Date: Sun, 18 Aug 2002 11:07:40 +0100 (BST) From: Duncan Barclay <dmlb@dmlb.org> To: Evren Yurtesen <eyurtese@turkuamk.fi> Cc: mobile@FreeBSD.ORG, "M. Warner Losh" <imp@bsdimp.com> Subject: Re: (2) OpenAP+ IBSS and BSS with multiple FreeBSD Wireless Gate Message-ID: <XFMail.20020818110740.dmlb@computer.my.domain> In-Reply-To: <Pine.A41.4.10.10208181153210.88330-100000@bessel.tekniikka.turkuamk.fi>
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On 18-Aug-2002 Evren Yurtesen wrote: > Well it seems there is no way but getting this OpenAP compatible wireless > access points. > > I was also wondering about the pc cards with multiple antenna. Do they > share the same circuit or they are seperate? Can they work at the same > time? If they are not different circuit. Does it lower the transmit power > since we would be transmitting to 2 antennas at the same time? > > Evren Cards with two antennas have them to implement a radio technique called antenna diversity. Put simply, the two antennas are arranged to see slightly different versions of the radio signals (through pointing is slightly different directions or by simply being placed in different places). This technique is used to get round a radio problem called multipath fading; you are likely to have experienced this when listening to FM radio in the car. The signal strength can go up and down in a very short distance because of constructive/destructive mutiple reflections (I often notice this sitting in traffic, and moving the car a metre or two gives me a better signal). The use of the diversity in most 802.11 cards is this. For receive, the receiver takes a signal strength measurement on antenna A and then on antenna B (in about 5us). This occurs during a preamble to the data pacaket. The radio then chooses the strongest antenna to receive the whole packet with. For transmit, the radio will transmit a packet out on antenna A. If it does not receive the ACK from the destination station, it will retry (as specified in 802.11) but now it chooses antenna B. A simple cache can be used to improve the chance that a transmitted packet get through. Physically in the radio, there is one receiver, one transmitter and a little switch that routes the input/output of the radio to the desired antenna. Note, this is true for 802.11b systems - 802.11a may use two receivers becuase the time available to make the comparative measurements is very short. Duncan -- ________________________________________________________________________ Duncan Barclay | God smiles upon the little children, dmlb@dmlb.org | the alcoholics, and the permanently stoned. dmlb@freebsd.org| Steven King To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-mobile" in the body of the message
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