Engineer or Clark, any thoughts on the difficulty to implement these techniques. In particular, the Software-Defined Wideband Radios look to be rather challenging.
Regards, Scott
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telecommagazine.com
New Developments Expand TDMA Network Capacity
Focus On Wireless
By using technologies that move capacity where it?s needed, adjust signal strength to reduce interference and save power, control power on the downlink and point the transmission beam right to the cell boundaries, wireless providers get more band for the buck.
Peter Mastro
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More band for the buck--it?s desperately needed by every wireless service provider as customers fill available spectrum with voice and then demand data services, too. Help for time division multiple access (TDMA) operators is coming soon from a batch of new techniques that stretch network capacity and expand network capabilities, such as flexible channel allocation, dynamic downlink power control, intelligent phased-array antennas, discontinuous transmission and software-defined wideband radio.
These IS-136-compatible techniques can boost capacity on TDMA networks by up to 100 percent, usually with just a new software load. They make smart radios smarter. Some of them also deliver dynamic bandwidth so data callers can access unified messaging services or the Internet. Most are already in field trials. Because they depend on software, they make use of existing hardware, protecting providers? investments. Finally, the techniques expand capacity and thus give operators a leg up in planning for third generation (3G) wireless services requiring high data transmission to support multimedia services.
Flexible Channel Allocation
Why build infrastructure to handle every cell?s peak traffic? During the day Wall Street is full of movers and shakers, cell phones clasped to their ears. It?s largely empty at night when Madison Square Garden--empty during the day--is jammed with fans. Providers currently have to gear up for peak times in both areas. But what if they could move channels around, so Wall Street?s unneeded nighttime capacity could be switched to Madison Square Garden in the evening, and vice versa during the day?
Flexible channel allocation (FLCA) does just that--moves capacity where it?s needed. FLCA will let radios in both areas use the best frequencies available--even over an entire metro region--by determining which channel delivers the clearest signal for each new call. The result is an increase in network capacity of as much as 45 percent, with substantially reduced call blocking and call dropping. The smarts, incidentally, come from software; the computing is done by processors such as digital signal processing (DSP) chips already built into most radios and switching centers.
In the channel allocation process, a locate radio in the base station gathers real-time data on the uplink and downlink radio frequency strength from the mobile unit and on interference levels for various channels. The information is then passed along to the executive cellular processor in the mobile switching center. The processor assigns the call to a traffic radio and tells the radio to identify and use the channel with the best signal--the one with the combination of the strongest signal and least interference at that moment.
FLCA is essentially a technique for minimizing interference in the network as well as expanding use of channels. Because it is software-based--the locate and traffic radios and the cellular processor all use proprietary algorithms--the only new hardware needed is additional radios to handle the increased number of channels now available to each cell site.
It?s an automated process, giving FLCA the ability to make instant decisions about channel allocation over a wide geographic area and over the entire chunk of spectrum used by the service provider. These self-engineered and self-configured networks will greatly reduce RF engineering costs. FLCA is now in field trials with several major carriers.
Dynamic Downlink Power Control
Controlling power on the downlink as well as the uplink also increases network capacity by reducing interference. Uplink power control has been used for years to increase or decrease the strength of the signal sent by a mobile unit. But dynamic downlink power control has been difficult in TDMA networks because, while the three calls that share one timeslot originate at different distances from the base station, each needs to hear the station and synchronize with it. In addition, the IS-136 standard requires a certain level of transmission power. However, downlink dynamic power control (DDPC) can provide additional savings in mobile power use and minimize interference, boosting network capacity.
The trick to TDMA DDPC is to lessen, but not eliminate, the signal used on the downlink when calls are not in progress. The newer IS-136A standard permits downlink signal strength to vary depending on the distance between mobile and base station; IS-136A-compliant mobiles could thus tolerate greater variations in downlink transmission strength.
Software techniques have been developed to enable the base station to transmit, as well as receive, signals of varying strengths to each of the calls sharing a TDMA timeslot; they also are in field trials. The minimized signal still permits the mobile to ?hear? the base station, but interference with adjacent sites is minimized. The drop in interference can account for more than 10 percent of the total capacity gain from this collection of software techniques--a significant improvement.
Discontinuous Transmission
Discontinuous transmission also involves adjusting signal strength to reduce interference and save power. An important part of the IS-136 standard enables mobile units to operate in sleep mode: They turn their transmitters off when not making a call or checking in with the base station. The mobile is on only in its own timeslot and only every 10 to 15 seconds when not calling. Discontinuous transmission extends the downtime during intervals of no calling so the mobile is on only for one-half to one-third of the timeslot.
This briefer period is called a shortened burst, and it, too, is the result of software innovations. New algorithms loaded into the mobile?s DSPs detect when voice is not present and after a certain interval put the phone into shortened burst mode. Discontinuous transmission obviously saves considerable battery power for the mobile, but its savings in reduced frequency interference are also substantial enough to improve network capacity. It has been estimated that a typical consumer can save up to 30 percent of the battery life when an operator implements this feature.
Intelligent Phased Array Antennas
Another technique for minimizing interference is pointing the transmission beam right to the cell boundaries and not beyond. Many operators already aim beams strictly within cells to lessen interference with adjacent ones; this ability will become increasingly important as cells shrink so networks can accommodate more callers. The means for precise targeting is an array of small antennas that acts like one single larger one but is much more nimble and precise. Such an array can be easily steered. The individual antennas are guided by phase-shifting electronics and software that introduce minute delays in the timing of each one?s signal: Changing the timing pattern across the array changes the direction in which the signal is transmitted.
The first use will probably be in aiming a beam lower to confine it within the new boundary when a cell is split. This alone can reduce interference markedly and will be crucial as cells are divided and subdivided. Gains in capacity may be as much as 10 percent and may be doubled when deployed with the new capacity-enhancing software such as FLCA and DDPC.
In the future, movement will be horizontal as well as vertical. Horizontal steering might be used, for example, to improve signal quality. An antenna might have multiple downlinks from which to choose. The radio automatically would choose the best downlink for a given call, and the steerable beam could be fixed automatically and precisely upon the uplink from the mobile unit. A crucial feature for ease of use is the ability to steer the antenna beam from the ground or a remote location, rather than from atop the tower. Intelligent antennas are now in field trial.
Software-Defined Wideband Radio
Software-defined wideband radio is one of the most exciting new developments. It will further improve voice capacity while making wireless packet data possible. The concept is simple, although the technology is not. The cell site digitizes all 15 MHz in the wireless spectrum being used, and then selects any portion for a given call, ranging from the 30 KHz assigned to a voice channel to the 200 KHz needed for the transmission of enhanced data for global evolution (EDGE) and EDGE compact.
Digitizing the full 15 MHz at the cell site requires, for a start, extremely fast analog-to-digital (A/D) converters. The signal from the receive antenna is routed to a circuit pack, which samples it at around 50 to 60 million samples per second. Multiple DSPs then filter out chunks of the desired widths and do the baseband processing. A given radio channel doesn?t map to a given circuit pack, but rather is invoked by software. Other DSPs then assemble signals of varying bandwidth for transmission.
Because channels are created by software and can be of variable width, the wideband radio can accommodate any air interface as well as chunks of spectrum wide enough for packet data. It?s just a matter of the DSP calling different subroutines. After the incoming 15 MHz signal is digitized, some subroutines can produce TDMA signals, others AMPS signals; still others GSM or EDGE signals. One platform handles all. Service providers will eventually be able to provide both GSM and TDMA on the same network, allowing roaming across continents. Best of all, wideband radio makes wireless high-speed packet data a reality. Compact EDGE over wideband radio can deliver signals of 150 kbps to 200 kbps--half of what?s proposed in the ITU-2000 standard for walk-around use and equivalent to the rate proposed for automotive use.
Network operators can evolve the current wideband radio by building upon the software already developed, so one platform will be able to handle all air interfaces and all future generations of wireless communications. Customers will appreciate packet-based wireless services becoming a reality. They?ll be able to use unified messaging and voice over IP, even surf the Internet from any street corner.
Developments such as these expand network capacity and thus usage, boosting revenues. They are run by software, make use of a provider?s installed base and protect its investment in radios, antennas and switches.
They also point the way beyond 3G to an organic network that dynamically and in real time minimizes interference and maximizes frequency reuse; delivers just the bandwidth a given caller needs, even beyond the established 3G specifications; and automatically reconfigures channel allocation and other aspects of the network for optimal flexibility.
Peter Mastro is TDMA strategic team leader at Lucent Technologies, responsible for planning and delivery of its TDMA wireless network products and system features. Mastro has an MBA from Northern Illinois University and a bachelor?s degree in computer systems engineering from the University of Illinois. |
