Open interfaces at the wireless frontier


Tremendous industry cooperation has been behind the creation of the AISG open standard communications protocol; the development has much import for the future of cellular infrastructure.

The arrival of third generation (3G) technology brings the wireless industry to a turning point. Offering advanced services with significantly higher data rates, the first networks promise to herald in a new mobile multimedia age. But the detractors of 3G point to a number of challenges, not the least of which is that the performance demands on RF equipment are not only extremely high, but constantly changing. Despite this, there are indications that these doubts may soon be overtaken by events.

Recent cooperative industry efforts to provide standardization at the RF interface are bearing fruit. One of the highlights is the open standard communications protocol developed by the Antenna Interface Standards Group (AISG). In its specification for the means of connection and communication between antenna line equipment and base station (or Node B), the AISG standard paves the way for digital remote control and monitoring of RF infrastructure. Intelligent antenna systems, responding dynamically to the changing needs of the networks, appear set to become a reality.

Remote considerations

The main driving force behind the original establishment of the AISG, according to the group's Chairman, Brian Collins, was the advent of antenna systems with remote electrical tilt (RET) technology. Offering operators a dynamic RF optimization tool for their networks, RET is seen as a potential boon to the implementation of 3G services.

Conventional electrical tilt has long proved itself to be an important tool for fine-tuning RF coverage; the effective size of a given cell sector can be determined by directing the antenna's radiation pattern at a set angle towards the ground. But the means of adjustment has traditionally required technicians to go to each site to access the antennas, and this incurs costs. RET, on the other hand, eliminates the site access requirement, allowing tilt to be adjusted remotely from the network control centre and in a matter of minutes.

Radio Frequency Systems was one of the first manufacturers to begin producing RET systems. Its Optimizer RT solution, launched in 2002, essentially relies on the use of additional equipment: at the antenna, a motor-driven 'antenna control unit' (ACU) with electromagnetic phase shifters is used to adjust the angle of tilt; at the base station, the 'control network interface' (CNI) communicates the tilt angle requirements from the network to the antenna ACU.

But, as different vendors' RET solutions began to emerge, there was a widespread concern: without a common standard for the equipment, operators could become locked into inflexible, proprietary systems. Equipment makers saw this as being likely to inhibit the growth of the market. "If you have different systems up the tower, then you have the problem that the network operator cannot either switch suppliers or mix products from different suppliers. In other words, he can't be confident, especially with the way things change in this industry, that in five years time, what he has will continue to be useable. That is obviously not a good incentive for him to invest," says Collins.

Non-standard origins

The industry's RET standardization issues were soon addressed. In the United Kingdom, a group of major antenna manufacturers and OEMs got together to discuss a way ahead. What emerged was the need to agree on a common standard communications protocol between the antenna line equipment--such as the ACU and tower mounted amplifiers (TMA)--and the base station.

The group became known as the AISG, and over the following months, it worked closely with a range of industry representatives to determine the requirements for a standard digital interface. "Many hardware manufacturers had bemoaned the fact that there wasn't a standard. So when we began bringing people together to establish one, the industry regarded it as excellent news," says Collins.

One of the parties involved in the early AISG discussions was RFS. Although the company had been manufacturing complete RET solutions using its own protocols, it was happy to embrace the new standard. According to Paul Hackett, RFS AISG Project Manager, the change from the original Modbus protocol to AISG presented few difficulties. "The philosophy of our original system was already close to the philosophy of AISG. The impact has been predominantly on the software to change the communication layer within. But we've also done some enhancements along the way," he reveals.

The benefits from moving towards AISG-compliance have implications for the wireless industry as a whole. The end-user gains from greater flexibility in the arrangement and selection of equipment, while the amount of wiring needed to realize the system is reduced. And Hackett claims that interoperability is a strong point. "It means that the protocol between the top of the tower and the base transmitter station (BTS) is the same for all users," he says. "We've always espoused the open system. This means that when we provide certain items of equipment, we don't limit the end-user developing anything that sits on this equipment bus."

A draft of the first version of the AISG communications protocol was released for industry review in 2002. Inside the document, it specified the open hardware and software standards--including connector types and data transmission systems--for the interfaces of antenna line equipment with digital remote control functionality. Immediately, RF equipment manufacturers around the world rushed to launch AISG-compliant products and networks began to mandate its use, even before the first version of the standard was formally released in October 2003.

Collins remarks that the approach from the 'ground-up' was unusual, but has provided distinct advantages. "It began with the manufacturers getting together and demanding a standard," he says. "Everyone had a common interest in the technology and was keen to come to an agreement. The whole process has been marked by a very open approach from all the manufacturers involved--it has been a real joint effort."

Wiring alternatives

The AISG standard was also developed to accommodate an alternative means of high-level communications between the tower-mounted components and the base station CNI. In the past, the connecting coaxial cables generally carried only transmit and receive RF signals. The AISG protocol, however, optionally allows seeding of control and monitoring signals into the very same transmission lines, eliminating the need for separate digital communications wiring.

The solution requires the use of a 'modem' bias-tee (so-called because it comprises an AISG modem) at the base-station to combine the RF and digital control signals. Hackett's colleague, RFS's Erik Wille, the Product Manager for RF Conditioning Equipment, explains how it works: "You cannot just have a lot of different communications protocols in a coaxial cable. The bias-tee modulates the RS485 input to a subcarrier signal, which can run along the long feeder line." The signals are then separated out again at the mast, by either a second bias-tee, or by an AISG-compatible TMA with its own in-built bias-tee.

The use of the RF transmission line for inter-device communications not only affects the design of the TMA, but also its associated power distribution unit (PDU). To simplify its solution, RFS has developed a combined CNI and PDU--the resulting 'CNI-P' control network interface is designed to support up to three double TMAs and nine ACUs. In this way, all AISG-compatible antenna line equipment can be used to create a number of possible equipment configurations (see diagram).

The possibilities of the open standard go beyond just the needs of RET. "AISG is a protocol for how a base station can communicate with any active equipment in the mast," summarizes Wille. "For example, with TMAs in the past there was no real communication--if the amplifier went into alarm, then it raised the current a small amount, and the amplifier would go into bypass. With the AISG protocol, manufacturing details, alarm information and the gain set can all be accessed remotely."

The protocol has similar application to a number of other antenna line components, many of which are also being developed to be AISG-compliant. These include tower-mounted boosters (TMB), voltage standing wave ratio (VSWR) measuring units and various other sensors and monitoring devices. Future applications--including such things as antennas with azimuth steering or adjustable azimuth beam-width--are also being considered.

Moreover, Wille points out that, in the event of failure, replacement of such devices is simple. Rather than having to find out the proprietary communications specifications of the unit and then check whether there exist compatible items in stock, the technician can just replace it with an AISG-compliant unit.

Dynamic advantage

With the recent launch of 3G universal mobile telecommunication system (UMTS) networks around the world, AISG's Collins asserts that the arrival of the open standard has proved timely. "What I'm seeing at the moment is that the operators are evaluating the economics of applying remote tilt, especially to city centres," he observes. "I think the standard will remove the stumbling blocks that would otherwise have been there."

The view is shared by one of the world's leading OEMs. According to Thomas George, who is responsible for Product Solutions UMTS Access at Nortel Networks, the AISG standard will greatly help the marketplace. "Open standardization simplifies the interface and the technology for the operators, and allows them to use several antenna suppliers' equipment in the network," he says.

In mid 2002, Nortel Networks, in conjunction with RFS, began one of the world's first trials of RET, using its remote electrical tilt automatic (RETA) antennas for UMTS. George sees that AISG-compliant RET solutions will particularly benefit operators seeking to implement code division multiple access (CDMA) and wideband-CDMA networks. Unlike the global system for mobile communications (GSM), there is no prospect for changing the frequency plan to improve the network performance. The ability to effectively tilt antennas will thus 'make or break' the task of RF optimization.

"Centralized remote tilting allows the operators to take more steps towards having full control of the network and its performance. This amounts to more intelligent, more dynamic and more interactive RF optimization processes. The AISG protocol is an important step towards achieving this," says George.

The implications are significant. George speculates that futuristic scenarios might allow the automatic adjustment of coverage to suit daily urban migration patterns. Effectively the network would assign resources dynamically to suit the whereabouts of the population, depending on whether they are at work, home or even the beach.

Future interface

The success of the AISG is evidenced by the fact that the high-level communications standard has been now actively and widely embraced within the cellular industry. Although some OEMs still use proprietary protocols, in many parts of the world the AISG protocol is becoming, by default, the standard specified for antenna line products. Furthermore, its application need not be limited to the latest cellular systems.

"The driving force behind the AISG project really came from the wideband CDMA people who needed extra parameters to help them optimize their 3G networks. In fact, what we're now seeing is that a lot of operators are applying this to their second generation (2G) systems as well," says Collins.

Collins attributes much of the enthusiastic support to the fact that in setting up and maintaining the standard, the AISG is committed to keeping it both interoperable and backwards-compatible. For OEMs, such as Nortel Networks, this assurance means that the company's investment is not wasted in the event of changes in technology. For RFS, the door has opened to new innovations in RF devices and architecture. As it turns out, the key to tomorrow's wireless systems may, paradoxically, be in the wiring.

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