arXiv subjects

Aria Nosratinia

Publications and source records attributed to Aria Nosratinia.

At least 37 records · Page 2Linked to original sources

arXiv2018

Side Information in the Binary Stochastic Block Model: Exact Recovery

In the community detection problem, one may have access to additional observations (side information) about the label of each node. This paper studies the effect of the quality and quantity of side information on the phase transition of exact recovery in the binary symmetric stochastic block model (SBM) with $n$ nodes. When the side information consists of the label observed through a binary symmetric channel with crossover probability $α$, and when $\log(\frac{1-α}α) =O(\log(n))$, it is shown that side information has a positive effect on phase transition; the new phase transition under this condition is characterized. When $α$ is constant or approaches zero sufficiently slowly, i.e., $\log(\frac{1-α}α) = o(\log(n))$, it is shown that side information does not help exact recovery. When the side information consists of the label observed through a binary erasure channel with parameter $ε$, and when $\log(ε)=O(\log(n))$, it is shown that side information improves exact recovery and the new phase transition is characterized. If $\log(ε)=o(\log(n))$, then it is shown that side information is not helpful. The results are then generalized to an arbitrary side information of finite cardinality. Necessary and sufficient conditions are derived for exact recovery that are tight, except for one special case under $M$-ary side information. An efficient algorithm that incorporates the effect of side information is proposed that uses a partial recovery algorithm combined with a local improvement procedure. Sufficient conditions are derived for exact recovery under this efficient algorithm.

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arXiv2017

Degrees of Freedom of the Broadcast Channel with Hybrid CSI at Transmitter and Receivers

In general, the different links of a broadcast channel may experience different fading dynamics and, potentially, unequal or hybrid channel state information (CSI) conditions. The faster the fading and the shorter the fading block length, the more often the link needs to be trained and estimated at the receiver, and the more likely that CSI is stale or unavailable at the transmitter. Disparity of link fading dynamics in the presence of CSI limitations can be modeled by a multi-user broadcast channel with both non-identical link fading block lengths as well as dissimilar link CSIR/CSIT conditions. This paper investigates a MISO broadcast channel where some receivers experience longer coherence intervals (static receivers) and have CSIR, while some other receivers experience shorter coherence intervals (dynamic receivers) and do not enjoy free CSIR. We consider a variety of CSIT conditions for the above mentioned model, including no CSIT, delayed CSIT, or hybrid CSIT. To investigate the degrees of freedom region, we employ interference alignment and beamforming along with a product superposition that allows simultaneous but non-contaminating transmission of pilots and data to different receivers. Outer bounds employ the extremal entropy inequality as well as a bounding of the performance of a discrete memoryless multiuser multilevel broadcast channel. For several cases, inner and outer bounds are established that either partially meet, or the gap diminishes with increasing coherence times.

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arXiv2017

Ergodic Fading MIMO Dirty Paper and Broadcast Channels: Capacity Bounds and Lattice Strategies

A multiple-input multiple-output (MIMO) version of the dirty paper channel is studied, where the channel input and the dirt experience the same fading process and the fading channel state is known at the receiver (CSIR). This represents settings where signal and interference sources are co-located, such as in the broadcast channel. First, a variant of Costa's dirty paper coding (DPC) is presented, whose achievable rates are within a constant gap to capacity for all signal and dirt powers. Additionally, a lattice coding and decoding scheme is proposed, whose decision regions are independent of the channel realizations. Under Rayleigh fading, the gap to capacity of the lattice coding scheme vanishes with the number of receive antennas, even at finite Signal-to-Noise Ratio (SNR). Thus, although the capacity of the fading dirty paper channel remains unknown, this work shows it is not far from its dirt-free counterpart. The insights from the dirty paper channel directly lead to transmission strategies for the two-user MIMO broadcast channel (BC), where the transmitter emits a superposition of desired and undesired (dirt) signals with respect to each receiver. The performance of the lattice coding scheme is analyzed under different fading dynamics for the two users, showing that high-dimensional lattices achieve rates close to capacity.

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arXiv2017

Lattice Coding and Decoding for Multiple-Antenna Ergodic Fading Channels

For ergodic fading, a lattice coding and decoding strategy is proposed and its performance is analyzed for the single-input single-output (SISO) and multiple-input multiple-output (MIMO) point-to-point channel as well as the multiple-access channel (MAC), with channel state information available only at the receiver (CSIR). At the decoder a novel strategy is proposed consisting of a time-varying equalization matrix followed by decision regions that depend only on channel statistics, not individual realizations. Our encoder has a similar structure to that of Erez and Zamir. For the SISO channel, the gap to capacity is bounded by a constant under a wide range of fading distributions. For the MIMO channel under Rayleigh fading, the rate achieved is within a gap to capacity that does not depend on the signal-to-noise ratio (SNR), and diminishes with the number of receive antennas. The analysis is extended to the K-user MAC where similar results hold. Achieving a small gap to capacity while limiting the use of CSIR to the equalizer highlights the scope for efficient decoder implementations, since decision regions are fixed, i.e., independent of channel realizations.

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arXiv2016

Broadcast Coded Modulation: Multilevel and Bit-interleaved Construction

The capacity of the AWGN broadcast channel is achieved by superposition coding, but superposition of individual coded modulations expands the modulation alphabet and distorts its configuration. Coded modulation over a broadcast channel subject to a specific channel-input modulation constraint remains an important open problem. Some progress has been made in the related area of unequal-error protection modulations which can be considered single-user broadcast transmission, but it does not approach all points on the boundary of the capacity region. This paper studies broadcast coded modulation using multilevel coding (MLC) subject to a specific channel input constellation. The conditions under which multilevel codes can achieve the constellation-constrained capacity of the AWGN broadcast channel are derived. For any given constellation, we propose a pragmatic multilevel design technique with near constellation-constrained-capacity performance where the coupling of the superposition inner and outer codes are localized to each bit-level. It is shown that this can be further relaxed to a code coupling on only one bit level, with little or no penalty under natural labeling. The rate allocation problem between the bit levels of the two users is studied and a pragmatic method is proposed, again with near-capacity performance. In further pursuit of lower complexity, a hybrid MLC-BICM is proposed, whose performance is shown to be very close to the boundary of the constellation-constrained capacity region. Simulation results show that good point-to-point LDPC codes produce excellent performance in the proposed coded modulation framework.

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arXiv2016

Coherence Disparity in Broadcast and Multiple Access Channels

Individual links in a wireless network may experience unequal fading coherence times due to differences in mobility or scattering environment, a practical scenario where the fundamental limits of communication have been mostly unknown. This paper studies broadcast and multiple access channels where multiple receivers experience unequal fading block lengths, and channel state information (CSI) is not available at the transmitter(s), or for free at any receiver. In other words, the cost of acquiring CSI at the receiver is fully accounted for in the degrees of freedom. In the broadcast channel, the method of product superposition is employed to find the achievable degrees of freedom. We start with unequal coherence intervals with integer ratios. As long as the coherence time is at least twice the number of transmit and receive antennas, these degrees of freedom meet the upper bound in four cases: when the transmitter has fewer antennas than the receivers, when all receivers have the same number of antennas, when the coherence time of one receiver is much shorter than all others, or when all receivers have identical block fading intervals. The degrees of freedom region of the broadcast under identical coherence times was also previously unknown and is settled by the results of this paper. The disparity of coherence times leads to gains that are distinct from those arising from other techniques such as spatial multiplexing or multi-user diversity; this class of gains is denoted coherence diversity. The inner bounds are further extended to the case of multiple receivers experiencing fading block lengths of arbitrary ratio or alignment. Also, in the multiple access channel with unequal coherence times, achievable and outer bounds on the degrees of freedom are obtained.

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arXiv2016

Coherent Product Superposition for Downlink Multiuser MIMO

In a two-user broadcast channel where one user has full CSIR and the other has none, a recent result showed that TDMA is strictly suboptimal and a product superposition requiring non-coherent signaling achieves DoF gains under many antenna configurations. This work introduces product superposition in the domain of coherent signaling with pilots, demonstrates the advantages of product superposition in low-SNR as well as high-SNR, and establishes DoF gains in a wider set of receiver antenna configurations. Two classes of decoders, with and without interference cancellation, are studied. Achievable rates are established by analysis and illustrated by simulations.

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arXiv2015

The Degrees of Freedom of the Interference Channel with a Cognitive Relay under Delayed Feedback

This paper studies the interference channel with a cognitive relay (ICCR) under delayed feedback. Three types of delayed feedback are studied: delayed channel state information at the transmitter (CSIT), delayed output feedback, and delayed Shannon feedback. Outer bounds are derived for the DoF region of the two-user multiple-input multiple-output (MIMO) ICCR with delayed feedback as well as without feedback. For the single-input single-output (SISO) scenario, optimal schemes are proposed based on retrospective interference alignment. It is shown that while a cognitive relay without feedback cannot extend the sum-DoF beyond $1$ in the two-user SISO interference channel, delayed feedback in the same scenario can extend the sum-DoF to $4/3$. For the MIMO case, achievable schemes are obtained via extensions of retrospective interference alignment, leading to DoF regions that meet the respective upper bounds.

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arXiv2013

Rate-Compatible Protograph-based LDPC Codes for Inter-Symbol Interference Channels

This letter produces a family of rate-compatible protograph-based LDPC codes approaching the independent and uniformly distributed (i.u.d.) capacity of inter-symbol interference (ISI) channels. This problem is highly nontrivial due to the joint design of structured (protograph-based) LDPC codes and the state structure of ISI channels. We describe a method to design nested high-rate protograph codes by adding variable nodes to the protograph of a lower rate code. We then design a family of rate-compatible protograph codes using the extension method. The resulting protograph codes have iterative decoding thresholds close to the i.u.d. capacity. Our results are supported by numerical simulations.

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arXiv2013

Rate-Compatible Short-Length Protograph LDPC Codes

This paper produces a rate-compatible protograph LDPC code at 1k information blocklength with superior performance in both waterfall and error floor regions. The design of such codes has proved difficult in the past because the constraints imposed by structured design (protographs), rate-compatibility, as well as small block length, are not easily satisfied together. For example, as the block length decreases, the predominance of decoding threshold as the main parameter in coding design is reduced, thus complicating the search for good codes. Our rate-compatible protograph codes have rates ranging from 1/3 to 4/5 and show no error floor down to $10^{-6}$ FER.

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arXiv2013

Bilayer Protograph Codes for Half-Duplex Relay Channels

Despite encouraging advances in the design of relay codes, several important challenges remain. Many of the existing LDPC relay codes are tightly optimized for fixed channel conditions and not easily adapted without extensive re-optimization of the code. Some have high encoding complexity and some need long block lengths to approach capacity. This paper presents a high-performance protograph-based LDPC coding scheme for the half-duplex relay channel that addresses simultaneously several important issues: structured coding that permits easy design, low encoding complexity, embedded structure for convenient adaptation to various channel conditions, and performance close to capacity with a reasonable block length. The application of the coding structure to multi-relay networks is demonstrated. Finally, a simple new methodology for evaluating the end-to-end error performance of relay coding systems is developed and used to highlight the performance of the proposed codes.

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arXiv2012

Adaptive Interference Alignment with CSI Uncertainty

Interference alignment (IA) is known to significantly increase sum-throughput at high SNR in the presence of multiple interfering nodes, however, the reliability of IA is little known, which is the subject of this paper. We study the error performance of IA and compare it with conventional orthogonal transmission schemes. Since most IA algorithms require extensive channel state information (CSI), we also investigate the impact of CSI imperfection (uncertainty) on the error performance. Our results show that under identical rates, IA attains a better error performance than the orthogonal scheme for practical signal to noise ratio (SNR) values but is more sensitive to CSI uncertainty. We design bit loading algorithms that significantly improve error performance of the existing IA schemes. Furthermore, we propose an adaptive transmission scheme that not only considerably reduces error probability, but also produces robustness to CSI uncertainty.

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arXiv2012

Diversity of MMSE MIMO Receivers

In most MIMO systems, the family of waterfall error curves, calculated at different spectral efficiencies, are asymptotically parallel at high SNR. In other words, most MIMO systems exhibit a single diversity value for all fixed rates. The MIMO MMSE receiver does not follow this pattern and exhibits a varying diversity in its family of error curves. This work analyzes this interesting behavior of the MMSE MIMO receiver and produces the MMSE MIMO diversity at all rates. The diversity of the quasi-static flat-fading MIMO channel consisting of any arbitrary number of transmit and receive antennas is fully characterized, showing that full spatial diversity is possible if and only if the rate is within a certain bound which is a function of the number of antennas. For other rates, the available diversity is fully characterized. At sufficiently low rates, the MMSE receiver has a diversity similar to the maximum likelihood receiver (maximal diversity), while at high rates it performs similarly to the zero-forcing receiver (minimal diversity). Linear receivers are also studied in the context of the MIMO multiple access channel (MAC). Then, the quasi-static frequency selective MIMO channel is analyzed under zero-padding (ZP) and cyclic-prefix (CP) block transmissions and MMSE reception, and lower and upper bounds on diversity are derived. For the special case of SIMO under CP, it is shown that the above-mentioned bounds are tight.

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arXiv2012

Spectrum Sharing with Distributed Relay Selection and Clustering

We consider a spectrum-sharing network where n secondary relays are used to increase secondary rate and also mitigate interference on the primary by reducing the required overall secondary emitted power. We propose a distributed relay selection and clustering framework, obtain closed-form expressions for the secondary rate, and show that secondary rate increases proportionally to log n. Remarkably, this is on the same order as the growth rate obtained in the absence of a primary system and its imposed constraints. Our results show that to maximize the rate, the secondary relays must transmit with power proportional to n^(-1) (thus the sum of relay powers is bounded) and also that the secondary source may not operate at its maximum allowable power. The tradeoff between the secondary rate and the interference on the primary is also characterized, showing that the primary interference can be reduced asymptotically to zero as n increases, while still maintaining a secondary rate that grows proportionally to log n. Finally, to address the rate loss due to half-duplex relaying in the secondary, we propose an alternating relay protocol and investigate its performance.

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arXiv2012

Product Superposition for MIMO Broadcast Channels

This paper considers the multiantenna broadcast channel without transmit-side channel state information (CSIT). For this channel, it has been known that when all receivers have channel state information (CSIR), the degrees of freedom (DoF) cannot be improved beyond what is available via TDMA. The same is true if none of the receivers possess CSIR. This paper shows that an entirely new scenario emerges when receivers have unequal CSIR. In particular, orthogonal transmission is no longer DoF-optimal when one receiver has CSIR and the other does not. A multiplicative superposition is proposed for this scenario and shown to attain the optimal degrees of freedom under a wide set of antenna configurations and coherence lengths. Two signaling schemes are constructed based on the multiplicative superposition. In the first method, the messages of the two receivers are carried in the row and column spaces of a matrix, respectively. This method works better than orthogonal transmission while reception at each receiver is still interference free. The second method uses coherent signaling for the receiver with CSIR, and Grassmannian signaling for the receiver without CSIR. This second method requires interference cancellation at the receiver with CSIR, but achieves higher DoF than the first method.

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arXiv2012

Diversity of MIMO Linear Precoding

Linear precoding is a relatively simple method of MIMO signaling that can also be optimal in certain special cases. This paper is dedicated to high-SNR analysis of MIMO linear precoding. The Diversity-Multiplexing Tradeoff (DMT) of a number of linear precoders is analyzed. Furthermore, since the diversity at finite rate (also known as the fixed-rate regime, corresponding to multiplexing gain of zero) does not always follow from the DMT, linear precoders are also analyzed for their diversity at fixed rates. In several cases, the diversity at multiplexing gain of zero is found not to be unique, but rather to depend on spectral efficiency. The analysis includes the zero-forcing (ZF), regularized ZF, matched filtering and Wiener filtering precoders. We calculate the DMT of ZF precoding under two common design approaches, namely maximizing the throughput and minimizing the transmit power. It is shown that regularized ZF (RZF) or Matched filter (MF) suffer from error floors for all positive multiplexing gains. However, in the fixed rate regime, RZF and MF precoding achieve full diversity up to a certain spectral efficiency and zero diversity at rates above it. When the regularization parameter in the RZF is optimized in the MMSE sense, the structure is known as the Wiener precoder which in the fixed-rate regime is shown to have diversity that depends not only on the number of antennas, but also on the spectral efficiency. The diversity in the presence of both precoding and equalization is also analyzed.

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arXiv2011

Relay-Assisted Interference Channel: Degrees of Freedom

This paper investigates the degrees of freedom of the interference channel in the presence of a dedicated MIMO relay. The relay is used to manage the interference at the receivers. It is assumed that all nodes including the relay have channel state information only for their own links and that the relay has M (greater than or equal to K) antennas in a K-user network. We pose the question: What is the benefit of exploiting the direct links from the source to destinations compared to a simpler two-hop strategy. To answer this question, we first establish the degrees of freedom of the interference channel with a MIMO relay, showing that a K-pair network with a MIMO relay has K/2 degrees of freedom. Thus, appropriate signaling in a two-hop scenario captures the degrees of freedom without the need for the direct links. We then consider more sophisticated encoding strategies in search of other ways to exploit the direct links. Using a number of hybrid encoding strategies, we obtain non-asymptotic achievable sum-rates. We investigate the case where the relay (unlike other nodes) has access to abundant power, showing that when sources have power P and the relay is allowed power proportional to O(P^2), the full degrees of freedom K are available to the network.

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arXiv2011

Opportunistic Wireless Relay Networks: Diversity-Multiplexing Tradeoff

Opportunistic analysis has traditionally relied on independence assumptions that break down in many interesting and useful network topologies. This paper develops techniques that expand opportunistic analysis to a broader class of networks, proposes new opportunistic methods for several network geometries, and analyzes them in the high-SNR regime. For each of the geometries studied in the paper, we analyze the opportunistic DMT of several relay protocols, including amplify-and-forward, decode-and-forward, compress-and-forward, non-orthogonal amplify-forward, and dynamic decode-forward. Among the highlights of the results: in a variety of multi-user single-relay networks, simple selection strategies are developed and shown to be DMT-optimal. It is shown that compress-forward relaying achieves the DMT upper bound in the opportunistic multiple-access relay channel as well as in the opportunistic nxn user network with relay. Other protocols, e.g. dynamic decode-forward, are shown to be near optimal in several cases. Finite-precision feedback is analyzed for the opportunistic multiple-access relay channel, the opportunistic broadcast relay channel, and the opportunistic gateway channel, and is shown to be almost as good as full channel state information.

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