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(Page 8) Cellular Telephone Basics continued . . .
Advanced features depend on digital but conserving bandwidth does not. How's that? Three conversations get handled on a single frequency. Call capacity increases. But is that a virtue of digital? No, it is a virtue of multiplexing. A digital signal does not automatically mean less bandwidth, in fact, it means more. [See more bandwidth] Multiplexing means transmitting multiple conversations on the same frequency at once. In this case, small parts of three conversations get sent almost simultaneously. This is not the same as NAMPS, which splits the frequency band into three discrete sub- frequencies of 10khz apiece. TDMA uses the whole frequency to transmit while NAMPS does not.
This is a good place to pause now that we are talking about digital. AMPS is a hybrid system, combing digital signaling on the setup channels and on the voice channel when it uses blank and burst. As well as tones to keep it on frequency and help it find a vacant channel. Voice traffic, though, is analog. That's AMPS. But IS-54, now folded into IS-136, is all digital. That's because it uses digital on its set-up channels, the same radio frequencies that AMPS uses, and all digital signaling on the voice channel. TDMA, GSM, PCS , and CDMA cellular are all digital. Let's look at some TDMA basics. But before we do, let me mention one thing.
More on digital: http://www.privateline.tv/PCS/Multiplexing.htm I wrote in passing about how increasing call capacity was the chief benefit of TDMA to cellular operators. But it is not necessarily of benefit to the caller, since most new digital routines play havoc with voice quality. An uncompressed, non-multiplexed, bandwidth hogging analog signal simply sounds better than its present day digital counterpart. If you have listened to talk radio you know what I mean, as one cell phone caller after another presents their version of digital noise. As the August, 2000 Consumers Digest puts it,
"Digital cellular service does have a couple of drawbacks, the most important of which is audio quality. Analog cellular phones sound worlds better. Many folks have commented on what we call the 'Flipper Effect." It refers to the sound of your voice taking on an 'underwater-like' quality with many digital phones. In poor signal areas or when cell sites are struggling with high call volume, digital phones will often lose full-duplex capability (the ability of both parties to talk simultaneously), and your voice may break up and sound garbled."
Getting back to our narrative, and to review, we see that going digital doesn't mean anything special. A multiplexed digital signal is what is key. Each frequency gets divided into six repeating time slots or frames. Two slots in each frame get assigned for each call. An empty slot serves as a guard space. This may sound esoteric but it is not. Time division multiplexing is a proven technology. It's the basis for T1, still the backbone of digital transmission in this country. Using this method, a T1 line can carry 24 separate phone lines into your house or business with just an extra twisted pair. Demultiplexing those conversations is no more difficult than adding the right circuit board to a personal computer. TDMA is a little different than TDM but it does have a long history in satellite working.
What is important to understand is that the system synchronizes each mobile with a master clock when a phone initiates or receives a call. It assigns a specific time slot for that call to use during the conversation. Think of a circus carousel and three groups of kids waiting for a ride. The horses represent a time slot. Let's say there are eight horses on the carousel. Each group of kids gets told to jump on a different colored horse when it comes around. One group rides a red horse, one rides a white one and the other one rides a black horse. They ride the carousel until they get off at a designated point. Now, if our kids were orderly, you'd see three lines of children descending on the carousel with one line of kids moving away. In the case of TDMA, one revolution of the ride might represent one frame. This precisely synchronized system keeps everyone's call in order. This synchronization continues throughout the call. Timing information is in every frame. Any digital scheme, though, is no circus. The actual complexity of these systems is daunting. You should you read further if you are interested.
Take a look into frames There are variations of TDMA. The only one that I am aware of in America is E-TDMA. It is or was operated in Mobile, Alabama by Bell South. Hughes Network Systems developed this E-TDMA or Enhanced TDMA. It runs on their equipment. Hughes developed much of their expertise in this area with satellites. E-TDMA seems to be a dynamic system. Slots get assigned a frame position as needed. Let's say that you are listening to your wife or a girlfriend. She's doing all the talking because you've forgotten her birthday. Again. Your transmit path is open but it's not doing much. As I understand it, "digital speech interpolation" or DSI stuffs the frame that your call would normally use with other bits from other calls. In other words, it fills in the quiet spaces in your call with other information. DSI kicks in when your signal level drops to a pre-determined level. Call capacity gets increased over normal TDMA. This trick had been limited before to very high density telephone trunks passing traffic between toll offices. Their system also uses half rate vocoders, advanced speech compression equipment that can double the amount of calls carried.
Before we turn to another multiplexing scheme, CDMA, let's consider how a digital cellular phone determines how to choose a digital channel and not an analog one. Perhaps I should have covered that before this section, but you may know enough terminology to understand what Mark van der Hoek has to say:
"The AMPS system control channel has a bit in its data stream which is called the 'Extended Protocol Bit.' This was designed in by Bell Labs to facilitate unknown future enhancements. It is used by both CDMA and TDMA 800 MHz systems."
"When a dual mode phone (TDMA or CDMA and AMPS) first powers up, it goes through a self check, then starts scanning the 21 control or setup channels, the same as an AMPS only phone. Like you've described before.When it locks on, it looks for what's called an Extended Protocol Bit within that data stream If it is low, it stays in AMPS. If that bit is high, the phone goes looking for digital service, according to an established routine. That routine is obviously different for CDMA and TDMA.
'TDMA phones then tune to one of the RF channels that has been set up by the carrier as a TDMA channel.Within that TDMA channel data stream is found blocks of control information interspersed in a carefully defined sequence with voice data. Some of these blocks are designated as the access or control channel for TDMA. This logical or data channel, a term brought in from the computer side, constitutes the access channel."
I know this is hard to follow. Although I don't have a graphic of the digital control channel in IS-54, you can get an idea of a data stream by going here. "Remember, the term 'channel' may refer to a pair of radio frequencies or to a particular segment of data. When data is involved it constitutes the 'logical channel'.' In TDMA, the sequence differentiates a number of logical channels. This different use of the same term channel, at once for radio frequencies and at the same time for blocks of data information, accounts for many reader's confusion. By comparison, in CDMA everything is on the same RF channel. No setting up on one radio frequency channel and then moving off to another. Within the one radio frequency channel we have traffic (voice) channels, access channels, and sync channels, differentiated by Walsh code."
Let's now look at CDMA. please see next page-->
Notes
[More bandwidth] "The most noticeable disadvantage that is directly associated with digital systems is the additional bandwidth necessary to carry the digital signal as opposed to its analog counterpart. A standard T1 transmission link carrying a DS-1 signal transmits 24 voice channels of about 4kHz each. The digital transmission rate on the link is 1.544 Mbps, and the bandwidth re-quired is about 772 kHz. Since only 96 kHz would be required to carry 24 analog channels (4khz x 24 channels), about eight times as much bandwidth is required to carry the digitally (722kHz / 96 = 8.04). The extra bandwidth is effectively traded for the lower signal to noise ratio." Fike, John L. and George Friend, UnderstandingTelephone Electronics SAMS, Carmel 1983 (back to text)
[TDMA] There's a wealth of general information on TDMA available. You won't have any problem looking it up, especially on the internet These books have snippets of information: Macario, Raymond Cellular Radio: Principles and Design McGraw Hill, Inc., New York 1993 161; Myers, Robert A. ed., Encyclopedia of Telecommunications Academic Press, Inc. San Diego 1989 321. (back to text)
