Youjian Eugene Liu
University of Colorado Boulder
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Featured researches published by Youjian Eugene Liu.
IEEE Transactions on Wireless Communications | 2014
Prasanna Madhusudhanan; Juan G. Restrepo; Youjian Eugene Liu; Timothy X. Brown; Kenneth R. Baker
In this paper, we analyze the signal-to-interference-plus-noise ratio (SINR) performance at a mobile station (MS) in a random cellular network. The cellular network is formed by base-stations (BSs) placed in a one, two or three dimensional space according to a possibly non-homogeneous Poisson point process, which is a generalization of the so-called shotgun cellular system. We develop a sequence of equivalence relations for the SCSs and use them to derive semi-analytical expressions for the coverage probability at the MS when the transmissions from each BS may be affected by random fading with arbitrary distributions as well as attenuation following arbitrary path-loss models. For homogeneous Poisson point processes in the interference-limited case with power-law path-loss model, we show that the SINR distribution is the same for all fading distributions and is not a function of the base station density. In addition, the influence of random transmission powers, power control, multiple channel reuse groups on the downlink performance are also discussed. The techniques developed for the analysis of SINR have applications beyond cellular networks and can be used in similar studies for cognitive radio networks, femtocell networks and other heterogeneous and multi-tier networks.
global communications conference | 2012
Prasanna Madhusudhanan; Juan G. Restrepo; Youjian Eugene Liu; Timothy X. Brown
In this paper, we consider the downlink signal-to-interference-plus-noise ratio (SINR) analysis in a heterogeneous cellular network with K tiers. Each tier is characterized by a base-station (BS) arrangement according to a homogeneous Poisson point process with certain BS density, transmission power, random shadow fading factors with arbitrary distribution, arbitrary path-loss exponent and a certain bias towards admitting the mobile-station (MS). The MS associates with the BS that has the maximum instantaneous biased received power under the open access cell association scheme. For such a general setting, we provide an analytical characterization of the coverage probability at the MS.
global communications conference | 2009
Prasanna Madhusudhanan; Juan G. Restrepo; Youjian Eugene Liu; Timothy X. Brown
This paper analyzes the carrier-to-interference ratio of the so-called shotgun cellular system (SCS). In the SCS, base-stations are placed randomly according to a two-dimensional Poisson point process. Such a system can model a dense cellular or wireless data network deployment, where the base station locations end up being close to random due to constraints other than optimal coverage. The SCS is a simple cellular system where we can introduce several variations and design scenarios such as shadow fading, power control features, and multiple channel reuse groups, and assess their impact on the performance. We first derive an analytical expression for the characteristic function of the inverse of the carrier-to-interference ratio. Using this result, we show that the carrier-to-interference ratio is independent of the base station density and further, we derive a semi-analytical expression for the tail-probability. These results enable a complete characterization of the cellular performance of the SCS. Next, we incorporate shadow fading into the SCS and demonstrate that it merely scales the base station density by a constant. Hence, the cellular performance of the SCS is independent of shadow fading. These results are further used to analyze dense cellular scenarios.
global communications conference | 2011
Prasanna Madhusudhanan; Juan G. Restrepo; Youjian Eugene Liu; Timothy X. Brown; Kenneth R. Baker
Abstract--- This paper studies the carrier-to-interference ratio (CIR) and carrier-to-interference-plus-noise ratio (CINR) performance at the mobile station (MS) within a multi-tier network composed of M tiers of wireless networks, with each tier modeled as the homogeneous n-dimensional (n-D, n=1,2, and 3) shotgun cellular system, where the base station (BS) distribution is given by the homogeneous Poisson point process in n-D. The CIR and CINR at the MS in a single tier network are thoroughly analyzed to simplify the analysis of the multi-tier network. For the multi-tier network with given system parameters, the following are the main results of this paper: (1) semi-analytical expressions for the tail probabilities of CIR and CINR; (2) a closed form expression for the tail probability of CIR in the range [1,infinity); (3) a closed form expression for the tail probability of an approximation to CINR in the entire range [0,infinity); (4) a lookup table based approach for obtaining the tail probability of CINR, and (5) the study of the effect of shadow fading and BSs with ideal sectorized antennas on the CIR and CINR. Based on these results, it is shown that, in a practical cellular system, the installation of additional wireless networks (microcells, picocells and femtocells) with low power BSs over the already existing macrocell network will always improve the CINR performance at the MS.
IEEE Wireless Communications Letters | 2012
Prasanna Madhusudhanan; Juan G. Restrepo; Youjian Eugene Liu; Timothy X. Brown; Kenneth R. Baker
An analytical tool based on usual stochastic ordering is developed to compare the distributions of carrier-to-interference ratio at the mobile station of cellular systems where the base stations are distributed randomly according to certain non-homogeneous Poisson point processes. The comparison is done by studying the base station densities without having to solve for the distributions of the carrier-to-interference ratio, which are often hard to obtain.
global communications conference | 2010
Prasanna Madhusudhanan; Juan G. Restrepo; Youjian Eugene Liu; Timothy X. Brown; Kenneth R. Baker
Cognitive radios (CR) must ensure that their transmissions do not cause harmful interference to the primary users that operate in the same band. Primary users with short communication range transmitters, such as wireless microphone systems, are considered. The CR network is distributed according to a Poisson process. The CRs detect the primary user either through primary transmitter signals or through beacons collocated with the primary receiver. Three levels of cooperative sensing by CRs are analyzed, namely, no cooperation, full cooperation and partial cooperation. The characteristic function of the interference at the primary receiver for all combinations of primary user detection and cooperative sensing schemes are derived for the system modeled along a line (1-D) and about a plane (2-D). The analysis in 1-D corresponds to a highway scenario and 2-D corresponds to a planar system. The carrier-to-interference ratio (C/I) at the primary receiver is evaluated using the characteristic function. The analysis clearly shows the need for cooperative spectrum sensing when the density of the CRs is high. It is shown that the effect of shadowing, small scale fading, and power control at the CRs can be captured into the density of the CR network. The results hold for the CRs that are wide-band devices as well as narrow-band devices.
IEEE Transactions on Wireless Communications | 2016
Prasanna Madhusudhanan; Juan G. Restrepo; Youjian Eugene Liu; Timothy X. Brown
In this paper, a comprehensive study of the downlink performance in a heterogeneous cellular network (or HetNet) is conducted via stochastic geometry. A general HetNet model is considered consisting of an arbitrary number of open-access and closed-access tiers of base stations (BSs) arranged according to independent homogeneous Poisson point processes. The BSs within each tier have a constant transmission power, random fading factors with an arbitrary distribution and arbitrary path-loss exponent of the power-law path-loss model. For such a system, analytical characterizations for the coverage probability are derived for the max-SINR connectivity and nearest-BS connectivity models. Using stochastic ordering, interesting properties and simplifications for the HetNet downlink performance are derived by relating these two connectivity models to the maximum instantaneous received power (MIRP) connectivity model and the maximum biased received power (MBRP) connectivity models, providing good insights about HetNets and their downlink performance in these complex networks. Furthermore, the results also demonstrate the effectiveness and analytical tractability of the stochastic geometric approach to study the HetNet performance.
global communications conference | 2014
Seungil You; Lijun Chen; Youjian Eugene Liu
The weighted sum-rate maximization in a general multiple-input multiple-output (MIMO) interference network has known to be a challenging non-convex problem, mainly due to the interference between different links. In this paper, by exploring the special structure of the sum-rate function being a difference of concave functions, we apply the convex-concave procedure to the weighted sum-rate maximization to handle non-convexity. With the introduction of a certain damping term, we establish the monotonie convergence of the proposed algorithm. Numerical examples show that the introduced damping term slows down the convergence of our algorithm but helps with finding a better solution in the network with high interference. Even though our algorithm has a slower convergence than some existing ones, it has the guaranteed convergence and can handle more general constraints and thus provides a general solver that can find broader applications.
global communications conference | 2012
Prasanna Madhusudhanan; Juan G. Restrepo; Youjian Eugene Liu; Timothy X. Brown
We study the downlink signal-to-interference ratio (SIR) in a multi-tier heterogeneous cellular network under the open and closed access schemes. Each tier is characterized by a base-station (BS) arrangement according to a homogeneous Poisson point process with certain density, constant transmission power and random shadow fading factors with arbitrary distribution. The mobile-station (MS) communicates with the BS that has the maximum instantaneous received power among those to which it is allowed access. The chosen BS provides coverage to the MS if the SIR is above a certain threshold determined by the BSs tier. The coverage probability and average rate expressions are derived for the open access scheme for any arbitrary SIR threshold assigned for each tier, and for the closed access scheme when the SIR thresholds are greater than one.
conference on information sciences and systems | 2007
Wei Dai; Youjian Eugene Liu; Brian Rider
This paper considers broadcast channels with L antennas at the base station and m single-antenna users, where each user has perfect channel knowledge and the base station obtains channel information through a finite rate feedback. The key observation of this paper is that the optimal number of on-users (users turned on), say s, is a function of signal-to-noise ratio (SNR) and other system parameters. Towards this observation, we use asymptotic analysis to guide the design of feedback and transmission strategies. As L, m and the feedback rates approach infinity linearly, we derive the asymptotic optimal feedback strategy and a realistic criterion to decide which users should be turned on. Define the corresponding asymptotic throughput per antenna as the spatial efficiency. It is a function of the number of on-users s, and therefore, s should be appropriately chosen. Based on the above asymptotic results, we also develop a scheme for a system with finite many antennas and users. Compared with other works where s is presumed constant, our scheme achieves a significant gain by choosing the appropriate s. Furthermore, our analysis and scheme is valid for heterogeneous systems where different users may have different path loss coefficients and feedback rates.