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AD6423 Datasheet(PDF) 2 Page - Analog Devices |
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AD6423 Datasheet(HTML) 2 Page - Analog Devices |
2 / 8 page AD20msp415 –2– REV. 0 ARCHITECTURE OVERVIEW Analog Devices and The Technology Partnership (TTP) pro- vide a cost effective and proven method of attaining the baseband processing subsystem and protocol stack software. This data sheet includes functional descriptions of the baseband process- ing subsystem and the Protocol Stack Layer 1. The Technology Partnership can provide licenses to software and reference de- signs in all areas of a GSM handportable terminal. For detailed information about the individual chipset compo- nents, please refer to the AD6421 (VBC) and AD6422 (GSMP) data sheets for electrical characteristics and timing information. FUNCTIONAL DESCRIPTION Figure 1 is a functional block diagram of the GSM baseband processing chipset. The chipset can be viewed as a functional block that contains a number of discrete functional units. The electrical and functional interfaces to the rest of the system are briefly described at the end of this section and described in detail in the individual data sheets for each component. VOICE ADC SPEECH ENCODE CHANNEL ENCODE INTER- LEAVE ENCRYPT VOICE DAC SPEECH DECODE CHANNEL DECODE DEINTER- LEAVE DECRYPT EQUALIZER BASEBAND ADC CONTROL + MMI + I/O VBC GSMP BASEBAND DAC Figure 1. Functional Block Diagram Uplink The uplink baseband processing functions include the following operations: Analog-to-Digital Voice Conversion (VBC) A conventional microphone, connected directly to the VBC, provides an analog input signal to the ADC. The analog voice signal is sampled at 8 kHz, producing 13-bit linear values corre- sponding to the magnitude of the input. The ADC includes all required filtering to meet the GSM specifications. The sampled voice data is passed to the GSMP through a dedicated serial port. Speech Encoding (GSMP) The GSMP receives the voice data stream from the VBC and encodes the data from 104 kb/s to 13 kb/s. The algorithm used is Regular Pulse Excitation, with Long Term Prediction (RPE- LTP) as specified in the 06-series of GSM Recommendations. Channel Coding (GSMP) The information received from the speech coder contains param- eters that have different levels of priority. These are protected to different levels within the channel coding. The encode protec- tion process incorporates block coding and convolutional encoding. In addition to the normal speech traffic channels, the channel coding function also supports data transmission at full rate and half rate. After the interleave process (if necessary) the data is encrypted using the required A5/1 or A5/2 encryption algorithm. Data is then formatted into bursts, with the required timing and training sequences, and sent to the VBC through a dedicated serial port. GMSK Modulation and D/A Conversion (VBC) The VBC receives data at 270 kb/s. The VBC uses an on-chip lookup table to perform GMSK modulation. A pair of 10-bit matched differential DACs convert the modulated data and pass I and Q analog data to the transmit section of the radio subsystem. Downlink The downlink baseband processing functions include the follow- ing operations: Analog-to-Digital Conversion (VBC) The receiver I and Q signals are sampled by a pair of ADCs at 270 kHz. The I and Q samples are transferred to the GSMP through a dedicated receive path serial port. Equalization (GSMP) The equalizer recovers and demodulates the received signal and establishes local timing and frequency references for the mobile unit 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 deci- sion to the channel codec’s convolutional decoder. The equal- izer outputs a sequence of bits including the confidence bits. Channel Decoding (GSMP) Data is decrypted as required, using the A5/1 or A5/2 decryption algorithm prior to the deinterleave process. The deinterleave process is an exact inversion of the interleave process used by the transmit section. The decode function then performs convo- lutional decoding and parity check. The convolutional decoder uses a Viterbi algorithm, with two soft decision confidence bits supplied by the equalizer. Error control mechanisms are used to ensure adequate bad frame indication. Speech Decoding (GSMP) Encoded speech data is transferred at 20 ms intervals in blocks of 260 bits plus 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 GSMP also implements control of talker side-tone and short term echo cancellation. The resulting data, at 104 kb/s, is transferred to the VBC through a dedicated serial path. Voice Digital-to-Analog Conversion (VBC) The Voice DAC function of the VBC operates at 8 kHz and includes all the needed filtering. The analog signal can be con- trolled in volume and directly drive a small earpiece as well as a separate auxiliary output. |
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