We propose a concept system termed distributed base station (DBS), which enables distributed transmit beamforming at large carrier wavelengths to achieve significant range extension and or increased downlink data rate, providing a low-cost infrastructure for applications such as rural broadband. We consider a frequency division duplexed (FDD) system, using feedback from the receiver to achieve the required phase coherence. At a given range, @math cooperating transmitters can achieve @math -fold increase in received power compared to that for a single transmitters, and feedback-based algorithms with near-ideal performance have been prototyped. In this paper, however, we identify and address key technical issues in translating such power gains into range extension via a DBS. First, to combat the drop in per-node SNR with extended range, we design a feedback-based adaptation strategy that is suitably robust to noise. Second, to utilize available system bandwidth, we extend narrowband adaptation algorithms to wideband channels through interpolation over OFDM subcarriers. Third, we observe that the feedback channel may become a bottleneck unless sophisticated distributed reception strategies are employed, but show that acceptable performance can still be obtained with standard uplink reception if channel time variations are slow enough. We quantify system performance compactly via outage capacity analyses.
The present paper builds on our prior conference paper @cite_5 , which introduced the DOST scheme, and also discussed extensions to wideband systems. However, it goes well beyond @cite_5 by presenting a DBS concept system built around DOST, including an OFDM system design roughly consistent with LTE, specific prescriptions for pilot design, and outage capacity analyses for downlink and uplink which compactly characterize system-level performance.
@cite_5
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Orthogonal Frequency Division Multiplexing (OFDM) uses pilot tones to estimate the channel's frequency response and perform equalization. It is commonly known that jamming pilot tones is more efficient than broadband attacks against an entire OFDM signal. This paper builds on this idea and introduces the pilot nulling attack, which is considerably more efficient than simple pilot jamming, by driving received pilot energy as close to zero as possible. This paper presents our channel and equalizer model, and then undertakes an analysis of OFDM under these attacks, verifying the assessment through simulation. For a target bit error rate of 0.4, QPSK underlying modulation, and pilot tone density of 1 8, we discover that pilot jamming is roughly 2 dB more efficient than barrage jamming, and pilot nulling is roughly 7.5 dB more efficient than barrage jamming. In all cases, pilot nulling is capable of fully denying the target signal at 4 dB of signal-to-jamming ratio by driving the QPSK bit error rate to 0.5.