Showing papers by "Martin Tomlinson published in 2010"

Magnitude modulation for VSAT's low back-off transmission

Abstract: This paper addresses the problem of controlling the envelope's power peak of single carrier modulated signals, band limited by root-raised cosine (RRC) pulse shaping filters, in order to reduce power amplifier back-off for very small aperture terminals ground stations. Magnitude modulation (MM) is presented as a very efficient solution to the peak-to-average power ratio problem. This paper gives a detailed description of the MM concept and its recent evolutions. It starts by extending the look-up-table (LUT) based approach of the MM concept to M-ary constellations with M ≤ 16. The constellation and RRC symmetries are explored, allowing considerable reduction on LUT computation complexity and storage requirements. An effective multistage polyphase (MPMM) approach for the MM concept is then proposed. As opposed to traditional LUT-MM solutions, MM coefficients are computed in real-time by a low complexity multirate filter system. The back-off from high-power amplifier saturation is almost eliminated (reduction is greater than 95%) with just a 2-stage MPMM system even for very demanding roll-off cases (e.g., α = 0.1). Also, the MPMM is independent of modulation in use, allowing its easy application to constellations with M > 16.

Analytical Analysis of Polyphase Magnitude Modulation Method's Performance

Abstract: An efficient multistage polyphase magnitude modulation (MPMM) scheme was recently proposed to solve the problem of controlling the envelope's power peak of single carrier modulated signals, band limited by root-raised cosine (RRC) pulse shaping filters, in order to maximize the efficiency of the transmitter's high power amplifier (HPA). This paper addresses the statistical analysis of MPMM coefficients and performs an analytical study of the average power and the peak-to-average power ratio (PAPR) of the magnitude modulated transmitted signals. It is shown that the average power losses due to MPMM are largely compensated by the outstanding back-off reduction gains, leading to an effective reduction on the transmitted signal's PAPR. The analytical study also shows that these losses are comparable to those observed with the clipping method (less than 1dB), without generating the undesirable spectral regrowth effect.

Decoding low-density parity-check codes with error-floor free over the AWGN channel

Abstract: We propose a new soft decision decoding arrangement for LDPC codes over the AWGN channel with error-floor free. The iterative belief propagation decoder is used as the initial decoder with the iterative output conditioned prior to OSD decoding. Improved results are obtained to break the corresponding error floors caused by the stopping sets. The basis of the conditioning of the iterative output is explained with supporting analysis. Some practical examples of performance are presented for some well known LDPC codes and it is shown that the proposed decoder with OSD-i does not only produce better results than a stand-alone OSD-(i + 1) decoder with considerable reduction in decoder complexity, but also guarantees the error-floor free.

Path loss measurements and comparisons of 433 MHz, 869 MHz and 1249 MHz within multi-floored buildings

Abstract: In this paper, an extensive set of propagation path loss measurements within multi-floored buildings at 433 MHz, 869 MHz and 1249 MHz are presented. Parameter for use in two indoor path loss prediction models, Distance-Dependant Model (DD) and Floor Attenuation Floor Model (FAF), are derived from measurement data of three multi-floored buildings. Buildings were chosen with typical features such as rectangle footprint, square footprint and existence of an atrium within the building, respectively. Comparison of model parameters has concluded that higher attenuation is experienced by the signal within a square footprint building than rectangle footprint. Building with an indoor atrium is found to have lower path losses than buildings without atrium, when considering multi-floor transmission. 869 MHz signal attenuated at slowest rate in most of the considered environments. 433 MHz signal is found to have better floor penetration compared to other frequencies. 1249 MHz is found to attenuate at slowest rate within a straight corridor with waveguiding and line-of-sight propagation path between the transmitter and the receiver. Path loss prediction within multi-floored buildings with indoor atrium is refined by considering type of propagation path between trnamsitter and receiver. It is found that path loss of areas with line-of-sight propagation path could be modelled using parameters of same floor environment. An attenuation factor is derived and added for areas with non line-of-sight propagation path. It is shown that using this refinement, better prediction accuracy is obtained. Standard deviations of path loss prediction error are reduced as a result.

Extended optimum decoding for LDPC codes based on exhaustive tree search algorithm

Abstract: A maximum-likelihood decoding algorithm inspired by information set decoding is realised for LDPC codes or linear block codes with sparse parity-check matrices of moderate codeword lengths over the AWGN channel. The extension involves an exhaustive branch and bound tree-search based algorithm for finding all small Euclidean distance error vectors. It provides an approach of searching on the bounded n-bit positions, which involves 2k combinatorial dual codes based on the corresponding parity-check matrix, to achieve the near-optimum decoding output with attainable computational complexity. Soft decision decoding results are presented for some well-known LDPC codes demonstrating near-optimum maximum-likelihood performance.

A variation of the McEliece public key cryptosystem

Abstract: This invention provides an improved security variation of the McEliece Public Key encryption system first published in 1978, which uses Goppa codes and contains features which make full use of random number generation for given message and cryptogram parameters. Unlike most other encryption methods, using this invention the encrypted message is a truly random function, not a pseudo-random function of the message so that even with the same message and the same public key, a different, unpredictable cryptogram is produced each time. It is shown that with the knowledge of the private key, the random function may be determined from the cryptogram and the message decrypted. Different embodiments of the invention are described which enable the level of security to be traded-off against cryptogram size and complexity. A number of different applications are given. In comparison to the original McEliece cryptosystem the arrangement described differs as the number of bit errors introduced into each (deliberately) corrupted codeword is not fixed, but is also randomly chosen. In addition a number of bits may be deleted from the codeword in random positions, adding to the security of the cryptogram and reducing the size of the cryptogram, without shortening the message.

Short range information transfer transmitter and receiver system

Abstract: Information transfer systems, with a transmitter and receiver, are disclosed comprising a range of features including: local storage of data at the transmitter, for remote control user selection, to be decoded into baseband components and filtered according to user preferences; transmitter encoding of the baseband components or a bit stream, using a (forward) error correcting code (FEC), into an information sample stream as well as a second stream of sequences of parity symbols or bits plus synchronisation sequences; frequency or time division multiplexing (FDM, TDM) of data streams; data scrambling at the transmitter to produce a signal power spectral density which has a low peak to average ratio (PAR, PAP, PAPR); and digital modulation of the scrambled signal onto a carrier for wireless transmission. In addition, there are broadly equivalent receiver-end descrambling, demultiplexing and user preference filtering steps, which may decode received codewords to generate information to substitute that received in error. The system may be applied to radio, music or other data streamed over the Internet, serving to extend the communication range of a wireless network.

Internet radio retransmission system and apparatus

Abstract: The invention is concerned with providing a wireless transmitting and receiving system such that a radio station streamed over the internet may be received wirelessly using a receiver which to the user may resemble a traditional domestic analogue radio set combining features of robustness and high audio quality. The unlicensed low power Instrument Scientific Medical (ISM) and Short Range Devices (SRD) frequency bands are well suited to be used by the invention. The invention matches the baseband bandwidth and number of baseband channels to the propagation conditions of the wireless link, provides low peak to average spectral power density and automatic error correction without synchronisation dependency. These features make the invention extremely tolerant to the high variability in propagation conditions which are commonly experienced in indoor wireless reception. A return control channel provides an automatic means of adapting to changing propagation conditions of the wireless link as well as providing the user with the ability to select different radio stations and to alter user preferences.

A tree-based ML decoding algorithm with adaptive thresholding

Abstract: In this work, we propose a maximum-likelihood (ML) soft-decision decoding algorithm of linear block codes based on an enhanced tree-based search algorithm. The algorithm considers each codeword as a tree branch and explores the subtree corresponding to the more reliable basis (MRB) of received vectors. By making use of the pre-defined ensemble branches and their costs, the branch evaluation process is effectively controlled by adaptive thresholding. This paper details the implementation of the proposed algorithm, compares the performance with the classic ordered statistic decoding (OSD) algorithm, and discusses the general decoding complexity and search effort in comparison to that of the OSD algorithm.

A variation of the McEliece public key cryptosystem using a reduced public key

Abstract: Improved security and improved throughput of the McEliece public key encryption system first published in 1978 using a relatively reduced public key size. Even though the public key is reduced, in some embodiments of the invention the ensemble of cryptograms produced is identical to the ensemble of cryptograms produced by the original system for a given Goppa code and the same private key. It is possible using this invention that the encrypted message, the cryptogram, is a truly random function not a pseudo-random function of the message so that even with the same message and the same public key a different, unpredictable cryptogram is produced each time. Other embodiments of the invention use a shortened error correcting code allowing the length of the generated cryptogram to match exactly the available transmission or storage media. One embodiment described relates to a method of encrypting data by constructing a digital cryptogram by means of a public key comprising:(a) Constructing a first generator matrix of a binary code with dimension k with a pre-selected Galois field whose base field is 2, and a Goppa polynornial whose degree is such that the corresponding binary code provides a t error correcting capability by utilising n-k parity bits;(b) Constructing a scrambled kxn generator matrix by matrix multiplication, said scrambled generator matrix being the product of a non-singular matrix, said first generator matrix and a first permutation matrix;(c) Constructing a reduced echelon kxn generator matrix by randomly selecting k independent columns of the scrambled kxn generator matrix according to a second permutation matrix;(d) Converting a message to be sent into binary form and formatting it by appending dummy bits as necessary into an integral number r of binary message vectors of length k bits each;(e) Scrambling each binary message vector using a k bit to k bit invertible scrambler;(f) Encoding each scrambled binary message vector by adding rows of said reduced echelon generator matrix according to the 1's in each scrambled message vector to form r codeword vectors of length n bits;(g) Adding to each codeword vector using modulo 2 arithmetic an error vector of length n bits, containing s bit errors where s≤t;(h) Forming a cryptogram from the r said corrupted codeword vectors.