"John R. Covert 08-Apr-1990 1031" <covert@covert.enet.dec.com> (04/08/90)
Re: Al Ginbey's reply concerning Cellular channel capacity: >The specific limit and the method used in the detection and use of the >next available channel differs by city/system. I believe the limit of >U.S. West in the Omaha area is 10 channels. The next available >channel is marked with a tone. You're describing the old IMTS (non-cellular) mobile system. One of the many major advantages of cellular technology is a drastic increase in channel capacity. U.S. West informs me that Omaha had three cell sites as of last November, and may have a few more by now. I am certain that the channel capacity of _each_ of these sites is at least 12 channels, and more likely is two to four times that. In larger cities, each of the two carriers has between fifty and one-hundred cell sites, with each carrier planning the addition of new sites in 1990 at the rate of about two per month. The FCC has allocated 832 channels for use in cellular systems, although few cities have expanded their systems beyond the 666 channels initially allocated. This spectrum is divided in half, with the "A" and "B" carriers each receiving half the channels. Each channel is a duplex channel using separate frequencies for transmit (from the cellular phone) and receive (at the cellular phone). In the initial channel allocation, channels were numbered 1-666. The "A" carriers had 1-333 and the "B" carriers had 334-666. In the 832 channel system, the additional channels are numbered 667-799 and 991-1023. The 33 channels from 991-1023 are allocated _below_ channel 1 in frequency. Channels 800-990 are not assigned. I'm not sure exactly how the 166 additional channels were allocated by carrier, but each carrier received 83 additional channels for a total of 416. The following formulas compute the phone's transmit and receive freqs: receive_freq = (if channel<991 then 870.030 MHz else 869.04) !chan 1/991 + 30kHz x (channel - 1 or 991) transmit_freq= (if channel<991 then 825.030 MHz else 824.04) !chan 1/991 + 30kHz x (channel - 1 or 991) A cellular phone scans for the strongest set-up channel (334-353 on the "B" carrier and 333-314 on the "A" carrier). This channel transmits a continuous 19.2 kbps data stream containing information such as the system ID (a 16-bit number), sign-in requirements, incoming call requests, and initial channel assignments for each call. Cellular phones transmit on the set-up channel using a contention protocol when they want to initiate an outgoing call or accept an incoming call. The cell site then sends a message in the data-stream to tell the cellular phone which channel it should switch to for processing the call. Further channel switch requests or power assignments during the call are sent to the phone on the same channel as is being used for the voice connection (thus not every blip you hear while using a cellular phone is a cell switch; many of them are commands to increase or decrease transmit power). The maximum channel capacity in any system will depend on the actual engineering requirements of that system, determined by the terrain, the cell placement, and marketing considerations. The theoretical maximum capacity of a single cell in a fully built-out system of honeycomb-shaped cells over perfectly flat terrain would be one seventh the total capacity available to each carrier, or about 56 channels per cell (after removing the set-up channels from the calculation). Cell size can be made almost arbitrarily small, since transmit power can be limited by command from the cell site to as little as 4.8 milliwatts measured at the antenna connector. In practice, cell sites tend to have either less than or more than the number above. The system must be designed so that co-channel interference is held to acceptable minimums. The terrain and placement of each cell will determine in which nearby cell it first becomes reasonable to re-use a frequency used in some other cell. Determination of the number of customers to accept requires a traffic analysis considering the local market data. People in Los Angeles spend more time in their cars than people in New York; thus the amount of traffic each customer offers to the network is greater. On the other hand, people in Hong Kong carry portable phones and use them while walking down the street and while eating in restaurants, because the system is well-designed for portables, the cost is less than 16 cents per minute, and fewer people have cars. It should be obvious that a reduction in the cost of making cellular phone calls in an existing system without an increase in the associated channel capacity will quickly affect the system loading. It should also be obvious that a lower call completion rate may be more acceptable in some countries than in others. For example, in Germany, where it is often necessary to redial several times to complete a normal land-line call from Stuttgart to Munich, customers will be more willing to retry calls to cellular phones, put up with recordings announcing that the call is in a holding queue, or accept a time limitation on the length of calls. /john