Chun-Hao Liao

Revisiting the So-Called Constructive Interference in Concurrent Transmission

Many works attributed the successful reception of concurrent transmission (CT) to the constructive interference. However, due to the inevitable carrier frequency offset (CFO) and the resulted beating effect, the claim of constructive interference is actually not valid. To clarify the reason behind the successful receptions under CT, we conduct extensive evaluations and identify the following findings. 1) We show that the IEEE 802.15.4 receivers survive the beating effect mainly because of the direct sequence spread spectrum (DSSS), while systems without the protection of DSSS are not applicable to CT. 2) We identify a counterintuitive phenomenon that the IEEE 802.15.4 receiver could survive only the beating results from large CFO, while performing poorly when CFO is small. 3) We demonstrate that, even if the receivers survive, CT links lead to little performance improvements compared to conventional signal transmission links from the SNR point of view.

Latticing the interference: Non-linear pre-coding for non-orthogonal multiple access in downlink multi-user MIMO system

Spectrally efficient multiple access technologies, such as Multi-User Multiple-Input-Multiple-Output (MU-MIMO) system and Non-Orthogonal Multiple Access (NOMA), are the key enablers for the next generation communication systems to accommodate tremendous devices. Combining NOMA in MU-MIMO systems (NOMA-MIMO) is expected to boost the spectral efficiency even further. However, in order to solve the Inter-Antenna-Interference (IAI) and the Inter-Cluster-Interference (ICI) resulted from multiple-antenna and non-orthogonal transmissions, computation-intensive interference cancellation schemes are usually required in the receivers, which prohibit the employment of NOMA-MIMO system in systems with simple receivers, such as sensor networks. In this work, we propose a non-linear pre-coding method for two-cluster NOMA-MIMO systems that allows simple receiver implementation with only one antenna. Our main contribution is twofold: first, we adopt IAI-oriented pre-coding to spare the receiver from IAI cancellation; second, we propose to regulate the conventionally unsolvable IAI-corrupted ICI onto lattice points, so that the ICI can be removed by simple modulo operations instead of computational-intensive Successive Interference Cancellation (SIC) schemes. The ergodic sum capacity and whole system bit-error-rate (BER) performance are provided to show the validity of the proposed approach.

A high-performance RACH detection scheme for random access overload in LTE-Advanced

The overload of Random Access Channel (RACH) has been always considered as one of the largest obstacles to accommodate massive Machine-to-Machine (M2M) devices. While several congestion control mechanisms have been proposed to alleviate the RACH overload, increasing the number of Random Access (RA) slots and/or RACH preamble signatures is considered as an essential solution to enlarge the RACH capacity. However, the amount of RA slots directly affects the uplink data capacity and the orthogonality between preambles deteriorates along with the steep rise of the preamble usage number (i.e., simultaneous access from M2M devices after a wide area blackout), which results in heavy interference and performance degradation at the preamble detector. In order to cope with the issue, an inter-user interference cancellation (IUIC) scheme with a low latency Parallel Interference Cancellation (PIC) architecture is proposed to guarantee the preamble detection performance in large-preamble-number scenarios. The simulation results show that the proposed scheme significantly increases the supported preamble number over the conventional matched-filter based scheme while providing satisfactory preamble detection performance. Particularly, given a target miss detection rate (MDR) of 1%, the supported preamble number increases more than 5-fold by applying IUIC.

Speaker-less sound-based 3D localization with centimeter-level accuracy

Positions of sensor nodes are essential information for many UbiComp applications. In this demo, we present an accurate but low-cost 3D localization system based on audible sound. By utilizing the tiny-size MEMS microphone and the high accuracy, lightweight synchronization technology enabled by concurrent transmission flooding (CTF), we developed a tiny and low-cost localization system with centimeter-level accuracy. Our experiment show that this system achieves an average accuracy of 5cm in a 3m × 2m × 6m room.

Beating the Beat: RSSI-Based Packet Combining in Concurrent Transmission Sensor Networks: Poster Abstract

This paper presents an RSSI-based packet combining (PC) scheme to improve the packet reception reliability affected by a beat interference in wireless sensor networks (WSN) using concurrent transmission (CT). To overcome the beat problem, our PC scheme exploits multiple receptions to perform simple post-detection selection combining based on RSSI values. An implementation verifies its feasibility in resource-limited sensor nodes with time-sensitive CT. Evaluations using computer simulations and hardware experiments show significant reliability improvement.

Toward Robust Concurrent Transmission for sub-GHz Non-DSSS Communication: Poster Abstract

Many researches have shown that concurrent transmission (CT) helps to improve the energy efficiency and latency of 2.4GHz IEEE-802.15.4-based multi-hop networks. However, it has been shown that the success of CT in IEEE 802.15.4 is mainly due to the DSSS. In view of the great potential of long range sub-GHz technologies for future smart meter applications, we assess the feasibility of applying CT to non-DSSS sub-GHz receivers. In this work, we evaluate the FSK-based receiver performance in the critical case - the scenario that concurrent transmitted signals are received with the same power such so the mutual interference become worst. Our evaluations are based on the 2-FSK mode of TI CC1120 chips. The results show that the basic 2-FSK mode is vulnerable to CT. On the other hand, if the FEC and the interleaver are adopted, the receiver can survive in the heavy interfering region as long as the timing offset is smaller than 1 μs and the CFO is larger than 5 KHz.

Making preamble shorter in OFDM real-time wireless control systems

Real-time and robust communication is essential for wireless control systems (WCS). Preambles are considered as significant communication overheads in WCS where payloads are typically small. To shorten the overhead in OFDM systems, previous works have considered fixed subcarrier sampling to obtain subcarrier-fixed short preambles (SFSP) and adopted frequency-direction interpolation (FDI) for channel estimation. Despite of advantage in fast fading channel, it results in poor performance in frequency-selective fading channel. In this work, we propose a subcarrier-selectable short preamble (SSSP) which provides full sampling coverage of all subcarriers with several preamble transmissions. By exploiting the periodicity of transmissions in WCS, the SSSP scheme obtains the channel estimation of non-selected subcarriers by not only FDI but also the time-direction extrapolation (TDE), which is robust in frequency-selective fading. In addition, in order to make the channel estimation robust in both fast fading and frequency-selective fading we adopt an adaptive algorithm altering between FDI or TDE mode. The feasibility of the proposed method is validated by evaluating symbol error rate (SER) in fast frequency-selective fading. Simulation results verify that unlike the SFSP the adaptive SSSP scheme shows no error floor in SER curve when the channel is frequency-selective fading, and performs similar to the SFSP scheme when the channel is fast fading.

Inter-user interference cancellation improvements for LTE-Advanced random access detector using dynamic thresholding mechanism

In LTE-Advanced, a device which tries to connect to a basestation randomly selects one signature from predefined 64 signatures as a device ID, known as a preamble, and transmits it over Random Access Channel (RACH). Those preambles collide to each other when multiple devices transmit the same preamble over the same random access (RA) slot, which leads to a deterioration of the preamble detection rate at the basestation. This issue is often referred to as RACH overload. An essential solution can be derived by increasing the preamble number but it causes noticeable inter-user interference (IUI). This paper studies an improvement technique to the proposed IUI cancellation (IUIC) scheme by introducing a simple thresholding mechanism to ameliorate the robustness of channel estimation adopted in IUIC. The simulations indicate the interference power significantly decreases as the iteration number increases and the channel nulling threshold reduces dynamically according to the iteration number. Comparing to the fixed threshold, the proposed dynamic thresholding mechanism provides robust and satisfactory performance in both large preamble number and large Max-to-min Power Ratio (MPR) between devices.