ierre Dogan, David Patton and Ray Nettleton
Two observations can be drawn from the recent personal communications services license auctions. First, PCS participants are willing to assume enormous financial risk, on the order of tens of billions of dollars-an unprecedented level of risk-taking by the telecommunications industry-for license acquisition and network development costs. Second, each of the PCS participants has committed, or soon will commit, to a digital wireless modulation technique among an alphabet soup of options: CDMA (Code Division Multiple Access), TDMA (Time Division Multiple Access), up-banded GSM (Global System for Mobile communications) and various “hybrids.”
Clearly the latter is an attempt to mitigate the risk of the former, but the digital technology choice by itself does not suffice to justify the economic gamble. The question for PCS participants is: What else in the realm of prudent management and technology preparedness should PCS participants do to significantly further reduce risk?
Many solutions have been offered to reduce risk, but few, if any, are comprehensive enough to address the root causes of the risk and not merely the symptoms. A “back to basics” approach leads one to search for innovations in RF technology that address one or more root causes of PCS risk (see Figure 1). Such innovations, incubated by the industry for several years, now are emerging to provide PCS participants with new risk-reducing capabilities.
The nature of PCS risk
The sources of PCS business risk originate in the profound differences between today’s PCS industry and its predecessor, the cellular industry, at its beginning. The most conspicuous difference, regulation, has created two hurdles for the PCS industry: a very high cost for access to spectrum and strong competition both from within and from outside the nascent PCS industry. The up-front PCS license cost is an enormous financial burden that service providers must amortize across future revenues in addition to the burden of amortizing the much larger infrastructure build-out and marketing costs. Using the cellular industry as a reference, the following summarizes the sources of risk that presently loom large on the PCS scene:
1) Poorer RF propagation at PCS frequencies imposes a much greater (from two to three times greater) number of base stations needed to provide required coverage. This causes an enormous increase in infrastructure and backhaul costs and in build-out risk, compared to cellular.
2) Poorer RF penetration at PCS frequencies results in weaker in-building and in-vehicle capability compared with cellular.
3) RF interference from adjacent-band service providers and incompatibility between the various air interfaces can lead to severe quality loss, capacity loss (the number of interference-free channels gets smaller), flexibility loss in base station siting and tower sharing difficulty.
4) Strong competition from up to five other PCS carriers, two existing cellular carriers, specialized mobile radio and enhanced SMR carriers, public and private networks based on unlicensed spectrum, data-only services, etc. leads to the need to differentiate services. In order to differentiate service, PCS operators must come up with the right combination of price, coverage, capacity, quality and other service features.
5) The need to be fully operational with capacity and coverage on the first day of service is driven by the need to meet minimum customer expectations and to compete with incumbents.
6) Limited time frame to complete the build-out and begin commercial operations leads to the need to produce revenue quickly. The clock is ticking for PCS license holders because of the FCC mandated coverage deadline and competitive pressure to be the first to market.
7) Local zoning restrictions make it difficult and expensive to install new base stations or to increase the height of antenna towers.
8) The necessity and the cost of relocating private operational fixed service (POFS) microwave operators from the PCS band imposes additional financial burdens on the service provider.
Digital’s dark side
The adoption of digital air interfaces by PCS participants solves the crucial problem of capacity, but also creates serious PCS-specific RF interference control and spectrum efficiency challenges that the cellular industry never faced. Digital modulation generates significant out-of-channel noise that can severely contaminate adjacent channels. Because of this, digital modulation requires guardbands against interference from adjacent channels that use other air interface standards or that belong to competing service providers, thus idling precious spectrum.
Furthermore, broadband digital RF networks are susceptible to out-of-channel interference sources powerful enough to saturate the base station receiver front end. Saturation is caused by narrowband interference sources and by tower sharing-an almost inevitable trend due to the simultaneous crowding of real estate and of spectrum. No amount of digital signal processing can compensate for signal degradation caused by front end saturation. These “dark side” issues of digital must be explicitly addressed.
Searching for cures
These sources of PCS risk can be mitigated by addressing root causes for which there are practical, technical solutions in the RF domain. The controllable root causes are:
1) Poor RF propagation at PCS frequencies, either through the atmosphere, natural obstacles or buildings.
2) Difficulty of providing practical, sharp RF filters to effect “spectrum containment,” a generic challenge that encompasses minimizing guard bands required by digital RF modulation schemes, rejecting RF interference and maximizing revenue-producing bandwidth.
3) Difficulty of dynamically tuning RF circuitry to mitigate RF interference sources; to accommodate shifts in network capacity demand; or to dynamically re-engineer the system as additional cell sites are turned on.
4) Limitations of linear amplifiers, both transmitter linear power amplifiers (LPAs) and receiver front-end low noise amplifiers (LNAs), which often cause distortions and interference.
5) The difficulties in providing compact, cost-effective and reliable electronically steerable antennas and associated control electronics, which are needed to provide improved range and interference rejection.
Some solutions
In the last few years, numerous companies in the RF industry have addressed the described root causes of risk, taking advantage of technical innovations to increase receiver sensitivity and selectivity, and combat RF interference. For example, a lack of transmit power by the subscriber terminal (reverse link limitation) can be compensated for by increased receiver sensitivity at the base station. There are several ways to achieve this, including increasing antenna height, smart electronically steerable antennas and cryoelectronics. These efforts now are coming to fruition. The categories of practical solutions that emerge are:
1) Ultra-low-noise base station receivers to increase the radius of coverage cells, to minimize transmit power (and increase battery life) for subscriber terminals operating in either coverage cells or capacity cells, to improve in-building penetration and to improve the tradeoff between coverage and capacity in certain digital RF schemes (e.g., CDMA).
2) Sharp, low loss RF filters for minimizing guardbands, maximizing the number of channels and rejecting interference in the receiver channel.
3) Adaptive filtering and dynamic channel allocation to combat fixed and intermittent sources of interference by frequency agile RF filters.
4) High performance linear power amplifiers (LPA) that allow the base station transmitter to cleanly combine the signals of many channels without distortion and saturation in the allowed bandwidth.
5) Electronically steerable “smart” antennas that can track mobile users and sele
ctively null out spatially wandering sources of interference, in essence establishing an optimized dedicated spatial communication path per user with increased range and in-building penetration.
6) Some companies are focusing their core competencies on solving a single challenge among the five mentioned above; others are taking a more systems-level approach. A few companies are focusing on improving LPA performance (increased power handling and linearity, lower cost and complexity, and easier maintainability). Electronically steerable smart antennas are being developed by several vendors using technology originally developed for military applications. Other companies with competencies in superconductivity are offering products to address one of the challenges listed above, low loss RF filters.
7) Superconducting Core Technologies Inc. combines electronic RF tuning with superconducting RF filters and ultra low noise receiver front ends in a rugged, low-cost miniature platform. Synergistic combination of this technology with smart antennas looks promising. Cryoelectronic solutions have matured to the point of successful field trials under the auspices of both RF equipment manufacturers and PCS carriers, with initial commercial availability scheduled for 1996.
Economic benefits
The physics and the engineering of the above-mentioned solutions are well understood and the pioneering phase is finished. The availability of PCS risk-reducing solutions will depend on the answer to a number of questions: How complete is the development of these solutions? How much time is needed to assure volume production? Has the targeted functionality and reliability been field proven? How much will it cost and how does the cost compare to the benefits and risks? Will the companies that produce these solutions be reliable suppliers?
All these questions are now being answered. But there is no need to wait for all the answers to agree on the fundamental benefits that these solutions, once validated, will provide, either singly or in combination. The benefits described below overwhelm the associated costs (see Figure 2).
Base station sensitivity
RF range enhancement dramatically reduces infrastructure cost and build-out completion time. Site availability is less of an issue and the industry’s limited capacity to supply, install and integrate coverage cell infrastructure equipment is less taxed. For example, assuming a conservative noise figure improvement of only 3 dB in the base station receiver by any of the techniques now in final development would mean that as much as 30 percent savings in coverage cell sites could be achieved. In a hypothetical deployment of 500 base stations, each costing $1 million, the savings would amount to $150 million. The 500 cell, $500 million deployment of Mercury One 2 One in London provides an existing reference that confirms this estimate of base station costs.
RF range enhancement also reduces regulatory risk-meeting the Federal Communications Commission-mandated coverage becomes easier and zoning and site approval risks are significantly reduced. Marketing risk is impacted in a major way-lower cost allows lower prices, customers’ expectations on coverage and quality are met more easily and sooner, and a competitive weapon is provided in applications that critically depend on in-building capability: residential use, mixed residential and mobile use, and business applications such as field engineering/repair and parcel pick-up and delivery.
The burden of providing full service on day one is made easier to accomplish. Lower amortization and maintenance costs and consolidated backhaul requirements reduce operations risk. Finally, the need for immediately relocating POFS microwave operators decreases because of flexibility in siting RF base stations and the ability to decrease or eliminate the interference altogether.
Coverage vs. capacity
A 5 dB improvement in a CDMA base station receiver noise figure could provide either an 85 percent increase in coverage for the same capacity, or a substantial increase in capacity with the same coverage.
Adaptive RF filtering
Spectrum containment becomes manageable and can be adaptive. RF interference can be eliminated by reducing the system sensitivity at the right time, at the right frequency and in the right direction. The problem of wasted RF channels due to large guardbands and uncontrollable RF interference is mitigated, while network revenues are boosted. Marketing risk decreases because capacity is increased and high performance RF filtering improves signal quality. Operations risk is reduced because of the ease of reconfiguring a network. Frequency-agile RF filters reduce or eliminate interference as capacity cells are added or as competing adjacent operators enter the picture. Build-out risk is reduced because interference problems are solved more easily and sooner, at an acceptable RF filter unit cost made possible by high-yield manufacturing based on electronic tuning. Tower sharing in some cases becomes possible, and in all cases, easier. Finally, POFS operator relocation is either facilitated or conveniently postponed.
Making the right choice
The economics of PCS deployment encompasses a number of highly interconnected factors, and their associated costs and risks, that require careful consideration for each system. The risk is large, matching the large economic reward that PCS winners will reap. Measures that can be taken in the radio access part of the PCS network that will significantly reduce risk deserve to be carefully examined.
A number of rapidly maturing industry innovations that address the RF nature of the PCS root problems are attacking the interlocking PCS risks in a fundamental way. There is growing agreement on the benefits of these solutions throughout the industry. The issue remaining is the timing of their availability.
At the very least, all the previously mentioned technical approaches to radio access innovation should serve as additional, cost-effective insurance in a jungle of risks. The perceived risk of these “new” technologies pales in comparison with the staggering overall economic, marketing and management risks already assumed by participants in the PCS race. That perspective provides a compelling argument to first experiment with and then to adopt one or several of the emerging innovations for both large coverage rural areas and high capacity urban areas.
PCS providers cannot resign themselves to inferior performance as a result of their assigned spectrum. Instead, they must vigorously pursue the many RF options that are emerging in the marketplace, both from their chosen principal equipment providers as well as from innovative companies making “add-in” components. (Cellular incumbents need not be left behind in the quest for better quality and business performance-these technologies also work at 850 MHz. Those analog-to-digital transition issues could become much easier through the application of any or all of the new developments in technology we have discussed here.)
The new technologies needed to relieve a myriad of headaches for wireless service providers are here now or coming soon. As deployment plans and business cases are assembled for the coming competitive market, the PCS industry, like the Roman god Janus, should face in two directions: looking backward to benefit from the experiences of the cellular industry and looking forward to grasp the opportunities of today’s leading-edge technologies.
Pierre Dogan, David Patton and Ray Nettleton are members of Superconducting Core Technologies Inc.’s corporate and market development team. SCT, which specializes in the development and manufacture of microwave cryoelectronics for the wireless industry, is based in Golden, Colo.
