Technology boosts interest in 220MHz auctions
Linear modulation (LM), a mature voice and data technology, opens up new opportunities for SMRs, utilities and public-safety agencies at 220MHz. The possibilities make auction participation attractive.
Auctions are one device employed by the FCC in the race to get spectrum into the hands of operators who will maximize its use. The 2MHz block at 220MHz-222MHz is now targeted for an early 1998 auction. The 220MHz service may be the “new kid on the block,” but LM technology has been to college and has earned its degree.
So, why should you take notice? If you’re an operator, or aspiring to be one, auctions may offer new opportunities. If you’re a user, one of the most efficient wireless technologies available today, 5kHz LM (linear modulation), is bringing new business efficiencies with its high-speed data capability. Although other forms of linear modulation have been used for decades, the form first demonstrated in 1992 and type-accepted by the FCC in 1994 is an example of very narrowband linear modulation (VNBLM).
220MHz vs. other bands The 220MHz services started years ago as a result of the FCC’s allocating a 2MHz block for “spectrally efficient technologies.” Service for 220MHz has had a tortuous infancy (some even say this has been to its advantage), with uncertain rules and the previous lottery system of assigning licenses that put channels in the hands of speculators rather than operators. Nevertheless, 220MHz services provide considerable advantages over other services. Their operators and suppliers have demonstrated tenacity and are now ready to take these burgeoning services into auctioned spectrum where other services are offering challenging levels for operators and users alike. The range of 220MHz signals is superior to the higher-frequency signals of 800MHz specialized mobile radio (SMR), cellular and personal communications services (PCS). Fewer sites are needed to provide coverage, which makes the cost of service more attractive for operators because they can offer users a comprehensive package at competitive rates.
Types of operator Although SMR has been the primary use for 220MHz services so far, there are other possibilities. Public safety already has a number of allocated channels that are not subject to auction and that are available under a mutually exclusive, first-come basis. Any mobile operation needing new spectrum should consider bidding at the 220MHz auctions. Licenses will be available for economic areas (EAs), regional and nationwide blocks. With nearly 200 channel pairs available, 220MHz is probably more attractive than some of the PCS spectrum. The spectrum winners will also be allowed to disaggregate and partition their spectrum to enable the greatest flexibility of use. “Disaggregation” means taking some of the channels from the allocated block over the whole licensed geographical area and allowing another operator to use them or selling them outright. “Partitioning” means taking a sub-area within the licensed area and allowing and selling all the channels in that sub-area.
The auction will include both small and large players. Incumbent 800MHz players displaced by the new rule changes could well find the highly spectrum-efficient digital technology at 220MHz to be more attractive than the prospect of continuing with outdated and inefficient systems that are nearing the end of their economic and technological life. The competition for SMR is enhanced specialized mobile radio (ESMR), cellular and PCS. LM 220MHz can provide a service with high-quality voice and high-speed data in a single unit at a competitive end-user cost.
5kHz LM technology The 5kHz LM technology developed by Securicor, Bath, Avon, U.K., was described in my article “Technology Update: 5kHz Linear Modulation” in the January 1994 issue. As a result of a merger between Securicor Radiocoms, Bath, and Intek Diversified, Princeton, NJ, in late 1996, Securicor took a majority holding in Intek. Midland USA, part of the Intek group of companies, with its nationwide dealer and direct sales operations, now markets the LM 220MHz systems. As a further result of the merger, Intek now owns the LM technology and intends to market it worldwide.
LM has many advantages over other technologies. It is digital, yet it is also analog. It uses all the benefits of digital without the accompanying disadvantages. One problem with an all-digital system is the range-edge cutoff. Unlike analog, where the user can choose whether to listen and to try to decipher the information when the signal gets weak and noisy, a digital system doesn’t allow such an option. The user has no choice, because, at the edge of the signal range, the information is indecipherable. In fact it is annoying to try to listen to it. The good news is, there is a solution; the bad news is, it costs! To ensure that a user doesn’t experience the reception of weak signals, more sites have to be implemented; a roughly 2:1 ratio is the norm.
So, if a system costs more to install, guess who pays?
Because LM transmits an analog signal, it doesn’t suffer this range-edge signal cutoff, so the system is more economical to install and to use. LM uses digital signal processing (DSP), but instead of low-rate vocoding, or a higher rate that is time- or code-shared (the effect is the same_reducing the number of bits to represent the coded information signal), it uses a sample rate of 44.1kbps, equivalent to an audio compact disc, to retain the original quality. This sample rate delivers excellent voice quality and recognizability, at least as good as 25kHz FM.
Data performance with LM is equally impressive. Figure 1 on page 18 illustrates the three data speed levels. Currently running a raw, or gross, data rate of 16.8kbps with 128QAM (quadrature amplitude modulation) trellis-coded modulation (in a 5kHz channel), the system uses a dynamic rate modem to interact with the real world of mobile data. No forward-error correction (FEC) is applied to slow the data down unnecessarily. The system senses the path conditions and adjusts the rate accordingly. This means that a system can be designed for both voice and data with no difference in the design criteria, and the user gets his data. The data might slow down at the range edge, but the user still gets it. The signal can even be lost for part of the transmission, and the system will fully recover, automatically and with no user intervention. This applies equally to transmissions to a fast-moving vehicle, which is the real world of mobile data (particularly for public safety use). This performance is achieved by the use of the integral technologies of reference vector equalization (RVE) and feed-forward signal regeneration (FFSR) where the distortions of amplitude, frequency and phase caused by multipath and Doppler effect are corrected in real time, giving the wireless environment a wireline performance.
RVE was formerly referred to as transparent tone-in-band (TTIB). Reference vector equalization is more completely descriptive of the technology, wherein a pilot tone is created and used as a reference for decoding in the receiver. With FFSR, the RVE tone is extracted in the receiver, and the audio baseband is recombined exactly in phase and amplitude. The tone is then used as a reference for canceling the effects of multipath, Rayleigh fading and the Doppler effect.
A number of public safety agencies are looking to SMR operations to provide some of their non-critical communications. The use of SMR operations could solve some of public safety’s communications congestion.
LM, being fully linear, is an ideal platform for future mobile radio technologies, whether they are analog or digital.
New developments: networking The initial products were for single-site operation, where the majority of the channels in the Phase I channel groups at 220MHz were licensed. The amalgamation of systems to provide wide-area coverage created a need for networking solutions. A new multisite switch and computerized control is now available, allowing for regional operation required for the new Phase II channel groups licensing.
Multisite switching enables a user to roam throughout the entire multisite coverage area and, with no operation of any controls on his subscriber unit, to make and receive calls as if he were operating on a single site in the same way that cellular systems operate. The system logs the site on which a user is registered, so it always knows how to set up a call. Neither the mobile user nor the dispatcher has to have knowledge of which site to use. The system is programmed with many functions to enable the operator of the system to both optimize use and to allow or limit the use of functions and sites. This ability ensures that users get what they pay for, and it allows the system operator to tailor service for the individual, keeping costs down for everyone. Full 24-hour use is recorded in tabular form, allowing graphing that can be used to optimize system design and grade of service.
The soft-failure modes, together with alarm reporting, ensure minimum capacity and service loss under failure conditions, a vitally important consideration for users who make critical use of their mobile communications. Emergency calling is another facility that can be made available. For an extra monthly fee, a user could be given priority access to a channel, even if the whole system is busy. There normally is a heavy fee to guard against misuse, but at least the priority features could be made available for an operator who wanted to provide that facility.
Because the Phase II 220MHz licensing is issued on a geographic basis rather than a site-specific basis, the networking computer’s ability to allocate channels on a “where-and-when” basis ensures optimum use of channel resources. Its modular design keeps initial costs down but allows simple expansion as needed. It is fully compatible with the high-speed data facility, so again, a single system design accommodates both voice and data.
New developments: products A choice of repeater powers of 25W and 100W enables optimum systems to be designed for wide-area coverage. The current choice of two mobiles and a portable will be supplemented by a flexible architecture that enables easy customization of products at the least cost to the provider and user. New data applications are being investigated that will use the unique capabilities of the LM system. Already available are GPS and vehicle data collection systems, high-speed file transfer for direct computer connections and computer-aided dispatch systems with status messaging.
Point-of-purchase credit card authorization, data monitoring and email will enable business users to be connected to their office at least cost. Other possibilities include remote site picture transfer for secure locations. With high-speed data, the applications are almost limitless.
Conclusions The 2MHz of spectrum at 220MHz-222MHz is the new opportunity for both small and large operators and users. There is a growing choice of subscriber units and data options. With spectrum costs now an important part of the overall cost-of-provision equation, VNBLM is the future of mobile communications.
