Posted: Sat Jun 23, 2007 23:51 Post subject: Devices with two antennas: How to take advantage?
Hi,
I plan to use two WRT54' or two WHR-HP' in order to provide network access to several different places. The first will be connected to an ADSL modem and have an omni directional antenna (for access by clients). The second will receive and repeat the signal using a directional antenna looking to the first device, and highly directional antennas facing far away places with clients.
For this second device, I ask myself why these devices have two antenna ports and how I can take advantage of this "feature".
In other words: How are these ports used internally ? Is there a single receiver/transmitter switching between the ports to see which signal is bigger (diversity) ? Or are they phased in order to block noise ? Or can they be used simultaneously (two RX/TX) ?
If so, I would connect the antenna looking to the first device on one port, and the directional antenna for the clients to the second port.
If the repeater mode takes too much of the bandwidth, would it make sense to double the number of WLAN devices and split the work into AccessPoint and Bridging ?
Regards,
Klaus. _________________ How am I supposed to know what I think before I hear what I say ?!
Multiple antennas provide receiver and/or transmitter diversity, the principle being that at high frequencies, the RF signal propagation path to one or another antenna will differ due to reflections and other impairments. It's a very old technique.
non-MIMO devices use "switched diversity" This means that the antenna with the best signal is used, for a specific data frame from a given client device.
Switched diversity means that the one receiver is switched between the two antennas in making the decision on best-signal. This happens in a few microseconds.
As to what strategy is to be used in assuring that the "best' antenna is chosen, and if that is done on every frame vs. only when the error rate is high, the IEEE standard doesn't say. Vendors do it differently.
Some products are implemented such that you can use two different antennas, say one inside and one outside (while avoiding high coax cable loss). This changes the coverage, but takes away switched diversity's benefit.
Other product can't do the above, because they park the receiver on a given antenna. They don't "scan" both looking for weak signals. If the client's signal is OK on the OTHER antenna but not the one that the radio is parked on, outa luck. It's inconsisitent from product to product and vintage to vintage.
MIMO, per draft 2 of 802.11n "should" have the means to receive simultaneously on 2+ antennas without switching. Unlike switched diversity, this technique combines the results from 2+ receivers, to reduce the error, again, presuming the RF propagation is better on one antenna than the other. Adding another dimension, MIMO "should" use 2+ frequencies, since propagation may be better on one than the other. All of this adds cost to the product, so the vendors juggle compliance with the IEEE standards and do very misleading advertising.
Posted: Sun Jun 24, 2007 14:49 Post subject: Re: Devices with two antennas: How to take advantage?
Glider wrote:
...would it make sense to double the number of WLAN devices and split the work into AccessPoint and Bridging ?
Regards,
Klaus.
Yes. Do it that way for reliability, full security and full bandwidth.
Wireless ethernet bridge on one antenna/direction and Access point on the other antenna direction.
Steve
Last edited by seaweedsteve on Sun Jun 24, 2007 16:33; edited 1 time in total
Also, with the bridge --> AP approach for repeating, once you have the bridge set up, you can add more APs to it's switch for more directions.
Also - biggest reason you don't want to do this with a single radio: Using two antennas in different directions with a diversity system has been shown to cause problems as you add users in different directions.
If you use a WRT54GL as a bridge (with ethernet connection to a modem), which antenna socket is best for a directional external antenna, or are they both the same?
Is it normal practice to remove the other 'stub' antenna, to concentrate power into the directional external antenna?
My understanding is that the right antenna (facing the router front) is the preferred, but with DD-WRT you have antenna selection anyway. Just choose right antenna for transmit and receive.
Makes sense to remove the 2nd antenna though if it's turned off in the firmware, then it shouldn't matter.
