Lingjia Liu

Downlink MIMO in LTE-advanced: SU-MIMO vs. MU-MIMO

Single-user multi-antenna technologies are well upported in current standard specifications like LTE Release 8/9. Further development of the specification (LTE-Advanced) is expected to conform to the requirements for IMT-Advanced systems. One of the key enabling features of LTE-Advanced to meet IMT-Advanced downlink performance requirements is multi-user MIMO, where a transmitter serves multiple users simultaneously on the same frequency resource, primarily relying on spatial separation. In general, multi-user MIMO is beneficial for improving average user spectral efficiency. However, cell edge user spectral efficiency may be reduced if multi-user MIMO is used exclusively, due to residual inter-user interference arising from practical multi-user beamforming and reduced transmit power allocated to each user. Therefore, it should be possible to configure the UE-specific transmission mode to support dynamic switching between single-user MIMO and multi-user MIMO to balance the cell edge user spectral efficiency as well as the average cell user spectral efficiency. In this article, we study various aspects of multi-user MIMO including design philosophy, multi-user precoding, and control signaling. The associated feedback schemes, including those that facilitate dynamic switching, are discussed. Performance evaluation is conducted to demonstrate the gain of dynamically switched single-user and multiuser MIMO as opposed to traditional single-user MIMO.

Resource Allocation and Quality of Service Evaluation for Wireless Communication Systems Using Fluid Models

Wireless systems offer a unique mixture of connectivity, flexibility, and freedom. It is therefore not surprising that wireless technology is being embraced with increasing vigor. For real-time applications, user satisfaction is closely linked to quantities such as queue length, packet loss probability, and delay. System performance is therefore related to, not only Shannon capacity, but also quality of service (QoS) requirements. This work studies the problem of resource allocation in the context of stringent QoS constraints. The joint impact of spectral bandwidth, power, and code rate is considered. Analytical expressions for the probability of buffer overflow, its associated exponential decay rate, and the effective capacity are obtained. Fundamental performance limits for Markov wireless channel models are identified. It is found that, even with an unlimited power and spectral bandwidth budget, only a finite arrival rate can be supported for a QoS constraint defined in terms of exponential decay rate

Automated Residential Demand Response: Algorithmic Implications of Pricing Models

Smart energy management is an important problem in Smart Grid network, and demand response (DR) is one of the key enabling technologies. If each home uses automated demand response which would opportunistically schedule devices that are flexible to run at any time in a large time window, towards the slots with lower electricity prices, peaks at these slots may happen. We denote such peaks as rebound peaks. We address the potential rebound peak problems of automated DR algorithms, and provide possible solutions. We illustrate why a rebound peak is possible via the insights we obtain from the optimal automated DR algorithm. We show that if the utility electricity supply cost is assumed to be a homogeneous function in the energy consumption over a certain time span, a system of multiple homes and utility company has the lowest total electricity supply cost if the electricity consumption from all the homes is flat over the time span. We study multiple approaches to reduce the rebound peak, and accordingly propose algorithms for DR at each home. Effectiveness of the approaches is verified by numerical results.

On the effective capacities of multiple-antenna Gaussian channels

The concept of effective capacity offers a novel methodology to investigate the impact that design decisions at the physical layer may have on system performance at the link layer. Assuming a constant flow of incoming data, the effective capacity characterizes the maximum arrival rate that a wireless system can support as a function of its service requirements. Service requirements in this framework are defined in terms of the asymptotic decay-rate of buffer occupancy. This article studies the effective capacity of a class of multiple-antenna wireless systems subject to Rayleigh flat fading. The effective capacity of the multi-antenna Gaussian channel is characterized, and system performance is evaluated in the low signal-to-noise ratio regime. Additional to the power gain of the multiple receive antenna system, we show that there is a statistical gain associated with a multiple transmit antenna system. When the number of transmit and/or receive antennas becomes large, the effective capacity of the system is bounded away from zero, even under very stringent service constraints. This phenomena, which results from channel-hardening, suggests that a multiple-antenna configuration is especially beneficial to delay-sensitive traffic.

Quality of Service Analysis for Wireless User-Cooperation Networks

A wireless communication system in which multiple users cooperate to transmit information to a common destination is considered. The traffic generated by the users is subject to a stringent quality of service requirement, which is defined in terms of the asymptotic decay-rate of buffer occupancy. The performance of this communication system is analyzed, and the corresponding achievable rate-region for the two-user scenario is identified. A simple user-cooperation scheme that improves performance is proposed. This cooperative scheme is shown to significantly enlarge the achievable rate-region of the service constrained communication system, provided that the quality of the wireless link between cooperating users is better than the individual connections from the users to the intended destination. Numerical results further indicate that the gains of cooperative strategies can be substantial. This suggests that cooperation allows for a fair distribution of the wireless resources among active users.

Cooperative communication technologies for LTE-advanced

The LTE-Advanced (LTE-A) system is currently under development to allow for significantly higher spectral efficiency and data throughput than LTE systems. In a wireless system based on orthogonal frequency division multiplexing (OFDM) with frequency reuse factor one such as LTE, the achievable cell spectral efficiency is often limited by the inter-cell interference or coverage shortage of base stations. Hence in LTE-A, coordinated multi-point (CoMP) transmission/reception (a.k.a. multi-cell MIMO or base station cooperation) and relaying technologies are being introduced to clear these major performance hurdles. In this paper, overall picture of cooperative communication technologies being discussed in LTE-A systems including CoMP and relaying is presented, together with considerations on system design.

Proportional fair scheduling for multi-cell multi-user MIMO systems

In this paper, a multi-cell multi-user MIMO scheme with advanced user pairing and scheduling algorithm is proposed to effectively explore multi-user diversity and to combat both intra-cell and inter-cell interference in the downlink of wireless communication systems. The optimality of the proposed user paring and scheduling algorithm is shown under generalized proportional fair metrics. System-level simulations are conducted to verify the performance of the proposed scheme with the agreed LTE-Advanced (LTE-A) system parameters. The results suggest the proposed scheme is a promising candidate for improving both the cell-edge user throughput and average cell throughput, and furthermore for fulfilling the IMT-Advanced (IMT-A) requirements.

Intercell Interference Coordination through Limited Feedback

We consider the applications of multicell transmission schemes to the downlink of future wireless communication networks. A multicell multiple-input multiple output-(MIMOs) based scheme with limited coordination among neighboring base stations (BSs) is proposed to effectively combat the intercell interference by taking advantage of the degreesoffreedom in the spatial domain. In this scheme, mobile users are required to feedback channel-related information to both serving base station and interfering base station. Furthermore, a chordal distance-based compression scheme is introduced to reduce the feedback overhead. The performance of the proposed scheme is investigated through theoretical analysis as well as system level simulations. Both results suggest that the so-called “intercell interference coordination through limited feedback” scheme is a very good candidate for improving the cell-edge user throughput as well as the average cell throughput of the future wireless communication networks.

Performance analysis of wireless hybrid-ARQ systems with delay-sensitive traffic

The design of wireless communication schemes tailored to real-time traffic requires an analysis framework that goes beyond the traditional criterion of data throughput. This work considers an approach that relates physical system parameters to the queueing performance of wireless links. The potential benefits of multi-rate techniques such as hybrid-ARQ are assessed in the context of delay-sensitive traffic using large deviations. A continuous-time Markov channel model is employed to partition the instantaneous data-rate received at the destination into a finite number of states, each representing a mode of operation of the hybrid-ARQ scheme. The proposed methodology accounts for the correlation of the wireless channel across time, which is computed in terms of level-crossing rates. The tail asymptote governing buffer overflow probabilities at the transmitter is then used to provide a measure of overall performance. This approach leads to a characterization of the effective capacity of the system which, in turn, is applied to quantify the performance advantages of hybrid-ARQ over traditional schemes.

Cooperative communications for LTE-advanced—relay and CoMP†

SUMMARY The Long Term Evolution-Advanced (LTE-A) system is currently under development to allow for significantly higher spectral efficiency and data throughput than the LTE systems. In a wireless system based on orthogonal frequency division multiplexing with frequency reuse factor one, the achievable cell spectral efficiency is often limited by the inter-cell interference or coverage shortage of base stations. In LTE-A, coordinated multi-point transmission/reception (a.k.a. multi-cell MIMO or base station cooperation) and relaying technologies are being introduced to clear these major performance hurdles. In this paper, cooperative communication technologies being discussed in LTE-A systems are presented, together with considerations on system design. Copyright