Excavating
Patent
1997-09-24
1999-08-10
Grant, William
Excavating
371 45, 370330, 4552261, 4552263, G11C 2900
Patent
active
059369795
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to method for frame quality detection used in a radio system comprising a plurality of base stations and subscriber terminals transmitting and receiving signals via different channels, the quality of the signals being measured by the bit error rate, the terminals generating speech frames from voice and silence descriptor frames regularly from silence, the frames comprising bits, and each frame being interleaved with another frame, and each frame being transmitted in a known number of essentially successive bursts comprising a frame start and end identifier and bits that are decoded by soft bit decisions.
The invention also relates to a receiver arranged to be used in a radio system comprising a plurality of base stations and subscriber terminals arranged to transmit and receive signals via different channels, and arranged to measure the quality of the signals by the bit error rate, and comprising at least speech frames and silence descriptor frames, the frames comprising bits and each frame being interleaved with another frame and each frame being arranged to be transmitted in a known number of essentially successive bursts comprising a frame start and stop identifier and bits that are arranged to be decoded by soft bit decisions.
2. Description of Related Art
In a cellular radio system, and especially in the GSM system, discontinuous transmission (DTX) is used to reduce interference and subscriber terminal power consumption. This operates so that when the voice coder of the subscriber terminal observes silence in speech, the subscriber terminal transmits only periodically (once every 480 ms) a silence descriptor frame (SID), typically used for noise generation in a receiving subscriber terminal. Without noise the receiving user finds the silence unpleasant and may even think that the connection has been disconnected.
However, the silence descriptor frame causes problems. In a cellular radio system the channel decoder of a receiver attempts to correct mistakes caused mainly by the air interface between subscribers. However, correction does not always succeed. This means that the channel decoder detects a bad frame (BFI), and the substitution and muting algorithm in the speech decoder "plays" again the last good frame received. If several bad frames succeed each other, the speech decoder "plays" the last good frame received again and again and gradually attenuates it completely. Known bad frame detection methods include e.g. 3 bit CRC (Cyclic Redundancy Check) and the pseudober method. In the CRC method a check parity is calculated for a bit block according to a known code polynome. A bit block usually comprises the most significant bits of a speech block, giving a three-bit parity check. This is compared with the parity bits calculated by the channel coder, and if the parity bits are the same, the frame received is good. In the pseudober method, a received frame is channel coded again with a convolution code after Viterbi decoding. The coded frame is directly compared with a frame received from the channel, and the differences in the bit sequences of the frames indicate error corrections made by the Viterbi decoder. All errors cannot, however, be corrected by the Viterbi decoder. In this method a frame corresponding to e.g. a speech block is determined to be bad if the Viterbi decoder has changed more bits than allowed by a predetermined limit. Another known method of estimating the bit error rate is to calculate the bit error rate from a predetermined training sequence of each traffic channel during a measuring period. If the received signal comprises more erroneous bits than allowed by the predetermined limit, the frame is determined to be bad.
Reception of discontinuous transmission is the most susceptible to errors in detection of bad frames. If a speech decoder determines a bad frame to be good during silence, the speech decoder switches to speech mode and "plays" this false frame. As the frame was erroneously detected, the f
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Grant William
Marc McDieunel
Nokia Telecommunications Oy
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