Cellular Telephony
Cellular telephony (How It Works) requires two channel pairs, one for transmission and one for reception. There are 2 bands for analog cellular (800) Mhz service. The cellular industry is a government-mandated “duopoly”; there are only two 800 Mhz cellular licensees in each of the 734 metropolitan statistical areas (MSAs) and rural service areas (RSAs). One licensee is the “B-side” or “wireline” carrier affiliated with the local telephone company. The other cellular licensee is the “A-side” or independent “nonwireline” carrier. Each licensee is allocated 25 MHz of spectrum in the 800 MHz band.

Analog cellular has 624 voice channels. In Portland, the duopoly cellular providers are AT&T Wireless and Verizon Wireless. By converting 800MHz from analog to digital more subscribers can fit on the band. Nextel uses a nearby band but is not strictly considered either a PCS or cellular provider.

PCS
There are 6 bands in the 1900 Mhz range for PCS. All are 100% digital. The FCC divided the PCS band into six licensing blocks (designated as “A” through “F”) based on Rand McNally major trading areas (MTAs) and basic trading areas (BTAs) for two 30 MHz MTA, one 30 MHz BTA, and three 10 MHz BTA licenses.

The 6 PCS bands (A, B, C, D, E & F) are not all used. The “C” band, for example, was re-auctioned and the D,E and F bands are often available. The FCC’s wireless division has the lastest auction news.

The Air Interface
The air interface defines the modulation scheme.

• GSM is a circuit-switched system that divides each 200kHz channel into eight 25kHz time-slots. GSM operates in the 1.9GHz PCS band in the US.
• CDMA operates in the 800MHz band and 1.9GHz PCS band and supports data transfer speeds of up to 14.4 kbit/s.
• TDMA, used as a digital upgrade for analog cellular, it divides each cellular channel into three time slots, tripling the data capacity from analog technology.
• Cellular Digital Packet Data (CDPD): A wireless data-only packet overlay for the existing analog AMPS network in the U.S., CDPD shunts data over unused intervals in the voice channels. It has a data transfer rate of 19.2 kbit/s.

How it Works
A cell-phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into cells. Each cell phone uses two frequencies per call — a duplex channel — so there are typically 395 voice channels per carrier (the other 42 frequencies are used for control channels). A single cell, in an analog system, uses only one-seventh of the available duplex voice channels (to avoid interfering with neighboring cells. Therefore, each cell has about 56 voice channels available. With TDMA a single channel can carry three times as many calls (about 168) because the channel is simultaneously shared by multiple people. But two different frequencies are still required. Digital cellular prioritizes voice, using circuit switching, and slow data transfer. With EDGE, data rates may increase up to three fold over GPRS using PSK modulation on the same channel spacing. 3G will emphasize data, with packet switching.

3G Evolution
Cellular providers use two frequencies in all their “evolutions” to 3G. Sprint PCS and Verizon Wireless use CDMA, while Nextel is based on iDEN technology. AT&T Wireless, Cingular and VoiceStream are the three TDMA/GSM carriers. To reach 3G status, all three GSM carriers will have to upgrade their networks in stages, from GPRS (general packet radio service) to EDGE (enhanced data for GSM evolution) to the eventual 3G destination, UMTS (universal mobile telecommunications system).

True 3G cellular with 384Kbps or faster speed won’t happen for years in the US. Spectrum isn’t available and cost is high. Currently, Verizon’s Express Network and Sprint’s 2.5G network will deliver 40-60Kbps. AT&T is upgrading it’s GPRS service to EDGE this year while Cingular may spend $4 billion. Cellular data may top out at 60-144Kbps for years to come. The 3G Newsroom has the latest poop.

MMDS Systems
MMDS systems are licensed to operate on sixteen (16) channels - each 6MHz wide - with 200 MHz of spectrum (between 2.5 GHz and 2.7 GHz). It was originally conceived as “wireless cable” but later was allowed to provide two-way data for “wireless internet”. MMDS systems are often deployed in a full 360 configuration, and powered to cover the maximum distance allowed by the FCC (35 mile radius). In practice more than one base station is often required due to interference.

Sprint paid $448.8 million for American Telecasting MMDS licenses covering a potential 10 million households in Denver, Portland, Ore., Seattle and Las Vegas Sprint also got four (6 MHz) ITFS channels in Portland, and other communities. Sprint’s spectrum portfolio includes more than 1,100 channels in 90 markets. Those agreements allow for two-way, cellularized, broadband voice, video and data services.

Other licensed Frequencies
The 2150-2162 MHz are allocated to the original MDS band while the 2.3 GHz, Wireless Communications Service has 30 MHz of spectrum from 2305 to 2320 MHz and 2345 to 2360 MHz. Higher up, the Local Multipoint Distribution Service (LMDS) uses 27.35 - 28.35 GHz, 29.1 - 29.25 GHz and 31.0 - 31.3 GHz freqency bands. Broadband auction bandplans show who’s got what.

Licensed Frequencies More Costly
The fixed cost for the base station infrastructure (building, power, tower, heat, light, etc) can be $1 million and incremental wireless equipment can cost another million. MMDS systems are more expensive than unlicensed 2.4 GHz or 5 GHz systems but they have the advantage of more power and licensed frequencies. Smart antennas are now being applied to the MMDS band to concentrate the signal to multiple, individual users.

Combined with “smart antennas”, MMDS systems have the potential to deliver mobile data at 144Kbps to 1.5Mbps. Indoor receivers, the size of a PDA or even incorporated into a PC Card, have the potential of delivering point-to-multipoint wireless broadband without a truck roll.

By eliminating circuit-switched, dual-frequency overhead of cellular and using a single (6MHz) frequency with a powerful, N-LOS base station, MMDS providers claim they can deliver more cost/effective mobile broadband - “4G”.

Community LANs
Community LANs, using the unlicensed 2.4 and 5 GHz bands, are restricted in power and bandwidth and are more prone to interference since the frequencies are available for anyone. The 2.4 GHz band has “only” 85 MHz while 5 Ghz band has a total of 300 MHz - although only 5.25-5.35 and 5.725-5.825 (200 MHz total) can be used outdoors. The 5.8 GHz band may have the most potential to deliver “backbone” service because more power is allowed (up to 200 watts - EIRP). Combined with “smart antennas” and “mesh” networking, many companies are convinced that the unlicensed 5 GHz will provide the most cost/effective solution for community networks.

Time-Division Multiplexing
Community LANs, based on unlicensed WiFi gear, and MMDS systems, based on licensed frequencies, both use a single channel to communicate - Time Division. This works fine for data but is problematic for voice. With enough speed, however, the two-way voice is possible. Time Division Duplexing (TDD) provides a scheme where a single channel is used for both upstream and downstream transmissions. A TDD system can dynamically allocate upstream and downstream bandwidth depending on traffic requirements.

Instead of needing two spectrum channels (one for transmit and one for receive), companies like Arraycomm can do both over one. TDD is well suited to Internet access as there is much downloading and little uploading, but problematic for voice - where the split is more even. The use of this technique saves 3G bidders precious cash as they bid for less spectrum. That was what brought Arraycomm to Australia. Australia let Arraycomm bid on only “half” on the 3G spectrum. Most “4G” systems intend to use licensed frequencies, usually in the 2.5 GHz MMDS band. But the unlicensed 5.8 GHz band could become the proving ground for a range of last mile services.

“4G” Goes Operational
In March of last year CKW Wireless - an Australian company founded by Arraycomm - licensed a small piece of spectrum in the Australian 3G Auctions for a modest $9.5 million. They plan “4G” wireless using only one half of the frequencies. Of course grass-roots Wi-Fi networks are sprouting up in Australia, too.

Nextel May Try “4G”
In the United States, Nextel may be planning to use Flarion’s system before the end of the year. Nextel is already testing Flarion’s equipment.

Nextel’s spectrum realignment plan is being deliberated by the FCC. The proposal calls for Nextel to exchange 16 megahertz of spectrum spread around the 700 MHz, 800 MHz and 900 MHz bands for 6 megahertz in the upper 800 MHz band and 10 megahertz in the 2.1 GHz band. If approved, this would leave Nextel with 16 megahertz of contiguous spectrum in the upper 800 MHz band, on which the carrier could continue to offer its voice service, and 10 megahertz in the 2.1 GHz band to offer 3G services in the future.

Meanwhile, Verizon, an investor in Beamreach may do trials with the Beamreach system.

The TDD Coalition
The TDD Coalition includes companies like IPWireless Adaptive Broadband, Aperto Networks, ArrayComm, BeamReach Networks, CALY Networks, Clearwire Technologies, Harris Corporation, IPWireless, LinkAir, Malibu Networks, Radiant Networks, and Raze Technologies.

Members of the TDD Coalition include the following “4G” hopefuls:

Wireless Communications Members
Press releases from WCAI members show a variety of wireless approaches for the “last mile”.

Many of these companies believe they can deliver high-speed voice and data cheaper than cellular.

Time will tell.

Posted by Sam Churchill on Friday, June 28th, 2002 at 8:26 am.