Abhijeet Bhorkar

Medium access design for LTE in unlicensed band

In this paper, a medium access protocol suitably adapted for the operation of Long term evolution (LTE) in unlicensed band (LTE-U) is proposed. The MAC design considers the impact of LTE-U on the devices operating in unlicensed band, such as WiFi and legacy LTE. The LTE-U design differences from the legacy LTE system, an inherently a synchronized, tightly controlled system are explored. The performance of LTE-U is studied via a detailed system level simulator implementation and 3GPP simulation test scenarios. We show that our proposed MAC design enables high throughput in unlicensed band and achieves co-existence with the incumbent systems. Moreover, the use of channel reservation mechanisms is shown to improve LTE-U cell edge performance.

Performance analysis of LTE and Wi-Fi in unlicensed band using stochastic geometry

In this paper, the multi-channel performance of large scale deployment of Long Term Evolution (LTE) in Unlicensed (LTE-U) band and WiFi using stochastic geometry is studied. LTE-U and WiFi cell placement is modeled as a Poisson Point Process (PPP), while transmission activity is modeled as hardcore simple sequential inhibition (SSI) point processes, which have been shown to accurately model Carrier sense multiple access (CSMA) access. This stochastic geometry based framework is used to analyze the co-existence of LTE-U and WiFi systems operating in the same set of available channels in unlicensed band. Several interesting results are derived. First, it is shown that fair coexistence can be achieved by LTE-U implementing a Listen-Before-Talk (LBT) protocol. Different variations of LBT are explored to improve the co-existence. Secondly, it is shown that the classical hidden node problem can severely affect performance, and that channel selection mechanisms can significantly alleviate the problem.

On the throughput analysis of LTE and WiFi in unlicensed band

In this paper, the co-channel performance of large scale deployment of LTE in Unlicensed (LTE-U) band and WiFi is studied using stochastic geometry. Analytical expressions of LTE-U throughput in presence of WiFi are presented and are partly validated by the simulation results. The LTE-U Low Power Nodes (LPNs) are deployed as Poisson Point Process (PPP), and, the WiFi transmissions are modeled as hardcore Matérn point process. Using this analytical approach the impact of various parameters such as sensing threshold and transmission power on the co-existence of LTE-U and WiFi is studied.

Licensed-Assisted Access to Unlicensed Spectrum in LTE Release 13

Exploiting the unlicensed spectrum is considered by 3GPP as one promising solution to meet ever-increasing traffic growth. As a result, one major enhancement for LTE in Release 13 has been to enable its operation in the unlicensed spectrum via licensed-assisted access (LAA). In this article, we provide an overview of the Release 13 LAA technology including motivation, use cases, LTE enhancements for enabling the unlicensed band operation, and the coexistence evaluation results contributed by 3GPP participants.

A Waveform for 5G: Guard Interval DFT-s-OFDM

Delay spread can be detrimental to communication systems. Orthogonal Frequency Division Multiplexing (OFDM)/Discrete Fourier Transform-spread-OFDM (DFT-s- OFDM) based systems (for e.g., Long Term Evolution (LTE), WiFi, etc.) tackle delay spread by adding a cyclic prefix (CP) before the start of the OFDM symbol. The duration of the CP is pre-determined and is set according to channel delay spread characteristics or to fit a certain frame duration. Moreover, the CP does not carry any useful data and represents an overhead that needs to be minimized. In this paper, we propose the use of DFT-s-OFDM with flexible, configurable guard interval (GI) of variable size. We call the resulting waveform GI-DFT-s-OFDM. The length of the GI can be adjusted to accommodate the maximum delay spread in a given scenario or for a particular user (UE). Additionally, the GI may carry a sequence that can be used for time/frequency synchronization. We also discuss system design based on GI-DFT-s-OFDM, giving possible options for the GI sequence. The proposed waveform has good Peak-to- Average Power Ratio (PAPR) with out-of-band emissions similar to OFDM/DFT-s-OFDM.

Channel Selection for Licensed Assisted Access in LTE Based on UE Measurements

Licensed Assisted Access (LAA) represents a new paradigm for LTE, where unlicensed bands are used in conjunction with licensed bands to increase network capacity. The main challenge of LAA is ensuring that unlicensed bands are shared with WiFi systems in an efficient manner. In this paper, we study a coexistence solution based on channel selection. A centralized channel selection mechanism which increases network capacity of LTE in unlicensed bands is proposed. We also describe the implementation of our scheme using existing LTE signaling mechanisms. Simulation results show that channel selection can improve the performance of both LTE and WiFi by over 100\%.

Algorithm design and analysis for network coding in D2D underlay communications

Device to Device (D2D) communications are an important part of next generation wireless systems, and a key technology to improve local services and overall system throughput. This paper proposes Optimized Network Coding (ONC) to boost the performance of D2D communications. ONC leverages on using network coding combined with improved scheduling to increase the throughput of D2D links. We describe the proposed ONC approach and verify its performance through extensive system level simulations. We show that ONC-based use of dynamic relay selection and a flow-based proportional fair scheduling significantly improves the system-wide D2D performance.

Achieving Congestion Diversity in Multi-Hop Wireless Mesh Networks

This paper reports on a comprehensive study comparing congestion-aware routing algorithms for wireless mesh networks with a state-of-the-art shortest-path routing protocol: Link-Quality Source Routing (LQSR). In particular, a set of congestion-aware protocols in the literature, Backpressure (BP), Enhanced-Backpressure (E-BP) and Congestion Diversity Protocol (CDP) are suitably adapted for implementation on 802.11-compatible radios. A testbed consisting of 802.11g nodes is deployed to empirically compare the performance of these congestion-aware routing protocols against LQSR. The results show that, under moderate to heavy UDP traffic, CDP delivers significant improvement compared to LQSR in 80-90 percent of the instances studied, while backpressure-based routing algorithms (BP and E-BP) frequently show significant degradation with respect to LQSR for both UDP and TCP traffic.