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Technology Stocks : LAST MILE TECHNOLOGIES - Let's Discuss Them Here -- Ignore unavailable to you. Want to Upgrade?

To: Lance Bredvold who wrote (9924)	12/25/2000 3:31:08 PM
From: axial	Respond to of 12823

Hi, Lance - A pothole on the road to learning? I'm trying to grasp a lot of this stuff: books on networks, so I can at least understand what Frank and Justone are saying, and books on wireless, so I can understand what everyone else is saying. Your points were well taken, so I went back to my sources: my notes, and to the source text, an IEEE book by Raj Pandya: Mobile and Personal Communications Systems and Services. There it is: Chapter 3, Digital Mobile Cellular Systems, Sec. 3.5: IS - 95: The North American CDMA Digital Cellular Standard 3.5.5.5: Soft Capacity...page 84 'In the present U.S. cellular environment, each operator can deploy 12.5 MHz of spectrum, which provides 57 analog channels in a three-sector cell site. When demand for service is at a peak, the 58th request for a channel must be blocked and given the all-channels-busy signal. The CDMA systems, however, offer much softer relationship between the number of users and the (transmission) grade of service. In other words, an operator has the flexibility to admit additional users during peak periods by providing a somewhat degraded grade of service (increased bit error rates). This capability is especially important when calls might be dropped during handoff because of a lack of available free channels.' However, before that, in 3.5.5.4, page 83, he says (I can't get the formulas in here...) 'IS-95 CDMA system Capacity. The key parameters that determine the capacity of a CDMA digital cellular system are as follows: - processing gain (ratio of spreading code to information data rate) - ratio of energy per bit to noise - voice activity factor - frequency reuse efficiency - number of sectors in the cell-site antenna The theoretical capacity of the IS - 95 CDMA cellular system in terms of calls per 1.25 MHz channel per cell is provided by... [omitted: formulas and explanations...] ...The theoretical estimated capacity of the IS - 95 CDMA system is therefore in the order of 128 calls per 1.25 MHz channel per cell. The theoretical capacity figure above assumes perfect power control to counter interference from other mobiles and neglects the effect of thermal noise. In practice the system is operated at maximum capacity loading such that about half the receiver noise is from mutual interference, and the other half is from thermal noise, which reduces the theoretical capacity above to approximately half (i.e., to 64 calls per 1.25 MHz channel per cell). So, how do we reconcile the first quote with the second, and the 1.25 MHz figure with the 12.5 MHz spectrum allocation? My guess, and I welcome correction: 12.5 MHz is the aggregate spectrum allocation. CDMA was converted from AMPS in 1.25 MHz allotments. ' In the United States there is also a problem of transition. Most cellular carriers will add CDMA service to existing AMPS systems. With IS-95A, the approach to transition is to convert the spectrum in 1.25 MHz segments. The operator would turn off the AMPS radios in about 1.8 MHz (the 1.25 MHz bandwidth of a single CDMA channel, plus guard bands).' amug.org As you pointed out, Lance, 10 allotments of 1.25 MHz spectrum equals Pandya's quoted figure of 12.5 MHz. I would also like to better understand the question of channel aggregation for data transmission, which as you pointed out again, appears to resemble TDMA variants. But it's Christmas day, so I'll postpone that for a bit. Probably, by the time I have all this stuff figured out, they'll have 4G in Korea. >smile< Best wishes, Jim