dsp.rice.edu/)
Recent progress in code design has made it crucial to understand how quickly communication systems can approach their limits. To address this issue for the channel capacity C, we define the nonasymptotic capacity C/sub NA/(n, /spl epsi/) as the maximal rate of codebooks that achieve a probability /spl epsi/ of codeword error while using codewords of length n. We prove for the binary symmetric channel that C/sub NA/(n,/spl epsi/)=C-K(/spl epsi/)//spl radic/n+o(1//spl radic/n), where K(/spl epsi/) is available in closed form. We also describe similar results for the Gaussian channel. These results may lead to more efficient resource usage in practical communication systems.
Improved Achievable Rates for User Cooperation and Relay Channels
(2004-06-01) Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we derive a new achievable rate region for the user cooperation channel, which exceeds the best known result for this channel [Wil83]. Since the user cooperation channel includes many other known channels as a special case, the new rate region provides an improved achievable rates for many of the special cases (discussed later). The most notable example is that of the Gaussian relay channel [CG79], for which we present a new closed form inner bound higher than the only known result (Theorem 5 in [CG79]). The basic idea of the proposed encoding and decoding scheme is easier to grasp for a simpler case, the relay channel, by keeping in mind the following Markovian scheme from [CG79]. In Markovian encoding, the relay decodes everything in the current block and uses it to establish a basis for common information sent from the transmitter and relay in the next block. The fact that Markovian coding does not achieve capacity depends on to two inter-related facts: (a) the relay decodes each codeword completely, and (b) discards the residual after decoding; this limits the relay channel capacity to the capacity between the source and the relay. Our proposed encoding does not require the relay to necessarily decode the whole codeword from the transmitter. In addition, the relay \source" encodes the whole received signal some implicit embedding of with the decoded information. Note that the signal transmitted by the relay is not separable into a source code and a channel code. For the user cooperation channel, the above operation is performed by both users for each others' signal.
Lower Bounds on the Capacity of Gaussian Relay Channel
(2004-03-01) Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
It has been shown recently that the overall throughput of dense wireless networks can increase significantly when wireless nodes collaborate in transmission of different packets. In this paper, we focus on a simple network known as the relay channel, where there is a single relaying node in the system assisting a sender-receiver pair. We present a new achievable rate for the case of Gaussian relay channels, which outperforms the best known schemes in many cases of interest. The proposed scheme is a variation of the amplify and forward method, and is labelled as a scale and forward scheme, signifying that the relay does not use all its power amplifying the received signal. By controlling the scaling and time-correlation of the relay signal, we show that the proposed scheme can outperform all known schemes for several channel gains, especially those where the best known methods provide no improvement over direct transmission. Numerical results are presented to quantify in which regimes the proposed scheme is the best known coding scheme for the Gaussian relay channel.
On Capacity of Gaussian â Cheapâ Relay Channel
(2003-12-01) Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we derive the capacity of the Gaussian degraded â cheapâ relay channel, consisting of nodes using â cheapâ radios operating in TDD mode when transmitting and sending in the same frequency band. The TDD model captures a practical limitation of most of the RF radios used in commercial wireless systems. The proof of achievability relies on a combination of superposition encoding and list decoding, while the converse is derived using the min-cut max-flow theorem for networks with â cheapâ nodes (previously derived by the authors). Even with â cheapâ radios, our capacity analysis shows that cooperative coding is beneficial and has a capacity advantage over direct transmission.
On Power Control with Finite Rate Feedback for Cooperative Relay Networks
(2004-03-01) Ahmed, Nasir; Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
Power control strategies with finite rate feedback are studied for the cooperative channel. The main contribution of this paper is to show that quantized feedback information can lead to significant reduction in outage probability for the cooperative relay network. To obtain an increase in diversity order and significant reductions in outage probability over constant power cooperative signaling, we develop algorithms that exploit the channel states of all network links. With one feedback bit, the proposed power control algorithm is shown to double the diversity order of constant power transmission. To quantify the performance increase of using power control in the cooperative network, we derive a lower bound on the diversity order. Based on these results, it is evident that future network protocols should utilize feedback in order to exploit the potential gains of network coding.
On the Capacity of `Cheap' Relay Networks
(2003-04-20) Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We consider the communication problem in a multi-hop relay network where the intermediate relay nodes cannot transmit and receive at the same time. The motivation for this assumption comes from the fact that current radios operate in TDD mode when the transmitting and receiving frequencies are the same. We label such a node radio as a 'cheap' radio and the corresponding node of the network as a 'cheap' node. In this paper we derive the capacities of the degraded cheap relay channel and the multi-hop network with cheap nodes. The proof of the achievability parts in coding theorems are presented based on the jointly typical sequences, while the proof of the converses are derived from the direct application of the upper bounds derived in [7].
Outage Minimization and Optimal Power Control for the Fading Relay Channel
(2004-05-01) Ahmed, Nasir; Khojastepour, Mohammad; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this work, we show that in the wireless relay network, a tremendous savings in energy can be achieved by having side information at the transmitters and by employing power control. We present efficient protocols and the corresponding optimal power control policies that approach the universal lower bound on the outage probability of the block fading relay channel. Each of the proposed protocols have their own utility for specific channel conditions. However, a hybrid protocol between two known coding schemes is the best scheme for all channel conditions and is sufficient to approach the lower bound on outage probability. Unlike the single link channel, we show that exploiting the knowledge of the channel at the transmitters can significantly lower the outage even if the transmit powers at the source and relay have to be kept constant. In this case, it is also demonstrated that the lower bound on outage is closely followed by the outage probability of the hybrid protocol. Our results reveal that exploiting the right network protocol in conjunction with power control result in orders of magnitude savings in power over direct transmission for a target performance level.
Outage Minimization with Limited Feedback for the Fading Relay Channel
(2006) Ahmed, Nasir; Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this work, we consider practical methods to approach the theoretical performance limits in the fading relay channel under different assumptions of transmitter channel knowledge. Specifically, we consider two degrees of transmitter channel knowledge: (i) perfect feedback is available and power control is employed, and (ii) no channel state knowledge is available at the transmitters and only spatial power allocation is possible. First, when perfect feedback is available, the optimal power control policy determines the ultimate limits of performance for constant rate transmission in the slow fading environment. However, in practice, perfect channel knowledge is not possible at the transmitters due to the finite capacity of the feedback links. We find practical methods to approach this performance limit through the use of power control with finite rate feedback. The finite rate feedback results are shown for the low complexity, full diversity amplify-and-forward(AF) protocol. Interestingly, we see that only a few feedback bits are needed to achieve most of the gains of the optimal perfect feedback power control algorithm. Second, we consider the performance limit when the transmitters have no channel state knowledge, and derive the optimal spatial power allocation between the source and relay for a given sum power constraint for the AF protocol. For most practical cases of interest, equal power allocation between the source and relay is shown to be nearly optimal. Our work suggests that there is minimal power savings from using spatial power allocation at the transmitters. To obtain large performance improvements over constant power transmission, it is imperative to have feedback for each realization of the channel state to allow for temporal power control.
Power Optimal Scheduling with Maximum Delay Constraints
(2003-10-01) Khojastepour, Mohammad; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)
Most multimedia sources are bursty in nature, a property which can be used to trade queuing delay with the resulting average transmission power [2, 3, 4]. In this paper, we study the relation between average transmission power and strict delay constraints. Our main contributions are two-fold. First, we establish necessary and sufficient conditions on the service rates of the wireless transmitter, to meet the delay deadline of every packet in the queue. Second, the conditions are used to show that a scheduler which meets a delay guarantee Dmax for each of the packet over Gaussian channels is a time-varying low-pass filter of order no more than Dmax. This confirms the intuitive explanation for power reduction due to additional queuing delay provided in [3]. Using the relation between delay bounded scheduling and linear filtering, we construct schedulers without the knowledge of source statistics. This marks a significant departure from most information theoretic work on power efficient scheduling [2, 3]. We construct the optimal time-invariant scheduler, which does not require the knowledge of the source statistics.

Browsing by Author "Khojastepour, Mohammad"

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