Antennas are the Answer

An essential part of any design, todays antennas can meet the cost per bit challenge of next-generation wireless networks.

Dec 9, 2008Kent Heath, Vice President of Marketing and Business Development, Ubidyne Inc.|Mobile Dev DesignWith the introduction ofHSxPA3Gservice, cellular network operators have begun to introduce broadband wireless service models that are increasingly competitive with traditional fixed-lineDSLor cable access plans. In some markets, like Australia, operators are now competing aggressively both with each other and with the fixed-line networks.

These operators are offering flat-rate data plans that let consumers purchase broadband services at rates that are similar to or lower than fixed-line, but with the added benefit of anytime/anywhere mobility. This is driving an explosion in wireless data consumption, resulting in dramatic data traffic growth but with a much lower increase in revenue improvement for the operators.

Because of the popularity of the new fixed-price data plans, network capacity limitations are quickly becoming a concern in dense urban areas, driving operators to spend heavily on infill solutions. At the same time, consumers are expecting the same service in rural areas that they experience in the city, requiring increasing capital expenditure (CAPEX) spending for coverage even where density is low.

To compete successfully and profitably, network operators need to adopt next-generation broadband wireless technologies like WiMAX and Long Term Evolution (LTE) that can offer cost per second or cost per bit that scales down faster than ever before (Fig. 1). Yet this demands advancedRFand antenna techniques, and some significant challenges lie ahead when it comes to deploying these new techniques in a real network environment.

More Antennas are Better

Next-generation wireless systems require advanced antenna systems to deliver higher data rates at increasingly lower cost points. In fact, at recent industry conferences, the more antennas, the better has become a common theme within some groups focused on 3.9G network development and standardization. Proposed techniques include beam control, diversity solutions, multiple-input multiple-output (MIMO), and other active antenna schemes.

Currently shipping in volume today, remote electrical tilt (RET) systems are electromechanical systems that enable the network operator to adjust the passive antennas vertical orientation in response to changes in the surrounding environment and local call traffic patterns. Theyre good tools for network optimization compared to former fixed-tilt or older mechanical tilt systems

However, the transmit (Tx) and receive (Rx) tilt angles must always be aligned, so there is a suboptimal tradeoff between Tx and Rx performance in the network even when these systems are utilized correctly. Further tradeoffs will have to be made with new multistandard (GSM/UMTS/LTE) RF platforms if a single passive antenna is used for all carriers, forming a significant challenge for network planners.

Designers have considered advanced adaptive tilt systemssystems with discrete, complex electromechanical systems providing differentiated tilt by carrier, standard, or frequencyto overcome the need to trade off Tx versus Rx tilt optimization and to support dedicated tilt-by-standard in multistandard/equipment sharing applications with specialized antennas.

These systems may provide significant RF performance benefits by providing capability for a much more flexible vertical tilt angle solution for each cell within the operators network. But this comes at the cost of complex added hardware and software that need to be integrated by theOEMbasestation supplier and placed on the rooftop or at the cell site, increasing operational expenses (OPEX) as well as CAPEX costs.

Beam forming/steering systems are routinely used in other applications. In fact, azimuth control for changing the direction of the beam within the cell site is now common inTD-SCDMAandWiMAXapplications. Multicolumn antenna systems with multiple RF Tx and Rx signal paths and complex control systems to monitor and optimize the beam shape on either a subsector or per-user basis are commonly proposed. These systems have been well accepted in networks where the added cost and complexity are compensated by added capacity and throughput required by the operators service model.

Diversity schemes can be implemented in many domains, e.g., space, time, or frequency. The basic idea for all approaches is to transmit the same information over at least two independently fading channels. At the receiver, the independence in fading significantly reduces the risk that all channels are degraded concurrently.

Today, in most cellular network deployments, receive path diversity is already common with an independent receiver connected to each of the antennas two orthogonal polarizations. Yet due to the added cost of increasing the number of expensive amplifiers, Tx path diversity has rarely been employed.

Already commonly deployed in Wi-Fi applications for in-building signal enhancement, MIMO systems are now being seriously considered for commercial cellular network deployments, as well as with new 3.9G WiMAX and LTE systems where high data rates are a key market driver.

Although considered too expensive for commercial application in the past, the throughput advantages of MIMO systems are now generally accepted based on the work of the LTE/System Architecture Evolution Trial Initiative (LTSI), which has shown downlink peak rates of up to 326 Mbits/s with 4x4 MIMO systems (Fig. 2).

Implementation Challenges

Though active antenna techniques offer significant performance advantages to network operators in terms of coverage, capacity, and terminal battery life, these schemes havent been deployed widely due to a number of practical implementation considerations and economic limitations with traditional analog RF network equipment.

Significant additional equipment and installation costs are required to deploy most of these systems. As discussed above, RET and advanced adaptive tilt systems require electromechanical subsystems for the control and activation of the vertical tilt functions.MIMO, beam forming, and advanced diversity systems require the addition of multiple RF components (such as transceivers, power amplifiers, and masthead amplifiers), multicolumn passive antenna arrays, coax cables/feeders, and associated mounting hardware.

In addition to the purchase cost of these components, these elements add to the cost and complexity of the installation, with site license fees to install new equipment and increased installation costs to contractors due to the additional weight and wind load. In some cases, costly rooftop or tower re-enforcement may be required to support the additional network elements.

Increasingly, OPEX costs are becoming the dominant factor impacting their total cost of ownership. Unfortunately, with traditional analog RF systems, the OPEX costs typically scale directly with the added complexity of the active antenna approach taken. Additional active components like masthead amplifiers, power amplifiers, and advanced tilt systems consume more power.

Monthly site leasing costs are also increased based on the weight or wind load of these added elements, which can be a significant potential problem for MIMO antennas. These added costs are particularly acute in rural areas in emerging markets where sites are powered by costly trucked-in diesel fuel and in areas where site real estate/leasing costs are at a premium due to local community restrictions against unsightly masthead or rooftop equipment.

In general, cost of quality tends to scale with an increasing number of RF components within complex systems. This has led to significant concern by many operators that employing advanced antenna systems may degrade network reliability and increase repair and maintenance costs. Furthermore, electromechanical RET and advanced tilt systems add further complexity and are subject to higher potential failure rates than solid-state system elements.

And, the obvious concern with traditional active antenna systems is the added carbon footprint from both the manufacturing of added network elements and the fuel consumed to power them. In addition, an increasingly challenging problem for operators is related to the visual impact of adding new equipment to existing cellular sites.

For example, a local operator in Scottsdale, Ariz., was unable to get approval to install a traditional cabinet-mounted basestation on the ground below an existing antenna tower. So, the operator had to commit to dig a chamber underground and bury the new equipment to hide it from viewa very expensive solution!

Changing the Radio Architecture

So how can the performance benefits of advanced antenna solutions be achieved without the penalties of higher costs, reduced reliability, and negative environmental impact? One solution is the antenna embedded radio (AER), a fundamentally different radio architecture for mobile communications.

This new network radio architecture implements a novel signal-processing chain inside the AER (Fig. 3). A central basestation radio server exchanges data with the server over a fiber-optic link according to standards established by the Common Public Radio Interface/Open Base Station Architecture Initiative (CPRI/OBSAI). A central processing unit (C-hub) inside the AER controls all RF micro-radio (m-radio) units individually, each serving one antenna element.

This fully digital processing chain allows for the individual adjustments of amplitude and phase of each antenna element as well as the grouping of several elements into logical antennas with independently fading signals. The AER system is highly scalable and easily reconfigurable, permitting the groupings to be dynamically changed online through the radio sever.

Figure 4shows an example of the scalable configuration realizing up to four independent logical antennas. As indicated, with no change in hardware, the AER system can be configured to replicate the performance of traditional remote radio head + passive antenna systems (when configured for one Tx and two Rx signal paths). The AER has built-in Tx diversity and can be configured to support 2x2 or 4x4 MIMO signal paths, providing a future proof solution as the network software from basestation OEMs evolves to exploit these capabilities.

Figure 5illustrates the simplicity of deploying the AER system. Because of the high level of integration, the AER can be mounted up the tower or on a rooftop as if it were a traditional passive antenna. Then, operation only requires the CPRI or OBSAI fiber-optic cable and power supply to be connected the input of the AER.

The Power of Antenna Embedded Radios

One of the major advantages of the antenna embedded radio architecture is its high level of integration and simplicity.AERs eliminate all other RF discrete components and associated installation/site upgrade costs. Explicitly, no remote radio heads, masthead amps, RETs, advanced tilt systems, or coax cables are required. The integrated AER system replaces all of these functions.

In addition to the dramatic reduction in equipment and installation costs, significant reductions in the cost of operation can be expected where AER systems are deployed. This is due to the lower power consumption, reduced leasing costs, and reduced site repair and maintenance costs related to the higher effective mean time between failures (MTBF).

In fact, reliability is one of the key strengths of the AER architecture and is a result of a number of its unique features. First, the power devices within the AER operate at a lower temperature than typical basestation power amplifiers because they handle a fraction of the total system power and are distributed across the backplane of the passive antenna, which acts as a heatsink for thermal dissipation. The lower temperature increases the MTBF of each component m-radio.

Additionally, AER is a system with built-in redundancy. If any of the m-radios should fail, the phase and amplitude of the remaining transceivers can be adjusted in response, and in most cases, the site can continue to operate indefinitely until regularly scheduled maintenance is performed. This self healing or graceful degradation is only possible due to the distributed architecture of the AER system.

Finally, from an environmental point of view, the lower carbon footprint and reduced visual clutter of sites that incorporate antenna embedded radios will be significant benefits to operators. Due to the recent spike in global fuel costs, the desire to minimize fuel consumption isnt just an issue of social responsibility, its an economic business necessity.

Similarly, increasing not in my neighborhood community resistance to the placement of basestation equipment anywhere it can be seen is driving up the cost and reducing the availability of equipment sites. AER systems, with their unparalleled integration level, will let operators avoid much of the hassle and expense of securing licenses and leases compared to legacy analog RF basestation equipment.

In Summary

Antenna embedded radios are one example of new creative solutions that are required to provide the benefits of active antenna systems without the associated drawbacks. As flat-rate, broadband tariff schemes become increasingly more prevalent and operators accelerate competition on data service, the need persists for ever lower costs.

This competitive pressure at the operator level will continue to drive the infrastructure segment to find more creative solutions to satisfy consumer hunger for ever increasing data at attractive prices. To meet this challenge, more revolution than evolution is needed to continue the neverending race toward higher levels of service (capacity and coverage) while maintaining the trend of lower cost per bit.