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Wlan 802.11a System Design (Read 5843 times)
RC
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Wlan 802.11a System Design
Oct 18th, 2002, 4:58am
 
I am currently involving in a IC design group, designing Wlan 802.11a Single Chip solution.
However, we are lacking of good system designer.
I have a few enquiries,
1. Any system specification at circuit level available?
2. Designing from front-end to baseband, how to determine the individule blocks spec. efficiently? eg. IIP3, gain, noise figure, at RF; group-delay, image rejection, SNR, THD, at IF; BER, aquisition time, synchonous time, at Baseband; and etc.
3. If there are a working example of Wlan 802.11a design from system spec. to circuit spec. is available, it would be great.

I am working in CMOS analog.
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Eugene
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Re: Wlan 802.11a System Design
Reply #1 - Oct 18th, 2002, 11:10pm
 
Finding a detailed example of how to specify an RF front end will be hard to find since details are usually proprietary. However, there are some high level papers and some general text books available. I list those at the end of this message.

The main problem is in transforming DSP metrics such as EVM, PER, BER into RF metrics such as IIP3, noise figure(NF), gain, SNR, and bandwidth. Typically the systems guys perform Monte Carlo simulations to generate BER(SNR) curves. From the curves and the target BER, they determine the minimum SNR required. From there, the RF engineer starts selecting the number of ADC bits and noise figure. Adjacent and alternate adjacent channel interference will determine the filtering requirements and IIP3. The RF engineers usually work out the link budget. The link budget distributes the IIP3, NF, gain, and filtering along the receiver chain. You can find short courses on how to perform link budgets. Link budgets are usually performed on spreadsheets. Again, the trick is to relate end-to-end RF metrics to DSP metrics. One popular solution to checking BER is to build up a model library of RF blocks in the DSP design environment. With a candidate RF architecture, the systems team can simulate BER. To simulate BER, you must account for RF impairments as well as the DSP remedies (equalizers, encoders, acquisition algorithms etc.) The RF models should also account for frequency offsets, DC offsets, and phase noise. At this level the models do not have to be too complicated. The models should suppress the carrier lest the simulations run on geological time scales.

Here are some references that may help:

"A 5GHz CMOS Transceiver for IEEE 802.11a Wirelss LAN". David Su, Masoud Zargari, .....  All from Atheros.
2002 ISSCC

"An Integrated 802.11a Baseband and MAC Processor". John Thompson, Bevan Baas, .... All from Atheros.
2002 ISSCC

The Atheros papers are high level but they describe a superhet 802.11a design.

"Behavioral Modeling of Nonlinear RF and Microwave Devices". Turlington. ISBN 1-58053-014-1

Turlington's book describes spreadsheet methods.

"CMOS Wirelss Transceiver Design". Crols and Stayaert.
ISBN 0-7923-9960-9

This book describes a GSM CMOS example in detail.
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gatsby
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Re: Wlan 802.11a System Design
Reply #2 - Nov 14th, 2002, 8:55am
 
Some Equations may be useful...
From your BER spec. , find out what Eb/No you
need for each modulation type
(e.g. QPSK - Pb(Eb/No)=1/2*erfc(sqrt(Eb/No)) )
convert this to SNR....  SNR=(Eb/No)*(Data BW/Channel BW)


Now you have an SNR spec. at the RX output ... and you
know the SNR at the input (from the input signal level)
you can distribute the SNR degredation among Noise
Figure, IM3 distortion, phase noise, I/Q mismatch etc.
etc. depending on your architecture.
Friis wil help you here - or use a simulator such as ADS.

This is a very simple summary....
check out Agilent's 1380-2 App Note for BER help, check
out Razavi's RF Microelectronics if you need help on the level plan, or ask me!
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Feng
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Re: Wlan 802.11a System Design
Reply #3 - Jan 27th, 2004, 8:43am
 
you need first do the link budget analysis for system design. Understanding the spec is a must. Then verify it in some software like System View.

take a look at http://www.sss-mag.com/pdf/an9804.pdf
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