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HFA3860A Datasheet(PDF) 16 Page - Intersil Corporation |
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HFA3860A Datasheet(HTML) 16 Page - Intersil Corporation |
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16 / 39 page  2-146 conditions. The CCA in combination with the visibility of the various internal parameters (i.e., Energy Detection measure- ment results), can assist an external processor in executing algorithms that can adapt to the environment. These algo- rithms can increase network throughput by minimizing collisions and reducing transmissions liable to errors. There are two measures that are used in the CCA assessment. The receive signal strength (RSSI) which measures the energy at the antenna and carrier sense early (CSE). Both indicators are normally used since interference can trigger the signal strength indication, but it might not trigger the carrier sense. The carrier sense, however, becomes active only when a spread signal with the proper PN code has been detected, so it may not be adequate in itself. The CCA compares these measures to thresholds at the end of the first antenna dwell following RX_PE going active. “The state of CCA is not guaranteed from the time RX_PE goes high until the CCA assessment is made. At the end of a packet, after RX_PE has been deasserted, the state of CCA is also not guaranteed.” CCA should be sampled 16 µs after raising RX_PE. The receive signal strength indication (RSSI) measurement is an analog input to the HFA3860A from the successive IF stage of the radio. The RSSI A/D converts it within the baseband processor and it compares it to a programmable threshold. This threshold is normally set to between -70 and -80dBm. A MAC controlled calibration procedure can be used to optimize this threshold. The CSE (Carrier Sense Early) is a signal that goes active when SQ1 (after an antenna dwell) has been satisfied. It is called early, since it is indicated before the carrier sense used for acquisition. It is calculated on the basis of the integrated energy in the correlator output over a block of 15 symbols. Thus, the CCA is valid after 16 µs has transpired from the time RX_PE was raised. The Configuration registers effecting the CCA algorithm operation are summarized below (more programming details on these registers can be found under the Control Registers section of this document). The CCA output from pin 32 of the device can be defined as active high or active low through CR 1 (bit 5). The RSSI threshold is set through CR14. If the actual RSSI value from the A/D exceeds this threshold then ED becomes active. The instantaneous RSSI value can be monitored by the external network processor by reading the test bus in mode 3. It measures the signal 16 µs after the start of each antenna or data dwell. RSSI value is invalid after MD_RDY goes active if CR31 bit 1 is set to a “1”. Value is valid until MD_RDY drops if bit is set to a “0”. The programmable threshold on the CSE measurement is set through CR12 and CR13. More details on SQ1 are included in the receiver section of this document. In a typical single antenna system CCA will be monitored to determine when the channel is clear. Once the channel is detected busy, CCA should be checked periodically to determine if the channel becomes clear. CCA is stable to allow asynchronous sampling or even falling edge detection of CCA. Once MD_RDY goes active, CCA is then ignored for the remainder of the message. Failure to monitor CCA until MD_RDY goes active (or use of a time-out circuit) could result in a stalled system as it is possible for the channel to be busy and then become clear without an MD_RDY occurring. A Dual antenna system has the added complexity that CCA will potentially toggle between active and inactive as each antenna is checked. The user must avoid mistaking the inactive CCA signal as an indication the channel is clear. A time-out circuit that begins with the first busy channel indication could be used. Alternatively CCA could be monitored, a clear channel indication for 2 successive antenna dwells would show the channel clear on both antennas. Time alignment of CCA monitoring with the receivers 16 µs antenna dwells would be required. Once the receiver has acquired, CCA should be monitored for loss of signal until MD_RDY goes active. An optional CCA mode is set by CR31 bit 0. When set to a zero, the HFA3860A will perform the CCA monitoring for successive antenna dwells when dual antenna mode is selected. The external CCA signal will go active when a busy channel is detected, CCA will stay active until the channel shows clear for two successive antenna dwells. This allows the same simple algorithm to be used in both signal and dual antenna, namely, continuous monitoring of CCA for a clear channel until MD_RDY goes active. CR5 selects the starting antenna used when RXPE is brought active. CSE is updated at the end of each antenna dwell. After acquisition, CSE is updated every 64 symbols. In the event of signal loss after acquisition, CSE may go inactive. But because the accumulation is over 63 symbols instead of 15, it is more likely the SQ1 value will exceed the CSE threshold and CSE will remain active. Demodulator Description The receiver portion of the baseband processor, performs A/D conversion and demodulation of the spread spectrum signal. It correlates the PN spread symbols, then demodulates the DBPSK, DQPSK, BMBOK, or QMBOK symbols. The demodulator includes a frequency tracking loop that tracks and removes the carrier frequency offset. In addition it tracks the symbol timing, and differentially decodes (where appropriate) and descrambles the data. The data is output through the RX Port to the external processor. The PRISM baseband processor, HFA3860A uses differential demodulation for the initial acquisition portion of the message processing and then switches to coherent demodulation for the rest of the acquisition and data demodulation. The HFA3860A is designed to achieve rapid settling of the carrier tracking loop during acquisition. Rapid phase fluctuations are HFA3860A |
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