This invention improves the frequency acquisition range and the resistance to multipath interference. In a digital signal transmission system using OFDM, on the transmission side, some or all of a plurality of equidistant carrier positions are treated as reference carrier positions. The actual transmitted carriers are arranged in a predetermined pattern non-equidistant to the frequency carrier positions to form an OFDM symbol. This OFDM symbol is periodically transmitted as frequency reference symbols. On the reception side, the carrier arrangement pattern of the frequency reference symbols is detected, a carrier frequency offset is detected from the detected pattern offset, and the carrier frequency is compensated based on the frequency offset.

OFDM Transmission/Reception System and Transmitting/Receiving Apparatus

The quantized parameter bits are grouped into several categories according to their sensitivity to bit errors. More effective error correction codes are used to encode the most sensitive parameter bits, while less effective error correction codes are used to encode the less sensitive parameter bits. This method improves the efficiency of the error correction and improves the performance if the total bit rate is limited. The perceived quality of coded speech is improved. A smoothed spectral envelope is created in the frequency domain. The ratio between the actual spectral envelope and the smoothed spectral envelope is used to enhance the spectral envelope. This reduces distortion which is contained in the spectral envelope.

Methods for speech transmission

A forward error correction decoder receives a received word comprising a plurality of characters. The decoder also receives a reliability value for each character. The decoder calculates a syndrome for each member of the set of possible received words by transforming each member with the parity check matrix. Members of the set having the same syndrome are grouped together into cosets. A weight based on the reliability value is then assigned to each member in each coset. The weight is defined to be the number of low reliability, non-zero characters in each member of the set of possible received words. For each coset, the member that has the lowest weight is selected as a coset leader. A syndrome is computed for the soft codeword by transforming it with the parity check matrix. The coset leader with the same syndrome as the received word is subtracted from the received word resulting in a codeword estimate.

Digital data decoder that derives codeword estimates from soft data

The present invention relates to an apparatus for providing a digital radio link between at least one fixed unit and at least one mobile unit wherein the apparatus comprises a small number of Rake fingers, one or more of which is assigned at any given to perform a searching operation. Whenever a searching Rake finger or fingers finds a path with higher energy than the current lowest Rake finger, the roles of the Rake finger are exchanged.

Apparatus for use in equipment providing a digital radio link between a fixed and a mobile radio unit

A speech coding system employs measurements of robust features of speech frames whose distribution are not strongly affected by noise/levels to make voicing decisions for input speech occurring in a noisy environment. Linear programing analysis of the robust features and respective weights are used to determine an optimum linear combination of these features. The input speech vectors are matched to a vocabulary of codewords in order to select the corresponding, optimally matching codeword. Adaptive vector quantization is used in which a vocabulary of words obtained in a quiet environment is updated based upon a noise estimate of a noisy environment in which the input speech occurs, and the "noisy" vocabulary is then searched for the best match with an input speech vector. The corresponding clean codeword index is then selected for transmission and for synthesis at the receiver end. The results are better spectral reproduction and significant intelligibility enhancement over prior coding approaches. Robust features found to allow robust voicing decisions include: low-band energy; zero-crossing counts adapted for noise level; AMDF ratio (speech periodicity) measure; low-pass filtered backward correlation; low-pass filtered forward correlation; inverse-filtered backward correlation; and inverse-filtered pitch prediction gain measure.

Enhancement of speech coding in background noise for low-rate speech coder

A low-overhead method of protecting multi-pulse speech coders from the effects of severe random or fading pattern bit errors combines a standard error correcting code (convolutional rate 1/2 coding and Viterbi trellis decoding) for protection in random errors with cyclic redundancy code (CRC) error detection for fading errors. Compensation for detected fading errors takes place within the speech coder. Protection is applied only to the perceptually significant bits in the transmitted frame.

Method for protecting multi-pulse coders from fading and random pattern bit errors

A Time Division Multiple Access (TDMA) radio system achieves synchronization by performing a two-step synchronization. A simplified frame/slot synchronization is followed by a symbol synchronization of higher accuracy. This symbol timing is passed to a frequency offset unit which determines the amount of frequency drift between the transmitter and receiver and compensates for the frequency drift. This results in improved receiver performance for the TDMA digital radio system.

Sample timing selection and frequency offset correction for U.S. digital cellular mobile receivers

A computationally efficient method for computing the filter coefficients for a prefilter in a decision feedback equalizer solves linear equations using a fast Toeplitz algorithm. Computations performed to compute the filter coefficients for the right half burst may be used to compute the prefilter for the left hand burst, thereby reducing the number of computations. Also, a square root-free algorithm may be used to solve the system of linear equations, further reducing computational complexity.

Equalizer with adaptive pre-filter

In a packet-based data transmission including incremental redundancy (IR) protocols, the memory consumption of the IR protocol is reduced by compressing and storing failed data units in their punctured format. The failed data units are compressed using low complexity compression/decompression algorithms. The compression algorithm includes two parts: calculating and storing a scale factor for each transmission burst that estimates the soft values in the burst, and storing each soft values' sign in local memory instead of the complete soft value. If the currently received data unit is a retransmission, its compressed versions in the punctured format stored in the IR memory are decompressed, de-punctured and combined with the currently received data unit. The combined data unit is then decoded. The decompression restores an estimated soft-value by multiplying the sign value stored in the IR memory with its corresponding scale factor obtained from a mapping table.

Data compression with incremental redundancy

A digital radio communications system including a suboptimal soft decision (e.g., errors and erasures) channel decoder and a speech decoder can be upgraded to effectively bypass the hard decision channel decoder and allow a higher performance soft decision (e.g., maximum likelihood sequence estimate (MLSE)) decoder to be used--without requiring physical removal or disconnection of the hard decision channel decoder. The new decoder can feed the existing decoder the right choice of received vector r' to force the existing decoder to produce a certain codeword y' and a certain bit error estimate b'. This technique is useful to test new MLSE decoders on existing radios, or to add an MLSE decoder as an upgrade on existing receivers.

Raymond Toy

Papers

Method for protecting multi-pulse coders from fading and random pattern bit errors

Sample timing selection and frequency offset correction for U.S. digital cellular mobile receivers

Equalizer with adaptive pre-filter

Data compression with incremental redundancy

Effective bypass of error control decoder in a digital radio system