Lots of really sharp technical commentary on this list. Maybe there is room for a more basic answer on occasion. (Surely not every subscriber here goes to Stanford! <g>)
True Tx AND Rx time/space diversity is used by a number of brands. As noted though, it is most important for the Rx side. It is simply a means of mitigating multipath -- reflections resulting from signal encountering objects. It gives the radio two listening options, slightly out of phase from the other, to capture the most direct (least reflected) signal. It also prevents any chance of the radio getting caught in a null (dead zone that is basically a standing wave caused by waves that cancel each other out). Patrick J. Leary Chief Evangelist, Alvarion, Inc. Executive Committee Member, WCA/LEA [EMAIL PROTECTED] Ph: 760.494.4717 Cell: 770.331.5849 Fax: 509.479.2374 From: Big Bloke <[EMAIL PROTECTED]> To: [EMAIL PROTECTED] Date: Tuesday, December 10, 2002, 11:12:04 PM Subject: HostAP digest, Vol 1 #61 - 14 msgs ===8<==============Original message text=============== On Mon, 9 Dec 2002 [EMAIL PROTECTED] wrote: Mike Wrote. > I'm trying to understand how the tx/rx diversity works if anyone would > be kind enough to enlighten me about their experiences I would really > appreciate it. Hi Mike, as far as I know, (If I am wrong someone please correct me) TX diversity is not used in WLAN. The idea of RX diversity is to combat "Phasing" . If you have ever listened to FM car radio under weak signal conditions you may have heard "flutter" on the signal. Diversity is a method of countering that effect. When signals arrive at the receive antenna, they may do so from a number of directions, either direct from the transmitter, or via some object that has reflected the signal. If the signals arrive together "In time" (i.e. in phase) then the two will add together to produce a larger version of the original signal (additive mixing) ,if they arrive at different times, there is a chance that one of the Signals will reduce the other (known as "subtractive mixing" ) and the resulting signal may be considerably weaker. The two receive antennas are spaced a critical distance (a number of wavelengths) apart in such a position, that if a signal is out of phase with the one antenna, it should be in phase with the other. Given that one wavelength is around 13cms then the spacing is quite critical! The Diversity circuit of the receiver will compare the phase difference of the two received signals, delay the more advanced of the two so that it is phased up with the other signal, then combine them together to produce one composite stronger signal, which can be de-modulated more effectively. This technique has been used in the cellphone industry for around 20+ years. IIRC, in a typical 2 antenna Access point setup, one antenna is both TX and RX whereas the other is RX diversity only. > Here is what I have tried so far: > 1 -> Installed 2 Maxrad 10db omni's on standoff arms at about the top of > a 100ft tower spaced 15ft apart horizontally, ran 2 separate Andrew's > 1/2" Andrews Heliax cables to each antenna. WOW! Serious setup ;-)) I thought my Homebrewed 28 Element omni into about 20 foot of LDF 5-50 was "Going some! " , But.... sheesh !!! ;-) The spacing of the two antennas is quite critical, as is the feeder Length , which IIRC needs to be identical for the two antennas. Also, considering the small power levels in use the feeder length is rather long even for LDF/FSJ-450. I calculate you would be losing 5.4 db Minimum climbing that 100Ft Tower ! and 3db represents a 50% reduction in that signal level. Is there anyway you could site the AP at the top of the tower and just run Ethernet up to it ? There is commercial equipment available to provide DC power for the AP over the Ethernet cable and the increase in performance should be significant with only a few feet of heliax between the AP and the Antennas. <Snipped> !! I'm not "quite" sure what you want to achieve with sectorising, I am considering an OTSR solution (Omni Transmit, Sectored Receive) by hacking into the WLAN card. to boost the "uplink" from the client cards. as an alternative. You might like to look at www.wimo.de who produce very good quality "Power Splitters" for 2.4GHz that could be used for sectoring. Any other questions, by all means mail me directly. cheers B B _______________________________________________ HostAP mailing list [EMAIL PROTECTED] http://lists.shmoo.com/mailman/listinfo/hostap ===8<===========End of original message text=========== ---------------------------------------------------------------------------- ---------------------------------------- This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the originator of the message. This footer also confirms that this email message has been scanned for the presence of computer viruses. Any views expressed in this message are those of the individual sender, except where the sender specifies and with authority, states them to be the views of Alvarion Inc. 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