arXiv subjects

Sang-Woon Jeon

Publications and source records attributed to Sang-Woon Jeon.

32 records · Page 2Linked to original sources

arXiv2015

Opportunistic Interference Alignment for Random Access Networks

An interference management problem among multiple overlapped random access networks (RANs) is investigated, each of which operates with slotted ALOHA protocol. Assuming that access points and users have multiple antennas, a novel opportunistic interference alignment~(OIA) is proposed to mitigate interference among overlapped RANs. The proposed technique intelligently combines the transmit beamforming technique at the physical layer and the opportunistic packet transmission at the medium access control layer. The transmit beamforming is based on interference alignment and the opportunistic packet transmission is based on the generating interference of users to other RANs, which can be regarded as a joint optimization of the physical layer and the medium access control layer. It is shown that the proposed OIA protocol significantly outperforms the conventional schemes such as multi-packet reception and interference nulling.

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arXiv2014

Degrees of Freedom of Uplink-Downlink Multiantenna Cellular Networks

An uplink-downlink two-cell cellular network is studied in which the first base station (BS) with $M_1$ antennas receives independent messages from its $N_1$ serving users, while the second BS with $M_2$ antennas transmits independent messages to its $N_2$ serving users. That is, the first and second cells operate as uplink and downlink, respectively. Each user is assumed to have a single antenna. Under this uplink-downlink setting, the sum degrees of freedom (DoF) is completely characterized as the minimum of $(N_1N_2+\min(M_1,N_1)(N_1-N_2)^++\min(M_2,N_2)(N_2-N_1)^+)/\max(N_1,N_2)$, $M_1+N_2,M_2+N_1$, $\max(M_1,M_2)$, and $\max(N_1,N_2)$, where $a^+$ denotes $\max(0,a)$. The result demonstrates that, for a broad class of network configurations, operating one of the two cells as uplink and the other cell as downlink can strictly improve the sum DoF compared to the conventional uplink or downlink operation, in which both cells operate as either uplink or downlink. The DoF gain from such uplink-downlink operation is further shown to be achievable for heterogeneous cellular networks having hotspots and with delayed channel state information.

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arXiv2014

Linear Degrees of Freedom of MIMO Broadcast Channels with Reconfigurable Antennas in the Absence of CSIT

The K-user multiple-input and multiple-output (MIMO) broadcast channel (BC) with no channel state information at the transmitter (CSIT) is considered, where each receiver is assumed to be equipped with reconfigurable antennas capable of choosing a subset of receiving modes from several preset modes. Under general antenna configurations, the sum linear degrees of freedom (LDoF) of the K-user MIMO BC with reconfigurable antennas is completely characterized, which corresponds to the maximum sum DoF achievable by linear coding strategies. The LDoF region is further characterized for a class of antenna configurations. Similar analysis is extended to the K-user MIMO interference channels with reconfigurable antennas and the sum LDoF is characterized for a class of antenna configurations.

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arXiv2013

Computation Over Gaussian Networks With Orthogonal Components

Function computation of arbitrarily correlated discrete sources over Gaussian networks with orthogonal components is studied. Two classes of functions are considered: the arithmetic sum function and the type function. The arithmetic sum function in this paper is defined as a set of multiple weighted arithmetic sums, which includes averaging of the sources and estimating each of the sources as special cases. The type or frequency histogram function counts the number of occurrences of each argument, which yields many important statistics such as mean, variance, maximum, minimum, median, and so on. The proposed computation coding first abstracts Gaussian networks into the corresponding modulo sum multiple-access channels via nested lattice codes and linear network coding and then computes the desired function by using linear Slepian-Wolf source coding. For orthogonal Gaussian networks (with no broadcast and multiple-access components), the computation capacity is characterized for a class of networks. For Gaussian networks with multiple-access components (but no broadcast), an approximate computation capacity is characterized for a class of networks.

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arXiv2013

Interactive Computation of Type-Threshold Functions in Collocated Broadcast-Superposition Networks

In wireless sensor networks, various applications involve learning one or multiple functions of the measurements observed by sensors, rather than the measurements themselves. This paper focuses on type-threshold functions, e.g., the maximum and indicator functions. Previous work studied this problem under the collocated collision network model and showed that under many probabilistic models for the measurements, the achievable computation rates converge to zero as the number of sensors increases. This paper considers two network models reflecting both the broadcast and superposition properties of wireless channels: the collocated linear finite field network and the collocated Gaussian network. A general multi-round coding scheme exploiting not only the broadcast property but particularly also the superposition property of the networks is developed. Through careful scheduling of concurrent transmissions to reduce redundancy, it is shown that given any independent measurement distribution, all type-threshold functions can be computed reliably with a non-vanishing rate in the collocated Gaussian network, even if the number of sensors tends to infinity.

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arXiv2012

Approximate Ergodic Capacity of a Class of Fading 2-user 2-hop Networks

We consider a fading AWGN 2-user 2-hop network where the channel coefficients are independent and identically distributed (i.i.d.) drawn from a continuous distribution and vary over time. For a broad class of channel distributions, we characterize the ergodic sum capacity to within a constant number of bits/sec/Hz, independent of signal-to-noise ratio. The achievability follows from the analysis of an interference neutralization scheme where the relays are partitioned into $M$ pairs, and interference is neutralized separately by each pair of relays. When $M=1$, the proposed ergodic interference neutralization characterizes the ergodic sum capacity to within $4$ bits/sec/Hz for i.i.d. uniform phase fading and approximately $4.7$ bits/sec/Hz for i.i.d. Rayleigh fading. We further show that this gap can be tightened to $4\log π-4$ bits/sec/Hz (approximately $2.6$) for i.i.d. uniform phase fading and $4-4\log( \frac{3π}{8})$ bits/sec/Hz (approximately $3.1$) for i.i.d. Rayleigh fading in the limit of large $M$.

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arXiv2012

Degrees of Freedom Region of a Class of Multi-source Gaussian Relay Networks

We study a layered $K$-user $M$-hop Gaussian relay network consisting of $K_m$ nodes in the $m^{\operatorname{th}}$ layer, where $M\geq2$ and $K=K_1=K_{M+1}$. We observe that the time-varying nature of wireless channels or fading can be exploited to mitigate the inter-user interference. The proposed amplify-and-forward relaying scheme exploits such channel variations and works for a wide class of channel distributions including Rayleigh fading. We show a general achievable degrees of freedom (DoF) region for this class of Gaussian relay networks. Specifically, the set of all $(d_1,..., d_K)$ such that $d_i\leq 1$ for all $i$ and $\sum_{i=1}^K d_i\leq K_Σ$ is achievable, where $d_i$ is the DoF of the $i^{\operatorname{th}}$ source--destination pair and $K_Σ$ is the maximum integer such that $K_Σ\leq \min_m\{K_m\}$ and $M/K_Σ$ is an integer. We show that surprisingly the achievable DoF region coincides with the cut-set outer bound if $M/\min_m\{K_m\}$ is an integer, thus interference-free communication is possible in terms of DoF. We further characterize an achievable DoF region assuming multi-antenna nodes and general message set, which again coincides with the cut-set outer bound for a certain class of networks.

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arXiv2011

Capacity of a Class of Linear Binary Field Multi-source Relay Networks

Characterizing the capacity region of multi-source wireless relay networks is one of the fundamental issues in network information theory. The problem is, however, quite challenging due to inter-user interference when there exist multiple source--destination (S--D) pairs in the network. By focusing on a special class of networks, we show that the capacity can be found. Namely, we study a layered linear binary field network with time-varying channels, which is a simplified model reflecting broadcast, interference, and fading natures of wireless communications. We observe that fading can play an important role in mitigating inter-user interference effectively for both single-hop and multi-hop networks. We propose new encoding and relaying schemes with randomized channel pairing, which exploit such channel variations, and derive their achievable rates. By comparing them with the cut-set upper bound, the capacity region of single-hop networks and the sum capacity of multi-hop networks can be characterized for some classes of channel distributions and network topologies. For these classes, we show that the capacity region or sum capacity can be interpreted as the max-flow min-cut theorem.

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arXiv2010

Aligned Interference Neutralization and the Degrees of Freedom of the 2x2x2 Interference Channel

We show that the 2x2x2 interference channel, i.e., the multihop interference channel formed by concatenation of two 2-user interference channels achieves the min-cut outer bound value of 2 DoF, for almost all values of channel coefficients, for both time-varying or fixed channel coefficients. The key to this result is a new idea, called aligned interference neutralization, that provides a way to align interference terms over each hop in a manner that allows them to be cancelled over the air at the last hop.

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arXiv2010

Improved Capacity Scaling in Wireless Networks With Infrastructure

This paper analyzes the impact and benefits of infrastructure support in improving the throughput scaling in networks of $n$ randomly located wireless nodes. The infrastructure uses multi-antenna base stations (BSs), in which the number of BSs and the number of antennas at each BS can scale at arbitrary rates relative to $n$. Under the model, capacity scaling laws are analyzed for both dense and extended networks. Two BS-based routing schemes are first introduced in this study: an infrastructure-supported single-hop (ISH) routing protocol with multiple-access uplink and broadcast downlink and an infrastructure-supported multi-hop (IMH) routing protocol. Then, their achievable throughput scalings are analyzed. These schemes are compared against two conventional schemes without BSs: the multi-hop (MH) transmission and hierarchical cooperation (HC) schemes. It is shown that a linear throughput scaling is achieved in dense networks, as in the case without help of BSs. In contrast, the proposed BS-based routing schemes can, under realistic network conditions, improve the throughput scaling significantly in extended networks. The gain comes from the following advantages of these BS-based protocols. First, more nodes can transmit simultaneously in the proposed scheme than in the MH scheme if the number of BSs and the number of antennas are large enough. Second, by improving the long-distance signal-to-noise ratio (SNR), the received signal power can be larger than that of the HC, enabling a better throughput scaling under extended networks. Furthermore, by deriving the corresponding information-theoretic cut-set upper bounds, it is shown under extended networks that a combination of four schemes IMH, ISH, MH, and HC is order-optimal in all operating regimes.

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arXiv2009

Degrees of Freedom of Multi-Source Relay Networks

We study a multi-source Gaussian relay network consisting of $K$ source--destination pairs having $K$ unicast sessions. We assume $M$ layers of relays between the sources and the destinations. We find achievable degrees of freedom of the network. Our schemes are based on interference alignment at the transmitters and symbol extension and opportunistic interference cancellation at the relays. For $K$-$L$-$K$ networks, i.e., 2-hop network with $L$ relays, we show $\min\{K,K/2+L/(2(K-1))\}$ degrees of freedom are achievable. For $K$-hop networks with $K$ relays in each layer, we show the full $K$ degrees of freedom are achievable provided that $K$ is even and the channel distribution satisfies a certain symmetry.

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arXiv2009

Cognitive Networks Achieve Throughput Scaling of a Homogeneous Network

We study two distinct, but overlapping, networks that operate at the same time, space, and frequency. The first network consists of $n$ randomly distributed \emph{primary users}, which form either an ad hoc network, or an infrastructure-supported ad hoc network with $l$ additional base stations. The second network consists of $m$ randomly distributed, ad hoc secondary users or cognitive users. The primary users have priority access to the spectrum and do not need to change their communication protocol in the presence of secondary users. The secondary users, however, need to adjust their protocol based on knowledge about the locations of the primary nodes to bring little loss to the primary network's throughput. By introducing preservation regions around primary receivers and avoidance regions around primary base stations, we propose two modified multihop routing protocols for the cognitive users. Base on percolation theory, we show that when the secondary network is denser than the primary network, both networks can simultaneously achieve the same throughput scaling law as a stand-alone network. Furthermore, the primary network throughput is subject to only a vanishingly fractional loss. Specifically, for the ad hoc and the infrastructure-supported primary models, the primary network achieves sum throughputs of order $n^{1/2}$ and $\max\{n^{1/2},l\}$, respectively. For both primary network models, for any $δ>0$, the secondary network can achieve sum throughput of order $m^{1/2-δ}$ with an arbitrarily small fraction of outage. Thus, almost all secondary source-destination pairs can communicate at a rate of order $m^{-1/2-δ}$.

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arXiv2009

Sum capacity of multi-source linear finite-field relay networks with fading

We study a fading linear finite-field relay network having multiple source-destination pairs. Because of the interference created by different unicast sessions, the problem of finding its capacity region is in general difficult. We observe that, since channels are time-varying, relays can deliver their received signals by waiting for appropriate channel realizations such that the destinations can decode their messages without interference. We propose a block Markov encoding and relaying scheme that exploits such channel variations. By deriving a general cut-set upper bound and an achievable rate region, we characterize the sum capacity for some classes of channel distributions and network topologies. For example, when the channels are uniformly distributed, the sum capacity is given by the minimum average rank of the channel matrices constructed by all cuts that separate the entire sources and destinations. We also describe other cases where the capacity is characterized.

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arXiv2009

Capacity Scaling of Single-source Wireless Networks: Effect of Multiple Antennas

We consider a wireless network in which a single source node located at the center of a unit area having $m$ antennas transmits messages to $n$ randomly located destination nodes in the same area having a single antenna each. To achieve the sum-rate proportional to $m$ by transmit beamforming, channel state information (CSI) is essentially required at the transmitter (CSIT), which is hard to obtain in practice because of the time-varying nature of the channels and feedback overhead. We show that, even without CSIT, the achievable sum-rate scales as $Θ(m\log m)$ if a cooperation between receivers is allowed. By deriving the cut-set upper bound, we also show that $Θ(m\log m)$ scaling is optimal. Specifically, for $n=ω(m^2)$, the simple TDMA-based quantize-and-forward is enough to achieve the capacity scaling. For $n=ω(m)$ and $n=\operatorname{O}(m^2)$, on the other hand, we apply the hierarchical cooperation to achieve the capacity scaling.

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