Havish Koorapaty

A Lean Carrier for LTE

The next major step in the evolution of LTE targets the rapidly increasing demand for mobile broadband services and traffic volumes. One of the key technologies is a new carrier type, referred to in this article as a Lean Carrier, an LTE carrier with minimized control channel overhead and cell-specific reference signals. The Lean Carrier can enhance spectral efficiency, increase spectrum flexibility, and reduce energy consumption. This article provides an overview of the motivations and main use cases of the Lean Carrier. Technical challenges are highlighted, and design options are discussed; finally, a performance evaluation quantifies the benefits of the Lean Carrier.

License-exempt LTE systems for secondary spectrum usage: Scenarios and first assessment

This paper investigates the feasibility and broad implications of the use of 3GPP LTE systems in license-exempt situations. License-exempt use can occur in traditional unlicensed bands or in secondary usage of white space spectrum. Various scenarios for license-exempt use are listed with a particular focus on spectrum access as a secondary user. Regulatory requirements devised by Federal Communications Commission (FCC) for TV white space usage are described and tabulated. General principles and recommendations for license exempt spectrum usage are proposed with some important scenarios being identified. Additional requirements arise out of the need to share the spectrum with other potential users in a fair manner. System simulation results in an indoor setting are used to estimate spectral efficiencies under such conditions. We conclude that LTE can be used in license-exempt secondary usage scenario with only minor modifications, such as fair sharing procedures, and could benefit from support for higher order modulations.

Licensed-Assisted Access LTE: coexistence with IEEE 802.11 and the evolution toward 5G

LAA is a new operation mode of LTE in the unlicensed spectrum, which will be featured in LTE Release 13. Under LAA, licensed carriers will be aggregated with unlicensed carriers in order to opportunistically enhance downlink user throughput while still offering seamless mobility support. In order to coexist with WiFi, some of the new functionalities required of LAA LTE include a mechanism for channel sensing based on listen-before-talk, discontinuous transmission on a carrier with limited maximum transmission duration, and multicarrier transmission across multiple unlicensed channels. This article presents a detailed overview of the design agreements for LAA, the impact of unlicensed spectrum operation on the LTE physical layer architecture, and the scope of additional enhancements beyond LTE Release 13. A range of simulations for indoor and multicarrier scenarios show that fair coexistence between LAA and WiFi can be achieved, and that deployment of LAA can provide a boost in WiFi performance.

System architecture and coexistence evaluation of licensed-assisted access LTE with IEEE 802.11

Licensed-assisted access (LAA) is a new operation mode of Long-Term Evolution (LTE) in the unlicensed spectrum currently under study in the 3GPP standardization forum. In order to coexist with Wi-Fi, some of the new functionalities required of LAA LTE include a mechanism for clear channel assessment based on listen-before-talk (LBT), discontinuous transmission on a carrier with limited maximum transmission duration, and dynamic frequency selection (DFS) for radar avoidance in certain frequency bands. This paper presents a detailed overview of the impact of unlicensed spectrum operation on the LTE physical layer architecture, such as downlink physical channel design, scheduling, and radio resource management. System-level simulation results are then presented for indoor and outdoor scenarios, and show that fair coexistence between LAA and Wi-Fi can be achieved and that deployment of LAA can provide a boost in Wi-Fi performance.

Small-Cell Wireless Backhauling: A Non-Line-of-Sight Approach for Point-to-Point Microwave Links

In this paper we investigate the feasibility of using microwave frequencies for fixed non-line-of-sight wireless backhauling connecting small-cell radio base stations with an aggregation node in an outdoor urban environment, i.e. a typical heterogeneous network scenario. We study system level simulations for a point-to-point system where the wave propagation is based on diffraction over rooftops. We further investigate the effects of carrier frequency, interference, antenna height, rain, and tolerance to antenna alignment errors. It is found that the higher frequencies offer not only larger bandwidths but also higher antenna gains which would ideally work to their advantage. However, these advantages may be lost when taking antenna alignment errors and rain into account. Different frequencies simply have their different trade-offs.

Low Complexity Channel Estimation for Minimizing Edge Effects in OFDM Systems

Channel estimation is critical to receiver performance in an OFDM system. Pilot symbols are typically interspersed with data symbols among the sub-carriers to aid channel estimation. Standard algorithms to estimate the channel based on these pilot symbols include minimum mean-square error (MMSE) and maximum-likelihood (ML) estimation. With a uniform density of pilots in frequency among the sub-carriers, the ML estimate exhibits a boundary effect whereby the quality of the estimates on sub-carriers towards the edges of the signal band degrades. While the use of MMSE mitigates this effect significantly it requires precise knowledge of channel statistics which are difficult to obtain. Also, it is often too complex to implement for common numbers of sub-carriers. This paper proposes mitigation of the boundary effect described above by transmitting extra pilot symbols in the sub-carriers towards the edges of the band in addition to the regularly spaced pilots, thereby increasing pilot density at the edges. Channel estimation is performed using a two-step algorithm. ML estimates are first formed using the uniform pilot set. The output of the ML estimator is then used to bias an MMSE estimator which is applied only to the edge pilot set. The two-step approach enhances performance as compared to ML estimation while keeping complexity much lower than that incurred when MMSE is used across the entire band.

Design and Evaluation of Licensed Assisted Access LTE in Unlicensed Spectrum

Licensed assisted access (LAA) is a new feature for 3GPP LTE systems to operate in the unlicensed spectrum. Under LAA, licensed carriers will be aggregated with unlicensed carriers in order to opportunistically enhance user throughput, while still offering seamless mobility and outdoor coverage. In order to coexist with other technologies in the unlicensed bands, several new functionalities for LAA LTE have been introduced, including longterm channel selection, short-term channel sensing based on listen-before-talk (LBT), and discontinuous transmission on a carrier with limited maximum transmission duration. In this article, we present research findings behind the designs for LAA systems. We discuss the impact of several parameters of the LAA LBT framework on the channel access opportunities of LAA, and its coexistence performance toward co-channel networks based on extensive system-level simulation results. The investigation covers both single-channel as well as multi-channel operation and coexistence scenarios. In addition to the finalized designs for downlink LAA operations in Release 13, our findings in this article further shed light on the uplink LAA operations to be introduced in Release 14.

Evaluation of Control Channel Performance with Adaptive Radio Unit Activation in LTE

Link level performance results for physical antenna port muting in LTE are presented. In this paper we examine the block error rate performance of the LTE downlink control channels (PBCH, PCFICH, PDCCH, and the PHICH) for different antenna muting configurations. Also the throughput performance of the LTE downlink data channel (PDSCH) is studied for 2Tx and 4Tx transmit diversity formats. The results show that the combination of logical antenna port merging with physical antenna port muting provides a very minor performance loss. Since antenna muting should only be used when the traffic in a cell is low, this performance loss is not significant in practice. With antenna muting we can save energy by activating radio units only when motivated by the traffic load in the cell.

Performance of ML estimators for time of arrival estimation

The performance of a set of high-resolution algorithms for estimating the time of arrival of a band-limited signal received on a multipath channel is compared. The algorithms investigated use an increasing degree of known information about the channel and estimate the unknown elements using the maximum-likelihood criterion. In the first algorithm, no information on the amplitudes and phases of the multipath components is assumed. In the second algorithm, the statistics of the complex path gains are known to be jointly Gaussian and in the third case, the path gains are jointly Gaussian with a known correlation matrix. The three algorithms are compared using a simple two path channel model where the second path position is assumed known and the statistics of the error in measuring the delay of the first path are compared.

Reference Signals for Improved Energy Efficiency in LTE

Cellular wireless systems have focused, to date, on system capacity, user data rates, latency and coverage as primary design criteria. Energy efficiency has recently received increased attention as another important criterion to be considered. The potential for energy savings in 3GPP LTE systems has been investigated in [1]-[3]. An important design element that determines the potential for energy efficient transmission is the structure of reference signals that need to be transmitted independent of the load in the network, such as the cell-specific reference signals (CRS) in 3GPP LTE systems. This paper discusses modifications to the reference signal structure in LTE that can enable greater energy efficiency. The time and frequency estimation performance implications of such modifications are evaluated and discussed.