antenna diversity in LAP1242AG?

[niklas]

Is the antenna diversity functionality in the LAP1242AG described somewhere? I have checked that both antenna ports are used for both transmit and receive. Eg is there a memory that remembers what port a client was best heard/receieved on? What happens with mobile client in a multi path environment. Note: I plan to use dual polarised antennas.

[Rob Huffman]

Hi Niklas,

Here is some info on the use of Diversity Antennas;

The purpose of diversity is to overcome multipath reflections. Diversity antennas that share the same physical housing are placed at an optimum distance apart. The maker of the particular antenna determines that distance based on the characteristics of the antenna. When you use a pair of antennas with matching characteristics to provide diversity for cell coverage in your facility, the guideline is to put those matched antennas at a distance apart from each other that is equal to a multiple of the wavelength of the frequency that is being transmitted. The 2.4 GHz wavelength is approximately 4.92 inches. Therefore, to support diversity on a 2.4 GHz radio with two separate antennas, the antennas should be spaced approximately 5 inches apart. The antenna pair could also be spaced at multiples of 5 inches, but the distance between should not exceed 4 multiples: reflected waves farther apart than that are likely to be so distorted and different in delay spread that the radio could not work with them.Because each antenna is selected by itself, both antennas must have the same radiation characteristics and be positioned to provide similar cell coverage. Two antennas connected to the same access point must not be used to cover two different cells.

From this good doc;

Multipath and Diversity

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml

"You can relate this to a common occurrence in your car. As you pull up to a stop, you may notice static on the radio. But as you move forward a few inches or feet, the station starts to come in more clearly. By rolling forward, you move the antenna slightly, out of the point where the multiple signals converge.

A diversity antenna system can be compared to a switch that selects one antenna or another, never both at the same time. The radio in receive mode will continually switch between antennas listening for a valid radio packet. After the beginning sync of a valid packet is heard, the radio will evaluate the sync signal of the packet on one antenna, then switch to the other antenna and evaluate. Then the radio will select the best antenna and use only that antenna for the remaining portion of that packet.

On transmit, the radio will select the same antenna it used the last time it communicated to that given radio. If a packet fails, it will switch to the other antenna and retry the packet.

One caution with diversity, it is not designed for using two antennas covering two different coverage cells. The problem in using it this way is that, if antenna no. 1 is communicating to device no. 1 while device no. 2 (which is in the antenna no. 2 cell) tries to communicate, antenna no. 2 is not connected (due to the position of the switch), and the communication fails. Diversity antennas should cover the same area from only a slightly different location."

From this Antenna reference guide;

http://www.cisco.com/en/US/products/hw/wireless/ps469/products_data_sheet09186a008008883b.html

Here is the best description of why to use Diversity Antennas (in laymans terms) from a great NetPro ScottMac. When I read his reference to being at a stadium watching a sporting event, this all finally made sense.Have a look at his great explanation;

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1dd7905b/6#selected_message

Hope this helps!

Rob

[niklas]

Rob, thanks for a very good reply on antenna diversity. However, I don't quite buy it all... :-) Let me explain;

You say that the AP listens on both antennas for the pre-ample. That is, the AP switches through both antenna ports and decides which has the best signal and chooses that one to receive the packet. Using long preample should help in this decision even if it takes a bit longer (less throughput), right?

The Cisco documentation says that the APs are designed for spatial diversity. I don't buy that 1 wavelength (122 mm) is superior for spatial diversity. The whole concept of antenna diversity is to orthogonalize the two antennas as much as possible, ie increasing the odds of having good signal in one antenna if the other has a "null" or other reason for multipath signal loss. There should be substantial gain from moving the antennas farther apart, say 10 wavelengths..?

Also, what is not really expressed clearly is that the OFDM modulation used in IEEE 802.11a and 802.11g gives a great advantage in multipath environments. Even if some subcarriers are "lost", there will be redundant information in other carriers (to some degree of course). The more reason to use orthogonalized antennas..?

Finally, when space is limited, the best way should be to use dual polarized antennas. Does Cisco have any documentation on that?

Thanks!

-niklas-