Juyul Lee

High SNR Analysis for MIMO Broadcast Channels: Dirty Paper Coding Versus Linear Precoding

In this correspondence, we compare the achievable throughput for the optimal strategy of dirty paper coding (DPC) to that achieved with suboptimal and lower complexity linear precoding techniques (zero-forcing and block diagonalization). Both strategies utilize all available spatial dimensions and therefore have the same multiplexing gain, but an absolute difference in terms of throughput does exist. The sum rate difference between the two strategies is analytically computed at asymptotically high SNR. Furthermore, the difference is not affected by asymmetric channel behavior when each user has a different average SNR. Weighted sum rate maximization is also considered. In the process, it is shown that allocating user powers in direct proportion to user weights asymptotically maximizes weighted sum rate.

Energy-efficient scheduling of delay constrained traffic over fading channels

A delay-constrained scheduling problem for point-to-point communication is considered: a packet of B bits must be transmitted by a hard deadline of T slots over a time-varying channel. The transmitter/scheduler determines how many bits to transmit, or equivalently how much energy to transmit with, during each time slot based on the current channel quality and the number of unserved bits, with the objective of minimizing expected total energy. Assuming transmission at capacity of the underlying Gaussian noise channel, a closed-form expression for the optimal scheduling policy is obtained for the case T = 2 via dynamic programming; for T > 2, the optimal policy can only be numerically determined. Thus, the focus of the work is on derivation of simple, near-optimal policies. The proposed bit-allocation policies consist of a linear combination of a delay-associated term and an opportunistic (channel-aware) term. In addition, a variation of the problem in which the entire packet must be transmitted in a single slot is studied.

Symmetric Capacity of MIMO Downlink Channels

This paper studies the symmetric capacity of the MIMO downlink channel, which is defined to be the maximum rate that can be allocated to every receiver in the system. The symmetric capacity represents absolute fairness and is an important metric for slowly fading channels in which users have symmetric rate demands. An efficient and provably convergent algorithm for computing the symmetric capacity is proposed, and it is shown that a simple modification of the algorithm can be used to compute the minimum power required to meet given downlink rate demands. In addition, the difference between the symmetric and sum capacity, termed the fairness penalty, is studied. Exact analytical results for the fairness penalty at high SNR are provided for the 2 user downlink channel, and numerical results are given for channels with more users

Dirty Paper Coding vs. Linear Precoding for MIMO Broadcast Channels

We study the MIMO broadcast channel and compare the achievable throughput for the optimal strategy of dirty paper coding to that achieved with sub-optimal and lower complexity linear precoding (e.g., zero-forcing and block diagonalization) transmission. Both strategies utilize all available spatial dimensions and therefore have the same multiplexing gain, but an absolute difference in terms of throughput does exist. The sum rate difference between the two strategies is analytically computed at asymptotically high SNR, and it is seen that this asymptotic statistic provides an accurate characterization at even moderate SNR levels. Weighted sum rate maximization is also considered, and a similar quantification of the throughput difference between the two strategies is computed. In the process, it is shown that allocating user powers in direct proportion to user weights asymptotically maximizes weighted sum rate.

Asymptotically Optimal Policies for Hard-Deadline Scheduling Over Fading Channels

A hard-deadline, opportunistic scheduling problem in which B bits must be transmitted within T time-slots over a time-varying channel is studied: the transmitter must decide how many bits to serve in each slot based on knowledge of the current channel but without knowledge of the channel in future slots, with the objective of minimizing expected transmission energy. In order to focus on the effects of delay and fading, we assume that no other packets are scheduled simultaneously and no outage is considered. We also assume that the scheduler can transmit at capacity where the underlying noise channel is Gaussian such that the energy-bit relation is a Shannon-type exponential function. No closed form solution for the optimal policy is known for this problem, which is naturally formulated as a finite-horizon dynamic program, but three different policies are shown to be optimal in the limiting regimes where T is fixed and B is large, T is fixed and B is small, and where B and T are simultaneously taken to infinity. In addition, the advantage of optimal scheduling is quantified relative to a non-opportunistic (i.e., channel-blind) equal-bit policy.

Delay Constrained Scheduling over Fading Channels: Optimal Policies for Monomial Energy-Cost Functions

A point-to-point discrete-time scheduling problem of transmitting B information bits within T hard delay deadline slots is considered assuming that the underlying energy-bit cost function is a convex monomial. The scheduling objective is to minimize the expected energy expenditure while satisfying the deadline constraint based on information about the unserved bits, channel state/statistics, and the remaining time slots to the deadline. At each time slot, the scheduling decision is made without knowledge of future channel state, and thus there is a tension between serving many bits when the current channel is good versus leaving too many bits for the deadline. Under the assumption that no other packet is scheduled concurrently and no outage is allowed, we derive the optimal scheduling policy. Furthermore, we also investigate the dual problem of maximizing the number of transmitted bits over T time slots when subject to an energy constraint.

Millimeter-wave channel model parameters for urban microcellular environment based on 28 and 38 GHz measurements

To meet the increasing demand for 5G high datarate communications, millimeter-wave frequency bands are expected to be utilized. However, the existing channel models are not applicable to predict channel behaviors in these frequency bands. In this paper we investigate millimeter-wave channel model parameters including path loss exponents, delay and angular spread, and clustering parameters based on measurements collected in urban microcell environments at 28 and 38 GHz. Our measurement data were obtained with a wideband millimeter-wave channel sounder equipped not only an omnidirectional antenna and but also a steerable direction antenna. The measurement campaigns were held in Dunsan District area in Daejeon City, Korea, which is a typical downtown area surrounded with high-rise buildings. From our analysis, fitting results indicate that a path loss exponent of LoS is close to the free space path loss exponent of 2. In NLoS, the path loss exponent is about 3 at both frequencies. Furthermore, we observe that delay and angular spread, and clustering parameters at 28 GHz are similar to those at 38 GHz. We believe that these analysis results will facilitate developing millimeter-wave band 5G wireless systems.

Directional path loss characteristics of large indoor environments with 28 GHz measurements

For 5G communications, millimeter-wave frequency bands are considered strong candidates for new spectra. However, propagation studies in these bands are still in their initial stages. Using 28 GHz measurements collected in railway and airport passenger terminals, this paper investigates path loss characteristics of large indoor environments. In particular, we conducted measurement campaigns in passenger terminals at Seoul Railway Station and Incheon International Airport, which are representative of commercial areas with a dense population. By employing steerable directional antennas, we characterize path loss not only with respect to distance but also with respect to antenna orientation. Directional path loss exhibits near free space propagation characteristics for LOS conditions (path loss exponent is about 2.15-2.17) and a path loss exponent of about 2.68-3.03 for NLOS conditions. From the dependency on antenna orientation, we found the range of azimuthal orientations satisfying a certain power level from the peaks. Consequently, this information provides a tool for determination of the amount of power deviation due to beam mis-alignment.

Path loss measurement at indoor commercial areas using 28GHz channel sounding system

In this paper, we present path loss characteristics based on channel measurements in indoor commercial area at 28 GHz. The measurement campaign has been conducted in Seoul railway station and Incheon international airport terminal, which were selected to represent indoor hotspot regions in Korea. In order to compensate for the path loss increase due to higher frequencies, high-gain directional horn antennas are used to reliably establish channel links between a transmitter and a receiver. Based on the measurement results, we investigate directional path loss exponents and shadow fading factors using close-in free space path loss model.

NLOS Path Loss Model for Low-Height Antenna Links in High-Rise Urban Street Grid Environments

This paper presents a NLOS (non-line-of-sight) path loss model for low-height antenna links in rectangular street grids to account for typical D2D (device-to-device) communication link situations in high-rise urban outdoor environments. From wideband propagation channel measurements collected in Seoul City at 3.7 GHz, we observed distinctive power delay profile behaviors between 1-Turn and 2-Turn NLOS links: the 2-Turn NLOS has a wider delay spread. This can be explained by employing the idea that the 2-Turn NLOS has multiple propagation paths along the various street roads from TX to RX, whereas the 1-Turn NLOS has a single dominant propagation path from TX to RX. Considering this, we develop a path loss model encompassing 1-Turn and 2-Turn NLOS links with separate scattering and diffraction parameters for the first and the second corners, based on the Uniform Geometrical Theory of Diffraction. In addition, we consider the effect of building heights on path loss by incorporating an adjustable “waveguide effect” parameter; that is, higher building alleys provide better propagation environments. When compared with field measurements, the predictions are in agreement.