Rapid Prototype, United States of America
Rapid Prototype is a service bureau based in Michigan which translates computer aided design (CAD) data into functional prototypes for the car industry, engineering firms, toy manufacturers and medical equipment companies. The company uses the selective laser sintering (SLS) process to create prototypes from the CAD data. The final product is a solid model, formed out of a variety of dry, nylon-based powders. Customer production commitments dictate that Rapid's processes are completed in three to five days.
The company needed to increase the speed of its email and file downloads and improve file security in order to keep its business growing. It chose to install a multi-channel multi-point distribution service (MMDS) with broadband wireless digital internet access, to deliver data to the desktop at 1-2 megabits per second (Mbps). The bandwidth provided by the wireless data channel shortened the average file transfer time from ten minutes to ten seconds.
FIXED VERSUS MOBILE NETWORKS
Until recently, the mention of the wireless communications industry meant one thing: cellular telephone. This does not hold true anymore, making it necessary to distinguish between wireless data options. The US's cellular and PCS network uses highly cellularised architectures with low-density modulation schemes optimised for mobility. This means a great deal of system capacity is dedicated to ensuring that a moving vehicle can remain in contact. No matter how much ingenuity is applied to other uses of the network, its original and highest priority is to provide digitised mobile voice connections at about a 10 kilobits per second (Kbps) equivalent data rate.
The new wireless options are called 'fixed wireless', as opposed to mobile. Fixed refers to a fixed location. In this case, the antenna is highly directional, high gain and mounted on a building. System capacity that would otherwise have been used to ensure uniform coverage for mobility can now be redirected to provide a higher throughput. Denser modulation schemes requiring higher signal-to-noise ratios can also be used. Data rates can easily reach 10Mbps and higher - a 1,000-fold increase from the current mobile capacity. As a result, cost-per-bit-second is lower than in a mobile system. While mobile data access systems are being designed and built, they will attack a fundamentally different market opportunity, and be some years behind their fixed counterparts with regard to wide-scale deployment.
MMDS is an excellent solution for small-to-medium enterprises (SMEs), as it provides a higher bandwidth at a lower cost than using leased lines. But there are several problems with rolling out this technology on a global scale:
- The absence of a clear policy at national and European levels for the licensing of the LMDS spectrum
- The growing regulatory pressure across Europe for the unbundling of the existing local loop infrastructure, which seems to reduce the need to construct wireless alternatives, and operator uncertainty regarding the capabilities of the technology
- The unwillingness of network manufacturers to enter risk-sharing agreements with prospective operators
Many operators have signed frame agreements with LMDS manufacturers with clauses stating that the agreement is made void if the company fails to win a license.
MODULATION AND REUSE SCHEMES
At a given power receive, level and modulation scheme, the fundamental capacity of all spectrum is the same, bit-for-bit and hertz-for-hertz. For instance, 64QAM modulation provides 5bits per hertz, QPSK modulation 1.6bits per hertz, no matter what frequency band they are used in. However, in an effort to squeeze more distance out of system transmission specification, lower density modulation allows a greater distance at a given power, but sacrifices data throughput rates. Thus, MMDS can make use of 64QAM for its downstream links, giving a raw downstream capacity of about 1Gbps (gigabit per second) for its 200MHz of bandwidth. LMDS systems use QPSK, and therefore realise about 1.8Gbps of raw capacity, even though they have five times the MMDS bandwidth. This allows LMDS systems to use a cell radius of two miles, rather than the much smaller radius that would be required for them to use 64QAM.
The net effect of the small cell size forced on 24, 28, and 38GHz systems is that they must repeat the spectrum often, via many separate cells in the market. Since each cell repeats some portion of the spectrum in some form, this multiplies capacity. This is a fundamental spectrum reuse scheme, albeit one forced upon such high frequency systems by their propagation characteristics.
Another form of capacity multiplication is to sectorise the transmission pattern. Thus, in one simple version, instead of transmitting channel A on a 360-degree pattern, and channel B on a 360-degree pattern (effective coverage of two channels), channels A and B would be alternated in pie-shaped sectors. If, for instance, 12 30-degree sectors were used, there would be six channel As, and six channel Bs, each capable of carrying separate data. Thus, the effective coverage would be 12 channels, or more if sectors were allowed to overlap. All omni-directional wireless data systems make use of this sort of method, either for outbound transmission, inbound reception, or both.