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)
[1].
Finally, encoding schemes, such as the spread spectrum technology
in 802.11b use extra bits to reduce the effects of random noise
and interferers [2,3,4] |
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 [5,6].
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 [8]. |
|
FAQs on WLAN Receivers
FAQs on SignalMAX |
