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AD6426 Datasheet(PDF) 8 Page - Analog Devices |
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AD6426 Datasheet(HTML) 8 Page - Analog Devices |
8 / 50 page Preliminary Technical Information AD6426 This Information applies to a product under development. Its characteristics and specifications are subject to change without notice. Analog Devices assumes no obligation regarding future manufacture unless otherwise agreed to in writing. No responsibility is assumed by Analog Devices for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Revision Preliminary 2.3 (June 9, ´98) - 8 - Confidential Information A feature of the GSM system is the application, as part of the TRANSMIT process, of data encryption for the purpose of link security. After the INTERLEAVE module the data may be encrypted using the prescribed A5/1 or A5/2 encryption algorithm. The RECEIVE function requires unmodulated baseband data from the equalizer. As necessary the data is decrypted and written to the DEINTERLEAVE module. This is conducted at TDMA frame rate, although precise timing is not necessary at this stage. The DECODING process reads data from the DEINTERLEAVE module, inverting the interleave algorithm and decodes the error control codes, correcting and flagging errors as appropriate. The data also includes a measure of confidence expressed as two additional bits per received symbol. These are used in the convolution decoder to improve the error decoding performance. The resultant data is then presented to the original sources as determined by the control programming. The Channel Codec interfaces with the speech transcoder for speech traffic data and with an equalizer for recovered receive data. In the AD6426 the equalizer and speech transcoder are implemented in the DSP. Processor Sub-System The Processor Sub-System consists of a high performance 16- bit microcontroller together with a selection of peripheral elements. The processor is a version of the Hitachi H8/300H that has been developed to support GSM applications and which is well suited to support the Protocol Stack and Application Layer software. DSP Sub-System The DSP Sub-System consists of a high performance 16-bit digital signal processor (DSP) with integrated RAM and ROM memories. The DSP performs two major tasks: speech transcoding and channel equalization. Additionally several support functions are performed by the DSP. The instruction code, which advises the DSP to perform these tasks, is stored in the internal ROM. The DSP sub-system is completely self- contained, no external memory or user-programming is necessary. Speech Transcoding In Full Rate mode the DSP receives the speech data stream from the EVBC and encodes the data from 104 kbit/s to 13 kbit/s. The algorithm used is Regular Pulse Excitation, with Long Term Prediction (RPE-LTP) as specified in the 06-series GSM Recommendations. In Enhanced Full Rate mode, the DSP encodes the 104 kbit/s speech data into 12.2 kbit/s (speech) +0.8 kbit/s (CRC and repetition bits) as additionally specified in the Phase 2 version of the 06-series GSM Recommendations. In both modes, the DSP also performs the appropriate voice activity detection and discontinuous transmission (VAD/DTX) functions. Alternatively the DSP receives encoded speech data from the channel codec sub-system including the Bad Frame Indicator (BFI). The Speech decoder supports a Comfort Noise Insertion (CNI) function that inserts a predefined silence descriptor into the decoding process. The resulting data, at 104 kbit/s, is transferred to the EVBC. Equalization The Equalizer recovers and demodulates the received signal and establishes local timing and frequency references for the mobile terminal as well as RSSI calculation. The equalization algorithm is a version of the Maximum Likelihood Sequence Estimation (MLSE) using the Viterbi algorithm. Two confidence bits per symbol provide additional information about the accuracy of each decision to the channel codec’s convolutional decoder. The equalizer outputs a sequence of bits including the confidence bits to the channel codec sub- system. Audio Control The DSP subsystem is also responsible for the control of the audio path. The EVBC provides two audio inputs and two audio outputs, as well as a separate buzzer output, which are switched and controlled by the DSP. Furthermore the EVBC provides for variable gain and sensitivity which is also controlled by the DSP under command of the Layer 1 software. Tone Generation All alert signals are generated by the DSP and output to the EVBC. These alerts can be used for the buzzer or for the earpiece. The tones used for alert signals can be fully defined by the user by means of a description which provides all the parameters required such as frequency content and duration of components of the tone. The tone descriptions are provided by the Layer 1 software. Automatic Frequency Control (AFC) The detection of the frequency correction burst provides the frequency offset between the mobile terminal and the received signal. This measure is supplied to the Layer 1 software which then requests a correction of the master clock oscillator frequency via the AFC-DAC in the EVBC. In order to do so the Layer 1 software includes a transfer function for the oscillator frequency against the voltage applied. The DSP provides the measurements for the AFC. Automatic Gain Control (AGC) The DSP is also responsible for making measurements of the power in the received signal. This is used for a number of functions including RSSI measurement, adjacent channel monitoring and AGC. The Layer 1 software passes the requested gain level to the DSP, which then analyzes the received signal and generates an AGC control signal. Depending on the radio architecture, this control signal will be used in digital form or, converted by the AD6425 in analog form. |
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