1. Rishi Mohindra MAXIM Integrated Products
Proposal for IEEE802.11g Receiver Adjacent Channel Rejection Requirement of 11b Blocker when receiving OFDM packets
Rishi Mohindra
MAXIM Integrated Products
Rishi Mohindra, MAXIM

2. Why is Adjacent Channel Rejection of 11b jammer a concern for 11g receiver in OFDM mode ?
11b standard allows large out-of-band transmitter spectral power density.
Page 3 shows a possible worst case received 11b jammer spectral plot.
-35 dBm adjacent channel blocking level allowed for 11b receiver, at a minimum 25 MHz separation between center frequencies of signal and jammer.
The 11b adjacent channel jammer spills about -85 dBm power into the 11g OFDM subcarriers.
Assuming above conditions of 11b. Received signal spectrum is plotted on page 4.
S/Interference = 23 dB (simulated as average receiver constellation error including 2 dB implementation loss), assuming 11a based OFDM signal of -62 dBm (54Mbits/s, 64-QAM).
Graph on page 5 shows simulated OFDM S/Interference ratio as a function of center frequency separation between the OFDM signal and the 11b jammer.
For a -62 dBm 54Mbits/s OFDM received signal, the S/Interference ratio should be 30dB or more for a reasonable operating margin.
Spreadsheet calculation is shown on page 6.
Rishi Mohindra, MAXIM

3. Rishi Mohindra, MAXIM

4. 11b jammer
OFDM signal
Rishi Mohindra, MAXIM

5. Rishi Mohindra, MAXIM

6. Double click to activate Excel Spreadsheet
Rishi Mohindra, MAXIM

7. Proposal for IEEE802.11g Receiver Adjacent Channel Rejection Requirement of 11b Blocker when receiving OFDM packets:
Either of these options (in decreasing preference):
Reduce the 11b based jammer level to -45 dBm at 25 MHz or greater frequency offest, keeping the OFDM signal levels as in 11a (e.g. -62 dBm for 54 Mbits/s)
Use -35 dBm “clean” (linear PA) 11b jammer at 30 MHz or greater frequency offset for 11a based OFDM signal levels (e.g. -52 dBm for 54 Mbits/s)
Increase the 11b based adjacent channel jammer center frequency separation to 40 MHz, keeping its level at -35 dBm, and the OFDM signal levels as in 11a (e.g. -62 dBm for 54 Mbits/s)
Rishi Mohindra, MAXIM