Path: sparky!uunet!lll-winken!telecom-request From: das...@rpi.edu (Anindadeb Vijaykumar Dasgupta) Newsgroups: comp.dcom.telecom Subject: Digital Cellular Telephony Message-ID: <telecom12.17.6@eecs.nwu.edu> Date: 7 Jan 92 18:08:34 GMT Sender: Tele...@eecs.nwu.edu Organization: Rensselaer Polytechnic Institute, Troy NY Lines: 19 Approved: Tele...@eecs.nwu.edu X-Submissions-To: tele...@eecs.nwu.edu X-Administrivia-To: telecom-requ...@eecs.nwu.edu X-Telecom-Digest: Volume 12, Issue 17, Message 6 of 8 I read somewhere that in most large cities cellular carriers are switching to digital systems due to saturation of existing cells. I couldn't tell what the advantages of this would be: With analog transmission, each equipment would need 4 KHz. while with 8 bit PCM, 64 Kbps would be needed, which would surely translate to a higher frequency than 8 KHz. This definitely does not free up any bandwidth. Are these carriers using source coding? Won't that make the cellular phones more expensive/bulky? I am not a telecom person and maybe I am missing the point here. Could someone explain the advantages of digital over analog (other than improvement in voice quality, if any) in cellular systems? Thanks in advance. Aninda DasGupta (ani...@networks.ecse.rpi.edu)
Path: sparky!uunet!wupost!uwm.edu!lll-winken!telecom-request From: rdipp...@cancun.qualcomm.com (Ron Dippold) Newsgroups: comp.dcom.telecom Subject: Re: Digital Cellular Telephony Message-ID: <telecom12.21.2@eecs.nwu.edu> Date: 9 Jan 92 19:26:43 GMT Sender: Tele...@eecs.nwu.edu Organization: Qualcomm, Inc., San Diego, CA Lines: 83 Approved: Tele...@eecs.nwu.edu X-Submissions-To: tele...@eecs.nwu.edu X-Administrivia-To: telecom-requ...@eecs.nwu.edu X-Telecom-Digest: Volume 12, Issue 21, Message 2 of 12 das...@rpi.edu (Anindadeb Vijaykumar Dasgupta) writes: > I read somewhere that in most large cities cellular carriers are > switching to digital systems due to saturation of existing cells. They're moving towards it, as fast as they can. > I couldn't tell what the advantages of this would be: > With analog transmission, each equipment would need 4 KHz. while with > 8 bit PCM, 64 Kbps would be needed, which would surely translate to a > higher frequency than 8 KHz. This definitely does not free up any > bandwidth. Are these carriers using source coding? Won't that make > the cellular phones more expensive/bulky? Okay, you're assuming that the analog phone is much more efficient in bandwidth than it actually is. In actuality, each phone completely occupies a 30 KHz channel, and each phone is transmitting at thre watts for marginal quality. And it still sounds bad. With digital encoding done correctly (so you can use Viterbi decoding), you need much less power to get your data through (you're just looking for on-off instead of a FM sound waveform). In addition, if you're using CDMA, CDMA doesn't require that you divide up your bandwidth into bandwith wasting 30 KHz chunks. Intead, each phone uses exactly as much bandwidth as it needs. More phones just mean more "noise" (not noise heard by the user, but as in signal to noise ...) For example, if you had 50 users that needed computers, then the analog analagous way to do it would be to buy two mainframes, and let only one user at a time use each computer, even if all they needed to do was simple word processing. The CDMA way would be to buy one computer that was 20 times more powerful and give each user a terminal, so they only use as much of the computer power as they need, and everyone can use it at once. Even better, we use active power control (something we can do because it's digital). The mobile and cell channel elements transmit with only as much power they need. If you're close to the cell, you don't need much power. As you get farther away, it slowly boosts power. Finally, with digital we can (and do) use a variable rate vocoder. In this way CDMA makes use of the Voice Activity Factor of conversation: either of the parties involved is not saying something about 60% of the time. (Interestingly, even when we force the vocodoer to use a maximum rate of half its top rate, it still sounds better than my analog phone). What it works out to is that while an analog phone transmits at three watts, and the cell-site power requirements are truly horrible, CDMA phones transmit in the milliwatt range, and the entire cell site transmits with only as much power as a couple of the radios in the analog cell. Bottom line: We have an officially capacity tested (tests observed by the major companies in the industry) CDMA system in the field that gives a capacity improvement of 10 to 30 times (depending on conditions) over an AMPS system, with better voice quality, better handoffs, and less dropped calls. Efficient use of resources is the key (plus a lot of geniuses in the theory department, and then ignoring those who claimed we couldn't do it). One further advantage: The digital medium is a lot more flexible. When an AMPS phone has to transmit control information, the voice blanks out. With CDMA, we can just vocode at half the normal rate and send the control information in the other half of the frame. Result: undetectable loss of voice quality instead of complete loss of voice. In addition, we can divide the channel between different data sources, so you could send voice and data (from a modem, perhaps) on the same channel. What we can do is limited only by the messages we can think up to send back and forth. Another voice quality advantage: because it's a digital vocoder, rather than sending the analog waveform out, we can more easily do a lot more filtering on it. For example, continuous background noises (such as a car engine or the wind) can be severely reduced. There are further advantages, but that should be enough.
Path: sparky!uunet!spool.mu.edu!telecom-request From: ibbot...@rtsg.mot.com (Craig Ibbotson) Newsgroups: comp.dcom.telecom Subject: Re: Digital Cellular Telephony Message-ID: <telecom12.23.1@eecs.nwu.edu> Date: 9 Jan 92 21:23:47 GMT Article-I.D.: eecs.telecom12.23.1 Sender: Tele...@eecs.nwu.edu Organization: Motorola Inc., Cellular Infrastructure Group, RTSG Lines: 87 Approved: Tele...@eecs.nwu.edu X-Submissions-To: tele...@eecs.nwu.edu X-Administrivia-To: telecom-requ...@eecs.nwu.edu X-Telecom-Digest: Volume 12, Issue 23, Message 1 of 6 das...@rpi.edu (Anindadeb Vijaykumar Dasgupta) writes: > I read somewhere that in most large cities cellular carriers are > switching to digital systems due to saturation of existing cells. I > couldn't tell what the advantages of this would be: > With analog transmission, each equipment would need 4 KHz. while with > 8 bit PCM, 64 Kbps would be needed, which would surely translate to a > higher frequency than 8 KHz. This definitely does not free up any > bandwidth. Are these carriers using source coding? Won't that make > the cellular phones more expensive/bulky? > I am not a telecom person and maybe I am missing the point here. > Could someone explain the advantages of digital over analog (other > than improvement in voice quality, if any) in cellular systems? There are currently two digital cellular proposals. One is TDMA (Time Division Multiple Access) and the other is CDMA (Code Division Multiple Access). TDMA divides the existing 30kHz radio channels into six time slots, yielding three equivalent voice channels. TDMA proponents say this will offer a 3.7 time capacity of existing analog systems. CDMA is a spread-spectrum technology which calls for a channel bandwidth of 1.25 MHz. The entire bandwidth is reused in every cell site, and is shared by a number of customers with different codes. Proponents say it will offer a 20X capacity increase. The main advantages of digital cellular (regardless of technology) are capacity increase and the introduction of data services. It is going to be very interesting to see what happens in the digital cellular market. TDMA was the initial technology of choice, picked over FDMA after lengthy trials here in the US. After TDMA was chosen, however, a company called Qualcomm met with some of the larger cellular vendors and convinced them that CDMA could greatly increase their capacity and provide some additional features. Both technologies are in market trials now, and the initial indications are that both work. The problem here is that different vendors are aligning themselves with different technologies. LA Cellular is promising a cellular system using Ericsson TDMA technology sometime this year. PACTEL and one of the vendors in New York are heavily involved in CDMA. A third technology has been introduced by Motorola, called N-AMPS. N-AMPS stands for Narrowband Amps, and is a "digitally enhanced" analog system. It increases capacity by using 10kHz radio channels instead of 30 kHz radio channels, using the existing analog technology. It also has a digital sub-audio channel which allows data to be sent to the mobile. This technology is intended as a bridge between an analog and digital system. I believe all Motorola mobile and portable cellular phones now shipping are dual mode AMPS/NAMPS. All of these technologies have their pluses and minuses. TDMA is the basis for GSM, which is in commercial service in Europe, so it is a proven player. NAMPS is in commercial service in Japan, and can be used with Micro-TAC LITE sized phones (the other technologies cannot be used with portables at this time). CDMA promises the biggest capacity increase and possibly higher quality handoffs. The big problem I see is that CDMA and TDMA are incompatible; the standards currently call for all mobiles to be dual mode (digitial/analog), but there is no thought at this time for a mobile to support both TDMA and CDMA. For example, if you are an LA Cellular TDMA user and attempt to roam in a CDMA system, you will not be able to use the digital system (you should always be able to go back to standard AMPS service, however, no matter where you are). This has also made it very difficult for manufacturers. Ericsson and Northern Telecom are firmly behind TDMA. Motorola and AT&T are developing CDMA, TDMA and NAMPS. Hughes is pushing E-TDMA, a second generation TDMA system which uses a half-rate vocoder to get an eight-fold increase over today's analog capacities. And Qualcomm is pushing CDMA. One thing should be noted about digital cellular. Everyone is hot about anything digital nowadays. When cellular goes digital, however, you will not see a quality improvement like that from LP records to compact discs. I quote the editor of Cellular Business: "Cellular's digital technology is another beast altogether. It might sound better. But chances are good that the customer won't even notice a difference, except that some manufacturers phones might be bigger. And initially, they might be bulkier, too." (Cellular Business, "Moving Into Digital", supplement to October 1991 issue). Craig Ibbotson, Motorola, Inc. ...uunet!motcid!ibbotsonc Cellular Infrastructure Division, Radio Telephone Systems Group
Path: sparky!uunet!sun-barr!ames!elroy.jpl.nasa.gov!usc!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!dsinc!telecom-request From: r...@rtsg.mot.com (Bernard Rupe) Newsgroups: comp.dcom.telecom Subject: Re: Digital Cellular Telephony Message-ID: <telecom12.29.3@eecs.nwu.edu> Date: 10 Jan 92 15:10:04 GMT Sender: Tele...@eecs.nwu.edu Organization: Motorola Inc., Cellular Infrastructure Group, RTSG Lines: 46 Approved: Tele...@eecs.nwu.edu X-Submissions-To: tele...@eecs.nwu.edu X-Administrivia-To: telecom-requ...@eecs.nwu.edu X-Telecom-Digest: Volume 12, Issue 29, Message 3 of 8 das...@rpi.edu (Anindadeb Vijaykumar Dasgupta) writes: > I read somewhere that in most large cities cellular carriers are > switching to digital systems due to saturation of existing cells. I > couldn't tell what the advantages of this would be: No carriers in the US have a digital system up yet. In fact, the standard is still up in the air, with both TDMA (time division) and CDMA (code division) being talked about. A standard on TDMA is essentially complete. There is, however, a NAMPS (narrow band AMPS) system in operation in Las Vegas. NAMPS uses compression to squeeze three voice channels (10KHz each) into the band one used to occupy (30KHz). This system was developed by Motorola to provide a bridge between current technology and the digital technology of the future (it also costs much less). > With analog transmission, each equipment would need 4 KHz. while with > 8 bit PCM, 64 Kbps would be needed, which would surely translate to a > higher frequency than 8 KHz. This definitely does not free up any > bandwidth. Are these carriers using source coding? Won't that make > the cellular phones more expensive/bulky? In AMPS (current system) each channel uses FM and 30 KHz. TDMA uses digital compression so that three (or possibly more) channels fit into the 30 KHz. CDMA uses a spread spectrum technique to expand capacity 10 or even 20 times current. Trials with both are going on as you read this. The price of the new phones may very well go up. In the case of NAMPS, the phones don't have to be more bulky. Motorola's new MicroTac Lite (7.7 oz) has the added NAMPS capability. In the case of the digital technologies, the answer is yes the phones need to be much more bulky. This is another reason why Motorola introduced NAMPS. > I am not a telecom person and maybe I am missing the point here. > Could someone explain the advantages of digital over analog (other > than improvement in voice quality, if any) in cellular systems? The voice quality should be about the same. The main advantage is that digital signals are much more resistant to noise, so greater range can be obtained and interference can be minimized. Bernie Rupe 1501 W. Shure Drive Room 1315 Motorola, Inc. Arlington Heights, IL 60004 Cellular Infrastructure Group +1 708 632 2814 r...@rtsg.mot.com