G.J.R. Povey

Using a CMOS camera sensor for visible light communication

In this paper, a novel scheme for data reception in a mobile phone using visible light communications (VLC) is proposed. The camera of the smartphone is used as a receiver in order to capture the continuous changes in state (on-off) of the light, which are invisible to the human eye. The information is captured in the camera in the form of light and dark bands which are then decoded by the smartphone and the received message is displayed. By exploiting the rolling shutter effect of CMOS sensors, a data rate much higher than the camera frame rate is achieved.

Polynomial cancellation coding of OFDM to reduce intercarrier interference due to Doppler spread

Polynomial cancellation coding (PCC) is a coding method for orthogonal frequency division multiplexing (OFDM) in which the information to be transmitted is modulated onto weighted groups of subcarriers rather than onto individual subcarriers. It has previously been shown that PCC substantially reduces the sensitivity of OFDM to carrier frequency offset. In this paper, it is shown that PCC also reduces the intercarrier interference (ICI) due to Doppler spread. Results are presented for a frequency non-selective i.e. flat fading channel, subject to classical Doppler spread. By using weighted pairs of subcarriers the ICI due to Doppler spread can be reduced by approximately 15 dB. By using weighted groups of three subcarriers a further 15 dB reduction in ICI can be achieved.

An FFT-Based Approach for Fast Acquisition in Spread Spectrum Communication Systems

This paper describes a non-coherent technique forfast acquisition of direct sequence spread spectrum(DS/SS) signals in low earth orbit (LEO) satellitecommunication scenarios. Large Doppler offsets areinherent to such environments and are likely to causemajor problems during the code acquisition phase dueto the introduced frequency ambiguity. In the presentpaper we discuss the use of a set of partialcorrelators for code phase acquisition, combined witha fast Fourier transform (FFT) for the purpose of simultaneous Doppler estimation. We will show that theuse of this architecture largely accelerates thesynchronisation process compared to conventionaltechniques over a wide range of Doppler offsets.Furthermore we discuss the implementation of azero-padded FFT that increases the frequencyresolution to such an extent, that acquisition timesidentical to scenarios without existing Doppler offsetcan be achieved.

A review of time division duplex-CDMA techniques

The basic concepts of a time division duplex system are introduced. The advantages of using TDD-CDMA for mobile radio communications is shown to be related to its ability to handle asymmetric data and benefits resulting from the reciprocal nature of the channel. Synchronisation difficulties and the associated interference problems are seen as the primary limiting factors. The use of TDD-CDMA in UMTS is highlighted and the proposal for sharing FDD bandwidth allocations with the TDD system is explained and outage problems are identified.

A fast acquisition technique for a direct sequence spread spectrum signal in the presence of a large Doppler shift

A non-coherent fast acquisition system is described for a DS-SS signal in the presence of a large Doppler shift. This technique utilises N complex correlators of length x such that Nx=m where m is the length of the spreading sequence. These complex correlators produce partial correlations of the DS-SS signal. The outputs of these correlators are fed to an N point FFT. The maximum output of the FFT is then selected and compared against a threshold. In situations with high Doppler shifts the FFT acquisition system described can test all frequency offsets for a given code phase simultaneously. The theoretical probability of detection and false alarm are derived in a Gaussian channel. We determine the mean acquisition time for an 8-point FFT using a single dwell system in the presence of 32 kHz Doppler shift. It is shown by choosing a suitable threshold level that the mean acquisition time is over 16 times lower than a standard single dwell non-coherent decorrelator.

Doppler estimation for fast acquisition in spread spectrum communication systems

This paper describes a non-coherent technique which employs a fast Fourier transform (FFT) for the fast acquisition of direct sequence spread spectrum signals in the presence of large Doppler shifts. The application of an FFT to code acquisition results in decreased acquisition time and can improve the probability of detection. The technique discussed in this paper uses a set of partial correlators and a zero-padded FFT to reduce the search region for code acquisition whilst maintaining good frequency resolution for Doppler offset. This approach will prove most pertinent in future re-configurable terminals.

A serial-parallel FFT correlator for PN code acquisition from LEO satellites

A new receiver architecture for PN code acquisition of signals with high Doppler shifts is proposed, in which the partially correlated outputs of the parallel section of a serial-parallel correlator (SPC) are subject to an FFT process before being summed by the serial section. In this way a wider range of frequencies can be searched in a single dwell time, which is particularly useful in a LEOS channel due to the high Doppler shifts involved. The performance in terms of mean acquisition time is shown to be better over a wider bandwidth than either serial or parallel correlators.

The effects of interference between the TDD and FDD mode in UMTS at the boundary of 1920 MHz

The Universal Mobile Telecommunications System (UMTS) is composed of an FDD and TDD mode. The spectrum allocation is in such a way that both modes have an adjacent carrier at 1920 MHz. The implications thereof with respect to system capacity are investigated. In this context the separation distance of the TDD and FDD base station (BS) and frame synchronisation are varied. It is shown that the most detrimental effects for the FDD interface are for small BS separations. In contrast, the optimum with respect to TDD capacity is found for the co-location of both base stations. This yields a trade-off for the optimal BS locations. Moreover, an adjacent channel protection factor of 30 dB in an interference limited system such as CDMA is shown to be too low unless a significant capacity loss is acceptable.

Spread Spectrum PN Code Acquisition Using Hybrid Correlator Architectures

A hybrid correlator architecture is described which combines the serial structure of an active correlator with the parallel structure of a matched filter correlator. The mean PN code acquisition time performance of this hybrid serial-parallel correlator structure is analysed. Results are shown which compare the acquisition performance of the serial, parallel, and serial-parallel structures. The results are for a PN code length of 64 code chips and assumes a Gaussian channel with the receiver detection threshold set to obtain a constant false alarm rate. An enhancement to the serial-parallel acquisition algorithm is also described which can increase the acquisition time performance by about 15% for typical operating conditions. Overall the results demonstrate that the hybrid correlator can provide rapid code acquisition with a limited receiver complexity.

The effect of adjacent channel interference on capacity in a hybrid TDMA/CDMA-TDD system using UTRA-TDD parameters

The subject of this analysis is the effects of adjacent channel interference (ACI) on capacity in a hybrid TDMA/CDMA system using time division duplex (TDD). The parameters of the UTRA (UMTS terrestrial radio access) air interface were used for this investigation. A statistical approach assuming a spatially uniform user distribution was taken to investigate the capacity reduction due to ACI. The probability density function (PDF) of ACI at the base station (BS) of interest was calculated and by defining a certain outage threshold (P/sub out/), the interference used to calculate the capacity loss was determined. A path loss model which accounts for the correlation of log normal shadowing between the useful link and interference link was adopted. The impact of the following parameters were studied: (a) BS separation, (b) adjacent channel protection (ACP), (c) load in the interfering cell and (d) frame synchronisation. It was found that there are sub-optimal BS separations due to a tradeoff between MS/spl rarr/BS and BS/spl rarr/BS interference. Moreover, it became apparent that an intelligent dynamic channel assignment (DCA) algorithm is required.