Article written by John Scourias, with comments in maroon by Tom Farley
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4.6 Discontinuous transmission
Minimizing co-channel interference is a goal in any cellular system, since it allows better service for a given cell size, or the use of smaller cells, thus increasing the overall capacity of the system. Discontinuous transmission (DTX) is a method that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation [22], by turning the transmitter off during silence periods. An added benefit of DTX is that power is conserved at the mobile unit.
The most important component of DTX is, of course, Voice Activity Detection. It must distinguish between voice and noise inputs, a task that is not as trivial as it appears, considering background noise. If a voice signal is misinterpreted as noise, the transmitter is turned off and a very annoying effect called clipping is heard at the receiving end. If, on the other hand, noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased. Another factor to consider is that when the transmitter is turned off, there is total silence heard at the receiving end, due to the digital nature of GSM. To assure the receiver that the connection is not dead, comfort noise is created at the receiving end by trying to match the characteristics of the transmitting end's background noise.
Levine (link to his cellular .pdf file) says that Voice Activity Detection or VAD is the 'gimmick" that enables greater call capacity in CDMA based (IS-95) systems. Not anything special with CDMA. I will let the experts argue that point. The clipping that John mentions is just the thing that makes digital audio generally inferior to analog. Analog audio quality, where a signal mereley fades instead of cutting out, almost always sounds better than digital.
The chief benefit of TDMA to cellular operators is increasing call capacity by multiplexing. With GSM and conventional cellular you put eight calls on a frequency pair compared to one call per pair with analog. But increased capacity does not necessarily benefit the callers, since most digital routines play havoc with voice quality. An uncompressed, non-multiplexed, bandwidth hogging analog signal simply sounds better than its present day compressed, digital counterpart. As Consumers Digest put 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." Consumers Digest, August, 2000.
One more thing to think about when considering digital, is that a digital signal increases bandwidth compared to analog. It is only compression that makes digital comparable in bandwidth to analog. As Fike says:
"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)." Fike, John L. and George Friend, Understanding Telephone Electronics SAMS, Carmel 1983. p. 164
I write more about this here.
4.7 Discontinuous reception
Another method used to conserve power at the mobile station is discontinuous reception. The paging channel, used by the base station to signal an incoming call, is structured into sub-channels. Each mobile station needs to listen only to its own sub-channel. In the time between successive paging sub-channels, the mobile can go into sleep mode, when almost no power is used.
All of this increases battery life considerably when compared to analog phones.
4.8 Power control
There are five classes of mobile stations defined, according to their peak transmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co-channel interference and to conserve power, both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality. Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm (20 milliwatts).
We need only enough power to make a connection. Any more is superfluous. If you can't make a connection using one watt then two watts won't help at these near microwave frequencies. Using less power means less interference or congestion among all the mobiles in a cell.
The mobile station measures the signal strength or signal quality (based on the Bit Error Ratio), and passes the information to the Base Station Controller, which ultimately decides if and when the power level should be changed. Power control should be handled carefully, since there is the possibility of instability. This arises from having mobiles in co-channel cells alternatingly increase their power in response to increased co-channel interference caused by the other mobile increasing its power. This in unlikely to occur in practice but it is (or was as of 1991) under study.
Two points. The first is that the base station can reach out to the mobile and turn down the transmitting power the handset is using. Very cool. The second point is that a digital signal will drop a call much more quickly than an analog signal. With an analog radio you can hear through static and fading. But with a digital radio the connection will be dropped, just like your landline modem, when too many 0s and 1s go missing. You need more base stations, consequently, to provide the same coverage as analog
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