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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 1 Thoughts on TX Spectral Masks for 802.11n Christopher J. Hansen Broadcom Corporation chansen@broadcom.com

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 2 TX Spectral Masks for 802.11n Overview.11a/.11g mask for 20 MHz OFDM Limitations of this mask Suggested masks for 20 and 40 MHz.11n Adjacent Channel Performance

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 3 Why Spectral Masks are Important Spectral masks limit interference to adjacent channels –promotes: interoperability, coexistence, and system capacity The out of band mask places a lower bound on interference levels in receivers regardless of implementation. –Interference energy appears on top of the desired signal. –Goal to make out of band interference as small as possible subject to practical constraints.

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 4 802.11a/.11g OFDM Mask In band Transition Floor

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 5 Key Parameters for Masks 3 Regions –In Band encompasses the desired signal As close to channel bandwidth as feasible –Transition bounds adjacent channel interference limited by baseband / IF bandwidth post IF inter-modulation distortion (IMD) –Floor bounds other channel interference Outside range of filter and IMD limits limited by local oscillator phase noise

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 6 Transition Region Transition region determined by TX non-linearity –Assume IMD in TX path is dominated by 3rd order compressive non-linearity (higher order terms << 3rd order term): –y(t) = x(t) - f(Ax 3 (t)) x(t) is ideal, time domain OFDM signal, f(t) is a bandpass filter to remove harmonic components –A = 4/3 (1/OIP 3 ) 2 OIP 3 is the 3rd order intercept point in absolute units –Y(f) = X(f) - AX(f)*X(f)*X(f) Double convolution expands bandwidth into adjacent channels Y(f) is the frequency domain representation of the resulting signal Intermodulation distortion will expand the bandwidth of the resulting signal

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 7 Transition Region Example –Roll off shoulder height determined by IMD level –Slope is 6 dB per octave close in; falls off to zero at 3 times signal bandwidth Ideal Signal Spectrum Intermodulation Distortion

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 8 Mask Floor Floor region is limited by oscillator phase noise (assuming no sharp filtering at IF) Transmitted signal –baseband signal x(t) modulated by carrier at f 0 and phase noise –Power spectral density of desired signal:

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 9 Mask Floor PSD of the phase noise corrupted signal is the convolution of the ideal signal PSD and the phase noise PSD: For the phase noise levels and of interest, the phase noise spectrum floor is almost exactly equal to L (f), the phase noise spectrum in one sideband (IEEE Std. 1139-1999) At 10 MHz or more from the carrier, phase noise spectrum is relatively flat Acheivable floor in dBr is L (f) * Signal Bandwidth (assume 18 MHz for.11a) –.11a floor of -40 dBr requires L (f) of -112.6 dBc/Hz

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 10 Limitations of the 802.11a TX Mask.11a Mask –Adjacent channel signal approximately 25.9 dB down –Other channels 40 dB down –Very little margin, especially with near/far problem in wireless Adjacent channel peformance is difficult to improve –Additional constraints on IMD undesirable Floor should be easy to improve –Phase noise level is tolerable

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 11 Suggested Masks for.11n Retain transition shape and level –No additional filter or linearity requirements Lower floor by 10 dB –Approximate limits on phase noise –L (f) = -123 dBc/Hz for 20 MHz channels –L (f) = -126 dBc/Hz for 40 MHz channels Should not place additional burden on implementations

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 12 Suggested Masks for.11n

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 13 Adjacent Channel Interference.11a to adjacent channel = -25.9 dB.11a to alternate adjacent = -40 dB.11n (20 MHz #1) to adjacent = -26.1.11n (20 MHz #2) to adjacent = -25.9.11n (20 MHz either) to alternate adjacent = -50 dB.11n (40 MHz) to 20 MHz adjacent = -23.2 –2.7 dB worse than.11a –for same TX power, actually 0.3 dB less interfence than 20 MHz channel since TX PSD in band drops by 3 dB.11n (40 MHz) to 40 MHz adjacent = -26.1

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doc.: IEEE 802.11-04/0060r0 Submission January 2004 Christopher Hansen, BroadcomSlide 14 Conclusions PSD Mask floor can be improved for.11n –Reduce system interference –Improve system capacity Adjacent channel interference does not change 40 MHz channels are compatible with existing 20 MHz systems –0.3 dB adjacent channel interference improvement for same transmitter power

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