Technology

 

FAQs on WLAN

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This Frequently Asked Questions list reviews the background of wireless LAN sensitivity and range and then examines specific claims of the ParkerVision SignalMAX product. The internals of the SignalMAX and the NetGear routers are compared, and the internals of the SignalMAX and the Belkin Pre-N cards are compared.

 

For the SignalMAX product, the use of multiple receivers connected to multiple antennas in the router and the use of redundant traffic paths to the PCMCIA card can easily explain all of the improvement observed in the tests. These are applicable to any competitor willing to pay the higher component cost and suffer the reduced bitrate resulting from these techniques. Moreover, these are unrelated to D2D technology, as described in ParkerVision’s white papers. The current SignalMAX products do not clearly demonstrate any inherent advantage of D2D.

 

Basic Questions

Q
  What determines WLAN product maximum sensitivity?
A
  Maximum sensitivity is set by the minimum signal to noise ratio at the detector. Receiver front ends add their own noise, given as the noise figure of the receiver. The receiver noise figure is usually given in dB and describes how many decibels of noise the front end adds to the incoming signal. A zero dB noise figure would not add any noise, and chips such as the Maxim 2820 have only a 3.5dB noise figure. At the detector, the radio bandwidth and the data encoding also affect the receiver sensitivity. Broadband noise, due to thermal and atmospheric sources, reduces the sensitivity in dB as 10 * log (bandwidth). Finally, encoding schemes, such as the spread spectrum technology in 802.11b use extra bits to reduce the effects of random noise and interferers.

 

Q
  What affects WLAN radio sensitivity?
A
 

Radio sensitivity is not just the maximum front end sensitivity. In the real world, radios suffer from interference from other radios and multiple transmission paths, which cause signal strength to increase and/or decrease rapidly over short distances. The presence of other radios effectively adds noise to the detector. Because the IEEE 802.11 specification was designed for multiple radios in one location, an extremely low noise figure is not necessary. This is why it is so easy to improve upon the 802.11 sensitivity.

 

Q
  Is the 802.11 standard inferior?
A
 

No, the standard is designed for a close range environment with many interferers (such as you find at a Wi-Fi hotspot), making receiver sensitivity a low priority. The IEEE specification requires -76 dbm sensitivity at 11 Mbps and -80 dbm sensitivity at 1 Mbps. Several chip vendors claim to better this by up to 20 db.

 

Q
  What determines WLAN product range?
A
 

Noise figure, interference, multi-path, modulation bandwidth, and data encoding all affect product range. One of the more significant effects on the radio signal is reflections from objects. The multiple paths caused by the reflections allow the signal to interfere with the reflections, either enhancing or reducing the signal strength. These variations rise and fall every half wavelength, or about 2.4 inches at 2.5 GHz. Having multiple antennas connected to multiple receivers, also called full diversity, can increase the range. In addition, redundant transmit paths can increase the product range, but at the expense of data rate and manufacturing cost (twice the part count).

 

Q
  Do the Pre-N MIMO techniques improve open-field range?
A
 

No. The Pre-N MIMO technology used in the Belkin Pre-N router and card (using chips from Airgo) only increase range and bandwidth in cluttered environments. They do not affect range in completely open environments, where there is no scattering for the multiple antennas to work with. Open fields with no RF interferers is the environment where the approach used by the SignalMAX cards has the greatest advantage. Comparing Pre-N to SignalMAX in open field testing is comparing the worst case for Pre-N to the best case for SignalMAX.

 

Do the Pre-N MIMO techniques improve open-field range?

 

Q
  Can D2D combined with Pre-N MIMO further increase the range of Pre-N MIMO?
A
 

Not in any way demonstrated by the current SignalMAX cards. As explained above, the range increase of the SignalMAX cards is explained by receiver diversity in the router, and redundant transmit paths (significantly reducing the bitrate).

 

Q
  Do Superheterodyne receivers have poor sensitivity?
A
  No. Superheterodyne receivers can get under 1 dB noise figure. Superhets have given way to many other receiver technologies (zero-IF, VLIF, software radios) mainly because of the cost of filters and the need to make architectures which are more easily realizable on chip.