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ISL3874 Datasheet(PDF) 24 Page - Intersil Corporation |
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ISL3874 Datasheet(HTML) 24 Page - Intersil Corporation |
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24 / 33 page  24 DQPSK modulation as shown in the table. The symbols of the MPDU shall be numbered starting with “0” for the first symbol for the purposes of determining odd and even symbols. That is, the MPDU starts on an even numbered symbol. The last data dibits d2, and d3 CCK encode the basic symbol as specified in Table 18. This table is derived from the CCK formula above by setting ϕ2= (d2*pi)+ pi/2, ϕ3 = 0, and ϕ4 = d3*pi. In the table d2 and d3 are in the order shown and the complex chips are shown LSB to MSB (left to right) with LSB transmitted first. At 11Mbps, 8 bits (d0 to d7; d0 first in time) are transmitted per symbol. The first dibit (d0, d1) encodes the phase ϕ1 based on DQPSK. The DQPSK encoder is specified in Table 17. The phase change for ϕ1 is relative to the phase ϕ1 of the preceding symbol. In the case of rate change, the phase change for ϕ1 is relative to the phase ϕ1 of the preceding CCK symbol. All odd numbered symbols of the MPDU are given an extra 180 degree ( π) rotation in accordance with the DQPSK modulation as shown in Table 17. Symbol numbering starts with “0” for the first symbol of the MPDU. The data dibits: (d2, d3), (d4, d5), (d6, d7) encode ϕ2, ϕ3, and ϕ4 respectively based on QPSK as specified in Table 19. Note that this table is binary, not Grey, coded. TX Power Control The transmitter power can be controlled via two registers. The first register, CR58, contains the digitized results of power measurements by the ISL3874. By comparing this measurement to what is needed for transmit power, The MAC determines whether to raise or lower the transmit power. It does this by writing the power level desired to register CR31. Clear Channel Assessment (CCA) and Energy Detect (ED) Description The Clear Channel Assessment (CCA) circuit implements the carrier sense portion of a Carrier Sense Multiple Access (CSMA) networking scheme. The Clear Channel Assessment (CCA) monitors the environment to determine when it is clear to transmit. The CCA circuit in the ISL3874 can be programmed to be a function of RSSI (energy detected on the channel), CS1, SQ1, or various combinations. The CCA is used by the Media Access Controller (MAC) in the ISL3874. The MAC decides on transmission based on traffic to send and the CCA indication. The CCA indication can be ignored, allowing transmissions independent of any channel conditions. The CCA in combination with the visibility of the various internal parameters (i.e., Energy Detection measurement results), can assist the MAC in executing algorithms that can adapt to the environment. These algorithms can increase network throughput by minimizing collisions and reducing transmissions liable to errors. There are three measures that can be used in the CCA assessment. The receive signal strength indication (RSSI) which indicates the energy at the antenna, CS1 and carrier sense (SQ1). CS1 becomes active anytime the AGC portion of the circuit becomes unlocked, which is likely at the onset of a signal that is strong enough to support 11Mbps, but may not occur with the onset of a signal that is only strong enough to support 1 or 2MBps. CS1 stays active until the AGC locks and a SQ1 assessment is done, if SQ1 is false, then CS1 is cleared, which deasserts CCA. If SQ1 is true, then tracking is begun, and CCA continues to show the channel busy. CS1 may occur at any time during acquisition as the AGC state machine runs asynchronously with respect to slot times. SQ1 becomes active only when a spread signal with the proper PN code has been detected, and the peak correlation amplitude to sidelobe ratio exceeds a set threshold, so it may not be adequate in itself. A SQ1 evaluation occurs whenever the AGC has remained locked for the entire data ingest period. When this happens, SQ1 is updated between 8 µs and 9µs into the 10µs dwell. If CS1 is not active, two consecutive SQ1s are required to advance the part to tracking. The state of CCA is not guaranteed from the time RX_PE goes high until the first CCA assessment is made. At the end of a packet, after RXPE has been deasserted, the state of CCA is also not guaranteed. The Receive Signal Strength Indication (RSSI) measurement is derived from the state of the AGC circuit. ED is the comparison TABLE 17. DQPSK ENCODING TABLE DIBIT PATTERN (d(0), d(1)) d(0) IS FIRST IN TIME EVEN SYMBOLS PHASE CHANGE (+j ω) ODD SYMBOLS PHASE CHANGE (+j ω) 00 0 π 01 π/2 3 π/2 (-π/2) 11 π 0 10 3 π/2 (-π/2) π/2 TABLE 18. 5.5Mbps CCK ENCODING TABLE d2, d3 CHIPS 00 1j 1 1j -1 1j 1 -1j 1 01 -1j -1 -1j 1 1j 1 -1j 1 10 -1j 1 -1j -1 -1j 1 1j 1 11 1j -1 1j 1 -1j 1 1j 1 TABLE 19. QPSK ENCODING TABLE DIBIT PATTERN (d(i), d(i+1)) d(i) IS FIRST IN TIME PHASE 00 0 01 π/2 10 p 11 3 π/2 (-π/2) ISL3874 |
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