Wireless communication networks have evolved continuously over the last four decades in order to meet the traffic and security requirements due to the ever-increasing amount of traffic. However this increase is projected to be massive for the fifth generation of wireless networks (5G), with a targeted capacity enhancement of 1000× w.r.t. 4G networks. This enhanced capacity is possible by a combination of major approaches (a) overhaul of some parts and (b) elimination of overhead and redundancies of the current 4G. In this work we focus on OFDM reference signal or pilot tones, which are used for channel estimation, link adaptation and other crucial functions in Long-Term Evolution (LTE). We investigate two aspects of pilot signals pertaining to its evolution - (a) impact of targeted interference on pilots and its mitigation and (b) adaptation of pilot patterns to match the channel conditions of the user.
We develop theoretical models that accurately quantify the performance degradation at the user’s receiver in the presence of a multi-tone pilot jammer. We develop and evaluate mitigation algorithms to mitigate power constrained multi-tone pilot jammers in SISO- and full rank spatial multiplexing MIMO-OFDM systems. Our results show that the channel estimation performance can be restored even in the presence of a strong pilot jammer. We also show that full rank spatial multiplexing in the presence of a synchronized pilot jammer (transmitting on pilot locations only) is possible when the channel is flat between two pilot locations in either time or frequency.
We also present experimental results of multi-tone broadcast pilot jamming (Jamming of Cell Specific Reference Signal) in the LTE downlink. Our results show that full-band jamming of pilots needs 5 dB less power than jamming the entire downlink signal, in order to cause Denial of Service (DoS) to the users. In addition to this, we have identified and demonstrated a previously unreported issue with LTE termed ‘Channel Quality Indicator (CQI) Spoofing’. In this scenario, the attacker tricks the user terminal into thinking that the channel quality is good, by transmitting interference transmission only on the data locations, while deliberately avoiding the pilots. This jamming strategy leverages the dependence of the adaptive modulation and coding (AMC) schemes on the CQI estimate in LTE.
Lastly, we investigate the idea of pilot pattern adaptation for SISO- and spatial multiplexing MIMO-OFDM systems. We present a generic heuristic algorithm to predict the optimal pilot spacing and power in a nonstationary doubly selective channel (channel fading in both time and frequency). The algorithm fits estimated channel statistics to stored codebook channel profiles and uses it to maximize the upper bound on the constrained capacity. We demonstrate up to a 30% improvement in ergodic capacity using our algorithm and describe ways to minimize feedback requirements while adapting pilot patterns in multi-band carrier aggregation systems. We conclude this work by identifying scenarios where pilot adaptation can be implemented in current wireless networks and provide some guidelines to adapt pilots for 5G. / Master of Science / Wireless communications have evolved continuously over the last four decades in order to meet the ever-increasing number of users. The next generation of wireless networks, named 5G, is expected to interconnect a massive number of devices called the Internet of Things (IoT). Compared to the current generation of wireless networks (termed 4G), 5G is expected to provide a thousandfold increase in data rates. In addition to this, the security of these connected devices is also a challenging issue that needs to be addressed. Hence in the event of an attack, even if a tiny fraction of the total number of users are affected, this will still result in a large number of users who are impacted.
The central theme of this thesis is the evolution of <i>Orthogonal Frequency Division Multiplexing (OFDM) pilot signals</i> on the road from 4G to 5G wireless networks. In OFDM, pilot signals are sent in parallel to data in order to aid the receiver in mitigating the impairments of the wireless channel. In this thesis, we look at two perspectives of the evolution of pilots: a) targeted interference on pilot signals, termed as ‘Multi-tone pilot jamming’ and b) adapting pilot patterns to optimize throughput.
In the first part of the thesis, we investigate the (a) impact of multi-tone pilot jamming and (b) propose and evaluate strategies to counter multi-tone pilot jamming. In particular, we propose methods that (a) have the potential to be implemented in the Third Generation Partnership Project Long-Term Evolution (3GPP LTE) standard, and (b) have the ability to maintain high data rates with a multi-antenna receiver, in the presence of a multi-tone pilot jammer. We also experiment and analyze the behavior of LTE in the presence of such targeted interference.
In the second half of the thesis, we explore the idea of adapting the density of pilots to optimize throughput. Increasing the pilot density improves the signal reception capabilities, but reduces the resources available for data and hence, data rate. Hence we propose and evaluate strategies to balance between these two conflicting requirements in a wireless communication system.
In summary, this thesis provides and evaluates ideas to mitigate interference on pilot signals, and design data rate-maximizing pilot patterns for future OFDM-based wireless networks.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/77485 |
| Date | 28 September 2016 |
| Creators | Rao, Raghunandan M. |
| Contributors | Electrical and Computer Engineering, Reed, Jeffrey H., Marojevic, Vuk, Buehrer, R. Michael |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | en_US |
| Detected Language | English |
| Type | Thesis, Text |
| Format | application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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