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Chapter 2: Efficiency: Combining Data-Rates with the Shannon Formula

By combining the 802.11X standards for data-rate encoding over multiple spatial streams with the Shannon formula, it is possible to gauge the relative efficiency of the 802.11X protocols, and to see how much room for improvement remains to be squeezed out of future products.

By combining the 802.11X standards for data-rate encoding over multiple spatial streams with the Shannon formula, it is possible to gauge the relative efficiency of the 802.11X protocols, and to see how much room for improvement remains to be squeezed out of future products.

The old 802.11b standard (not used by modern WISPs!) was very inefficient. Over a 16 mhz channel bandwidth, it managed to provide a data-rate of 11 mbit/s. Assuming a signal-to-noise ratio of 7, over a single spatial stream one should be able to transmit 48 mbit/s of data at peak efficiency. The 802.11b standard is therefore only utilizing approximately 22% of the available spectrum efficiently. This was a good start for Wi-Fi, but a full 78% of the data-stream was taken up with signaling and error-correction!

802.11n, widely used by WISPs today, can provide an encoding rate of 65 mbit/s (for real world speeds closer to 22 mbit/s). The Shannon Formula shows that a 20mhz-wide band can provide a capacity of 69 mbit/s at 20mhz with a signal-to-noise ratio of 10. Therefore, given a clean spectrum, 802.11n can approach 94% encoding efficiency. With a signal-to-noise ratio of 15, Shannon shows us that 80 mbit/s is the theoretical peak; at such good signal levels, 802.11 can be 81% efficient.

802.11ac, emerging for WISPs today, can provide an encoding rate of 86 mbit/s on a 20mhz-wide channel. Shannon shows us that with a signal-to-noise ratio of 19:1, 802.11AC can theoretically utilize 99% of available encoding space!

The most important thing to derive from this is that the signal-to-noise ratio is improved for each of the subsequent calculations. For 802.11b, we assumed an easily attainable SNR of 7:1. For 802.11n, we assumed an SNR of 10 or 15. For 802.11ac, we had to assume an SNR or 19:1 to make the math work! This is indicative of two phenomena:

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