Re: [SOCALWUG] Phased Array WiFi Antennas

[Jim Thompson]

OK, building a switched array (or even phased array) for the client is 
much easier than
building one for the AP.

Clients move.  Typically APs don't.   Clients (in BSS/"Infrastructure" 
mode) talk to one other thing (the AP), APs never know where the next 
packet is coming from.

as for 'ms'


The wireless physical layer is split into two parts, called the PLCP 
(Physical Layer Convergence Protocol) and the PMD (Physical Medium 
Dependent) sublayer. The PMD takes care of the wireless encoding. The 
PLCP presents a common interface for higher-level drivers to write to 
and provides carrier sense and CCA (Clear Channel Assessment), which is 
the signal that the MAC (Media Access Control) layer needs so it can 
determine whether the medium is currently in use.

  The PLCP consists of a 144 bits preamble that is used for 
synchronization to determine radio gain and to establish CCA. The 
preamble comprises 128 bits of synchronization, followed by a 16 bits 
field consisting of the pattern 1111001110100000. This sequence is used 
to mark the start of every frame and is called the SFD (Start Frame 
Delimiter).

  The next 48 bits are collectively known as the PLCP header. The header 
contains four fields: signal, service, length and HEC (header error 
check). The signal field indicates how fast the payload will be 
transmitted (1, 2, 5.5 or 11 Mbps). The service field is reserved for 
future use. The length field indicates the length of the ensuing 
payload, and the HEC is a 16 bits CRC of the 48 bits header.

In a wireless environment, the PLCP is always transmitted at 1 Mbps. 
Thus, 24 bytes of each packet are sent at 1 Mbps. The PLCP introduces 
24 bytes of overhead into each wireless Ethernet packet before we even 
start talking about where the packet is going. Ethernet introduces only 
8 bytes of data. Because the 192 bits header payload is transmitted at 
1 Mbps, 802.11b is at best only 85 percent efficient at the physical 
layer.

Those 192 bits take 192us to transmit (at 1Mbps).  144us of this is the 
sync field, and you can probably miss half of that and still get a good 
reliable reception.   (There is a short preamble option in 802.11b that 
uses only 56 sync bits, and then sends about half the remaining 
preamble at 2Mbps.)

In other words, all fine and good.  If you're scanning a set of 
antennas, you have a lot of time to determine 'best'.

OFDM (802.11g) is another beast entirely.   There are modes that keep 
the full preamble, but we won't discuss these, because they're covered 
by the cases above.  A pure OFDM preamble consists of 10 short training 
symbols, each of which is 0.8 μs, followed by two long training 
symbols, each of which is 3.2 μs including a 1.6-μs prefix which 
precedes the long training symbol.

Now, at my last job (where I built phased array gear), we did a lot of 
empirical analysis of various 11g receivers using a pretty fancy 
Agilent ESG vector signal generator.  What we found was that no 
commodity 802.11g chipset could stand missing more than three of the 
short training symbols.   Thats 2.4us to scan all of your antennas, 
kids.

This, btw, is why most 802.11g (and 802.11a) gear won't support fast 
(per packet) switched receive diversity.  :-)

You could do it with a parallel receiver architecture, but then you 
might as well co-phase and combine, and have a "Real Smart Antenna 
System" (tm).   Since the receiver grabs the lions share of the gates 
in an 802.11x chip(set), it will be a while before these appear at 
Fry's.

Jim

On May 20, 2004, at 6:40 PM, Mike Outmesguine wrote:

> Well, I heard back.  Nice folks over there at Airgain.
>
> The product is about 4-6 weeks away from complete.  It doesn't have an 
> FCC
> cert.  And its not clear they will even get one.  Their model is to 
> provide
> these as OEM units for built-in systems.
>
> What surprised me is that it's actually quite small.  (I didn't look 
> at the
> dimensions first. :)  The unit is about the size of a large coke from 
> Carls
> Jr. - 3 inches square and 8 inches tall.
>
> It's designed to be a client-side bridge.  Not an access point like 
> Vivato.
>
> And while pricing was elusive as features are being refined, they are
> shooting for a consumer price target.  So, expect around $100. (Yes,
> dollar-sign-one-hundred.)
>
> In the Cerritos environment they are planning use as a customer 
> premises
> antenna.  They stated that the phased-array will pick up signals even 
> in
> multi-path environments.  Increasing gain in non-line-of-sight 
> installs,
> which are common in community wireless networks.
>
> As far as the way it works, I didn't learn much.  Except that it 
> switches
> through the 8, 9, or 10 antennas very quickly ("milliseconds" was the 
> term.
> But I hope it's not 1000 milliseconds.)  When a signal is found on the 
> edge
> between two panels, it will use both antennas.
>
> They are doing some demos and report good results (grain of salt.)  I 
> am
> hoping to get an eval and/or OEM unit when they become available.
>
> That's what I heard!
>
> -Mike O.
>
>
>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On 
> Behalf
> Of Mike Outmesguine
> Sent: Thursday, May 20, 2004 11:10 AM
> To: [EMAIL PROTECTED]
> Subject: RE: [SOCALWUG] Phased Array WiFi Antennas
>
> I'm waiting a callback from engineering or sales.  But I agree with the
> "switched antenna" theory.
>
> It looks like a great design - or plan anyway.  I think the way it 
> works is
> that it detects and transmits only on the panel on which the signal is 
> being
> received.  So, all transmit energy only goes in the direction of the 
> laptop.
>
>
> In the case of their "omni" tower antenna, it looks like there are at 
> least
> 8 different radiation patterns.  That would be like having 8 
> narrow-beam
> directional antennas on a pole.  The effect makes this a high-gain
> directional antenna that automatically adjusts direction 360 degrees 
> around
> the tower.  Neat.
>
> Comparing to Vivato is difficult - except that they both call their 
> antennas
> phased arrays.  Vivato is a 180 degree pattern and probably has the 
> same net
> effect.  That is, beaming the transmit energy towards the receiving 
> antenna
> instead of in a wide pattern.
>
> I'll send another message when I hear back.  Though it's always hard 
> getting
> pricing info unless the checkbook is open!
>
> -Mike O.
>
>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On 
> Behalf
> Of Jim Thompson
> Sent: Thursday, May 20, 2004 10:50 AM
> To: [EMAIL PROTECTED]
> Subject: Re: [SOCALWUG] Phased Array WiFi Antennas
>
>
> I'm still trying to determine what provides the co-phasing.
>
> At first blush, it looks like a switched antenna system to me.  Also,
> the gain ("10dBi over an omni with the same pattern"?) is fairly low.
>
> On May 20, 2004, at 10:06 AM, Ronan Higgins, Cafe.com wrote:
>
>> Frank,
>>
>> Have you tested these in the field?
>> How do they compare with other phased array systems such as Vivato?
>> How much do they cost?
>>
>>
>>
>> Ronan.
>>
>> -----Original Message-----
>> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
>> Behalf Of Frank Keeney
>> Sent: Thursday, May 20, 2004 1:25 AM
>> To: [EMAIL PROTECTED]
>> Subject: [SOCALWUG] Phased Array WiFi Antennas
>>
>>
>>
>> fyi
>>
>> "Based on phased array radar principles developed for the military,
>> Airgain'
>> s smart antenna solutions use patented antenna steering technology to
>> direct
>> the beam towards any active wireless device. Each smart antenna is
>> capable
>> of continuously monitoring network signal strength...."
>>
>> Nice photos and diagrams:
>>
>> http://www.airgain.com./AirgainAntennaConfigDS0304.pdf
>>
>> http://www.airgain.com./AirgainWirelessBridgeDS0304.pdf
>>
>>
>>
>>
>>
>> Frank Keeney
>> Tel: 888-259-5110 x0
>> Pasadena Networks, LLC
>> Wireless Antennas, Cables and Equipment:
>> http://www.wlanparts.com
>>
>>
>>