Wenjie Yang

Analysis of the Frequency Offset Effect on Random Access Signals

Zadoff-Chu (ZC) sequences have been used as random access sequences in modern wireless communication systems, replacing the conventional pseudo-random-noise (PN) sequences due to their superior autocorrelation properties. An analytical framework quantifying the ZC sequences performance and its fundamental limitation as a random access sequencein the presence of frequency offset between the transmitter and the receiver is introduced. We show that a ZC sequences perfect autocorrelation properties can be severely impaired by the frequency offset thereby limiting the overall performance of the random access signals formed from these sequences. First, we derive the autocorrelation function of these random access sequences as a function of the frequency offset. Next, we introduce the concept of critical frequency offsets and the spectrum associated with a ZC sequence set to characterize the frequency offset properties of the random access signals. Finally, we demonstrate that the frequency offset immunity of a ZC sequence set can be controlled by shaping the spectrum of the ZC sequence set.

Narrowband Wireless Access for Low-Power Massive Internet of Things: A Bandwidth Perspective

LPWAN is a type of wireless telecommunication network designed to allow long range communications with relaxed requirements on data rate and latency between the core network and a high-volume of battery-operated devices. This article first reviews the leading LPWAN technologies on both unlicensed spectrum (SIGFOX, and LoRa) and licensed spectrum (LTE-M and NB-IoT). Although these technologies differ in many aspects, they do have one thing in common: they all utilize the narrow-band transmission mechanism as a leverage to achieve three fundamental goals, that is, high system capacity, long battery life, and wide coverage. This article introduces an effective bandwidth concept that ties these goals together with the transmission bandwidth, such that these contradicting goals are balanced for best overall system performance.

The Evolution of LTE Physical Layer Control Channels

Physical layer control signals in cellular communications serve the purpose of delivering physical layer control messages in a timely fashion to support cell radio resource management and data transmissions between the network and the mobile users. This article describes the evolution of 3GPP LTE in control channel design. The legacy LTE control channels (Release 8) have proven to be well-designed robust channels for signaling physical layer messages. However, emerging wireless communication transmission technologies continue to challenge the traditional design of LTE control channels. We have reached a point where a totally new design of the control channels has to be provided in order for the new transmission features to be introduced into LTE. In this article we first give an overview of the legacy LTE control channel, and present the new challenges. We then describe in depth the new solutions provided by LTE (Release 11). Finally, we address the limitations of the new design, which are recently identified in certain newly-emerging applications, in particular, the machine-type communications. A new round of evolution is hence imperative. New solutions provided by next release of LTE (Release 13) are presented.

Cellular communications on license-exempt spectrum

A traditional cellular system (e.g., LTE) operates only on licensed spectrum. This article describes the concept of cellular communications on both licensed and license-exempt/unlicensed spectrum under a unified architecture. The purpose of extending a cellular system into the bandwidth-rich license-exempt spectrum is to form a larger cellular network for both spectrum types. This would result in an ultimate mobile converged cellular network. This article examines the benefits of this concept and the technical challenges, and provides a conceptual LTE-based design example that demonstrates how a traditional cellular system like LTE can adapt itself to a different spectrum type, conform to the regulatory requirements, and harmoniously coexist with the incumbent systems such as WiFi. In order to cope with the interference and regulatory rules on license-exempt spectrum, a special medium access mechanism is introduced into the existing LTE transmission frame structure to exploit the full benefits of coordinated and managed cellular architecture.

Enhanced System Acquisition for NB-IoT

Machine-type communication (MTC) is the key technology to support data transfer among devices (sensors and actuators) in Internet of Things (IoT). Although cellular communication technologies are developed mainly for “human-type” communications, enabling MTC with cellular networks not only improves the connectivity, accessibility, and availability of an MTC network but also has the potential to further drive down the operation cost. However, cellular MTC, especially when applied to low-power massive IoT (mIoT), poses some unique challenges due to the low-cost and low-power nature of an mIoT device. One of the most challenging issues is providing a robust way for an mIoT device to acquire the network under a large frequency offset due to the use of low-cost crystal oscillators and under extended coverage. Although differentiation is a well-known technique for removing impairments caused by frequency offset, its “noise amplification” effect limits its applications in cellular communications due to the fact that cellular communication is typically interference limited. Matched-filter-based detection is, therefore, almost unexceptionally used. We show that the differential technique can actually benefit system acquisition in mIoT, where the use of low-cost crystals is a default. Although the existing system acquisition design in a cellular mIoT system, i.e., NB-IoT, facilitates both techniques, there still remain issues that need to be solved in order to take full advantage of the design. We provide a comprehensive analysis on the performance of two most common techniques when applied in a typical NB-IoT environment based on two factors, the geometry factor and the frequency offset factor. Finally, we derive the operating regions for matched-filter-based detection and differentiation using these two factors, in which the system acquisition performance of the two types of techniques is maximized for NB-IoT.

Robust Synchronization Waveform Design for Massive IoT

Machine-type communication (MTC) is the key technology to support data transfer among devices (sensors and actuators) in Internet of Things (IoT). However, MTC, especially when applied to massive low-power IoT (mIoT), poses some unique and serious challenges due to the low-cost and low-power nature of an mIoT device. One of the most challenging issues is providing a robust way for an mIoT device to acquire the network under a large frequency offset/error (due to the use of a low-cost crystal oscillator) and a low operating SNR (due to the extended coverage). We address the issues in the existing mIoT system acquisition, particularly the initial synchronization waveform detection, and derive a new synchronization waveform that is more robust in an mIoT environment. The mathematical approach provides a useful analytical insight into the design of the synchronization signal waveform for the 5G mIoT system.

On LTE coding scheme inefficiency & potential improvement

Coding and decoding schemes are critical to the performance in wireless communication system. In Current coding scheme of LTE system, each transport block (TB) is segmented into several code blocks (CBs) which can be coded and decoded individually. If one of the CBs in a TB fails in decoding, the whole TB is required to be retransmitted. Channel estimation is very crucial for the correctness of the decoding. In LTE, the demodulation reference signal (DM-RS) used for channel estimation, are unevenly arranged in resource block pair (RBP). We first show how the arrangement of DM-RS can impair channel estimation of some CBs, which ultimately results in the inefficiency of the coding scheme. To solve the problem, we propose a design by commandeering the decoded data with higher accuracy channel estimation as reference signals to help to estimate the channel of the part which lacks of DM-RS. The performance is evaluated via simulations.

Peer-to-peer communications in a cellular network

This paper describes a practical peer-to-peer (P2P) communication architecture applied to the conventional cellular network. It enables efficient coexistence of P2P connections with cellular connections while sharing the same wireless spectrum. The design achieves both proximity discovery and P2P communication under a cellular architecture with a particular emphasis on building an RF blueprint that reflects the RF relationship between devices to ensure power and spectral efficiency, as well as interference and quality of service control of P2P communications.

Optimal band allocation for cognitive cellular networks

The FCC new regulation for cognitive use of the TV white space spectrum provides a new means for improving traditional cellular network performance. But it also introduces a number of technical challenges. This paper studies one of the challenges: given the significant differences in the propagation property and the transmit power limitations between the cellular band and the TV white space, how both bands can be jointly utilized such that the benefit from the TV white space is maximized for overall cellular network performance improvement. Both analytical and simulation results are provided.