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Technology Stocks : Winstar Comm. (WCII) -- Ignore unavailable to you. Want to Upgrade?

To: limtex who wrote (5772)	5/6/1998 10:04:00 PM
From: Bernard Levy	Read Replies (1) \| Respond to of 12468

limtex: I will try to find you a good book on wireless. Ira Brodsky has a few wireless books which are addressed to the general public, and which may contain what you need (will try to be more specific soon). I won't try to respond to all your questions at once, but the one regarding TDMA and CDMA is simple(you would need also to throw in frequency division multiple access--FDMA, and space division multiple access --SDMA for good measure). All concern how lots of users can share a set of frequencies to send separate messages without having all these messages mixed up into an unrecognizable mess. FDMA is the simplest and oldest: it divides the available frequency spectrum into smaller frequency bands and says: user A has the first band, user B has the second band, etc... This is the way AM and FM radio stations can be separated. TDMA does the same thing in the time domain: it divides time into fixed intervals, and each interval is divided into slots. During the first slot, user A can transmit, and the other users are silent, then in the second slot, it is the turn of user B, and so on. At the end of an interval, it is time to restart: user A gets his/her turn again, and so on. CDMA is more complex, since all users transmit information all the time and use the entire spectrum available. The trick is that each user has a characteristic spreading sequence (a key if you want), and superimposes the information he/she wants to transmit on this sequence. The sequences used by different users are orthogonal (or almost orthogonal) to one another, which means that at the receiver, when the superimposed signal is multiplied by a spreading sequence, only the message associated to this sequence is received. SDMA is newer and uses the direction from which the signals com from to separate users. Typically, it is used as a way of improving TDMA or CDMA. Right now WCII does not use either TDMA or CDMA, since it is based on point to point technology: an antenna at the base station sends a signal directly in the direction of a receiving antenna on the roof of a customer. In some sense, it is a mix of SDMA and FDMA (different frequency bands can also be used to serve different customers). All this will change with the rollout of Nortel's point to multipoint technology, since it includes a wireless ATM protocol, which I believe is TDMA based (I have not yet done my homework on Nortel's equipment). Regarding accounting versus electrical engineering, keep in mind that the grass is often greener on the other side. However, I must admit that the last 20 years have been rather exciting for EEs. Because of the microelectronics revolution, all sorts of things have become possible which could only be dreamt of in the past. For example, when the first RAKE receiver (used to combine multipath signals in CDMA) was built at Lincoln Labs in the late 50s (early 60s?), it required a whole room. Now, you will find it in each of QCOM's handsets. Best wishes, Bernard Levy

To: limtex who wrote (5772)	5/6/1998 11:36:00 PM
From: wonk	Read Replies (1) \| Respond to of 12468

Limtex: Regarding question number 4(F) . "On account of the basic interaction of electromagnetic waves with water in liquid form, raindrops cause absorption, scattering and depolarization phenomena. The first two result in signal attenuation (not to be confused with either water vapor attenuation or attenuation by rain clouds) and increase in sky noise temperature. These effects are quite noticeable above 10 GHz..." Source: Reference Data for Engineers: Radio, Electronics, Computer and Communications. (Page 27-34) Eighth Edition. Sams Publishing (1995) This book is similar to the Physicians Desk Reference. While much of the material goes rapidly out of date with the continuing advances in state of the art, it is still well worth having for the foundation information alone. The actual amount of attenuation (reduction in signal strength) caused by rain depends upon the frequency and the rain rate. You can make two generalizations: (1) the higher the frequency, the more attenuation and (2) the harder the rain the more attenuation. Point number two trips people up at times. For example, in which city would a microwave path be more effected by rain, Seattle or Dallas? Most people would say Seattle. However, the answer is Dallas because while it does not get as much annually as Seattle, the rain is much harder. (thunderstorm downpours versus constant drizzles). There is no magic to calculating this for any city. (the equations and graphs are in the above book). In the US, we've been collecting rain data for over 100 years and the attenuation effect by frequency and rain rate are known. The foregoing has large impact on valuation for P-MP broadband wireless. However, that's a post for another day (if I haven't already put everyone to sleep.) ww