Jaewon Kim

Interference coordination of heterogeneous LTE systems using remote radio heads

In this paper, we present an operational strategy to mitigate co-channel interference (CCI) by using geographically distributed remote radio heads (RRHs). The inter-node CCI becomes a dominant performance degradation factor for heterogeneous network (HetNet) systems. Recently, there are emerging attempts in Third Generation Partnership Project to adopt advanced techniques to Long Term Evolution Advanced systems to mitigate CCI problems for HetNet systems, namely, the coordinated multipoint transmission (CoMP). However, the CoMP scheme cannot control the CCI generated from outside coordination boundaries. To resolve this problem, we propose a partial activation strategy by using RRHs deployed near cell edge which results in moving coverage boundary effects. Based on Monte Carlo system level simulations, performance of the conventional strategies and the presented strategy is evaluated. Simulation results show that the proposed scheme outperforms the enhanced inter-cell interference coordination and CoMP schemes especially for users located near cell edge areas.

Performance of distributed MISO systems using cooperative transmission with antenna selection

Performance of downlink transmission strategies exploiting cooperative transmit diversity is investigated for distributed multiple-input single-output (MISO) systems, for which geographically distributed remote antennas (RA) in a cell can either communicate with distinct mobile stations (MS) or cooperate for a common MS. Statistical characteristics in terms of the signal-to-interference-plus-noise ratio (SINR) and the achievable capacity are analyzed for both cooperative and non-cooperative transmission schemes, and the preferred mode of operation for given channel conditions is presented using the analysis result. In particular, we determine an exact amount of the maximum achievable gain in capacity when RAs for signal transmission are selected based on the instantaneous channel condition, by deriving a general expression for the SINR of such antenna selection based transmission. For important special cases of selecting a single RA for non-cooperative transmission and selecting two RAs for cooperative transmission among three RAs surrounding the MS, closed-form formulas are presented for the SINR and capacity distributions.

Construction of phase tracking codebooks based on the Lloyd-Max vector quantization

In this paper, we propose a codebook construction method for multiple-input multiple-output (MIMO) beamforming systems operating under the per-antenna power constraint (PAPC). Under the PAPC which limits the individual power of each transmit antenna, it is known that only phase steering information is required to form the optimal beamforming vector at the transmitter. For systems with non-reciprocal uplink and downlink channels, the optimal phase steering information should be chosen at the receiver based on the estimated channel state information, and fed back to the transmitter through rate-limited feedback links. To efficiently encode the phase steering information in the feedback messages, phase steering codebooks have been previously proposed in the literature. In order to exploit the temporal correlation property of the wireless fading channels, we propose to use the phase tracking information between adjacent feedback slots as feedback messages, and provide the construction algorithm for the phase tracking codebook. The objective function for the phase steering codebook is reformulated as a new objective function for the phase tracking codebook to apply the generalized Lloyd-Max vector quantization (VQ) algorithm. The performance of the two codebooks is evaluated and the effectiveness of the proposed codebook is demonstrated by Monte-Carlo simulations.

Inter-cell interference mitigation using the moving coverage boundary of cellular systems employing remote radio heads

In this paper, we propose strategies to mitigate inter-cell interference by using remote radio heads (RRHs) installed at cell edge areas. For conventional systems, all user equipments (UEs) located in the coverage of a macro base station (BS) are assumed to be scheduling candidates for the corresponding cell. This makes UEs located near cell edge areas experience significant inter-cell interference. The proposed strategies solve the problem by virtually extending the coverage area to be larger than the area where the scheduling candidates are located. The advantage of the coverage extension is mainly obtained from a concept of moving cell boundaries, which is realized by partially activating the RRHs in a time-division fashion. Based on the Monte-Carlo simulations, performance benefits of moving boundary schemes over the static boundary schemes are presented.

Error probability expressions for frame synchronization using differential correlation

Probabilistic modeling and analysis of correlation metrics have been receiving considerable interest for a long period of time because they can be used to evaluate the performance of communication receivers, including satellite broadcasting receivers. Although differential correlators have a simple structure and practical importance over channels with severe frequency offsets, closed-form expressions for the output distribution of differential correlators do not exist. In this paper, we present detection error probability expressions for frame synchronization using differential correlation, and demonstrate their accuracy over channel parameters of practical interest. The derived formulas are presented in terms of the Marcum Q-function, and do not involve numerical integration, unlike the formulas derived in some previous studies. We first determine the distributions and error probabilities for single-span differential correlation metric, and then extend the result to multi-span differential correlation metric with certain approximations. The results can be used for the performance analysis of various detection strategies that utilize the differential correlation structure.

Capacity and outage performance of regional signal combining using distributed antennas

For cellular systems with base stations (BSs) equipped with distributed antennas, various cooperative signal transmission strategies can be employed. In this paper, we analyze the trade-off between the transmission capacity and the outage probability when downlink signals from two geographically separated antennas are combined. It is demonstrated a significant reduction in the outage probability can be achieved while simultaneously improving the capacity, when signal combining is selectively performed based on the carrier-to-interference ratio (CIR) measurement at a given mobile station (MS). Experimental results show the outage (CIR less than 0 dB) probability reduces by 54.1% with the capacity increase of 7.4% for the fading channel condition.

Weighted Sum Rate Maximization for the Two-User Vector Gaussian Broadcast Channel

In this letter, the problem of determining the maximum weighted sum rate (WSR) for given arbitrary weights is investigated for the vector Gaussian broadcast channel (GBC). Although the solution to the problem in general involves an iterative algorithm, we focus on the important special case of the two-user channel to provide a closed-form expression for the maximum WSR. The achievable rate region is known to be identical for both the vector GBC and its dual uplink Gaussian multiple access channel (GMAC) due to the uplink-downlink duality. We first derive the optimal power allocation in the dual uplink channel, which is then applied to the capacity formula to obtain the desired result. The presented result is a generalization of the (unweighted) sum rate maximization of the two-user GBC provided by Caire and Shamai .

Derivation of the Maximum Capacity Using the Jacobian Transformation Method for Systems Employing Adaptive Cooperation

The performance of cooperative transmission in wireless systems depends on the maximum signal quality from multiple transmission sources. The knowledge of the probabilistic distribution for the maximum statistics, therefore, plays an important role in determining the system performance. Obtaining the distribution in many cases relies on experimental results, especially when random variables of interest are mutually correlated. In this paper, we provide a derivation method for the maximum distribution using the Jacobian transformation rule, and investigate the performance of systems employing adaptive cooperation among remote antennas. A closed-form expression of the maximum achievable capacity is derived and verified by computer simulation.