This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.
In the past year, several US wireless carriers have spent almost $20 billion acquiring spectrum as part of the FCC's reallocation of the UHF (700 MHz) TV bands. The two biggest winners of spectrum, Verizon and AT&T, have both announced that they will use the spectrum to roll out LTE (Long Term Evolution) services.
Many industry leaders think that LTE is the 4G solution that will naturally follow today's 3G UMTS technology. LTE promises significantly higher data rates for both upload and download, thereby enabling a wide variety of Internet Protocol (IP) services such as VoIP and online gaming. The ability to deliver high data rates to a variety of consumer devices including Mobile Internet Devices (MIDs) and smartphones will largely drive the success of LTE.
The industry is painfully aware of the issues that surround implementing LTE in small mobile devices with already limited space. To make matters worse, LTE will likely be yet another service that the device must support. Therefore, LTE will compete with existing functions and services for PCB real estate and battery power, potentially increasing the cost of the handset.
For mobile handset antenna development, significant challenges arise when some of the complex implementation schemes of LTE are compounded by the longer wavelengths of the 700 MHz frequency band. This article explores these challenges and offers a solution.
700 MHz LTE ntenna Design Challenges
In the current LTE standard as defined by 3GPP, developers must implement multiple-in, multiple-out (MIMO) antenna technology and a number of advanced signal processing techniques to achieve the maximum data rate.
For effective MIMO, multiple independent antennas must operate simultaneously in the 700 MHz frequency band. Although there are several possible implementation schemes, this article focuses specifically on a 2×2 MIMO configuration with two independent transmit and receive paths.
The typical antenna specification for 2×2 MIMO has the following goals:
1. Number of independent antenna ports: 2
2. Radiation efficiency: As high as possible
3. Gain balance ratio (ratio of the gain at each antenna port): As high as possible, approaching 1
4. Correlation coefficient (envelope correlation coefficient between the two antenna ports): As low as possible, approaching 0
Other typical antenna parameters such as impedance bandwidth, compliance to SAR, and HAC are a given. Following is a detailed look at the implications for each of these requirements.
Requirement #1: Number of independent antennas – 2
There are several types of MIMO supported by the standard. This article looks at the implementation of 2×2 MIMO since it has a reasonable data rate benefit.
Requirement #2: Radiation Efficiency: As high as possible
The volume of the antenna has the largest effect on the maximum attainable antenna efficiency in a given bandwidth. In reality, the maximum antenna volume is a function of the size of the platform on which it is implemented.
In fact, the platform, in this case the handset itself, represents the counterpoise for the antenna excitation element. In other words, the handset is the antenna.
The typical handset is not the proper size to resonate at 700 MHz for LTE. A device is resonant where its electrical length is half of a wavelength. A typical smartphone is 50 mm wide x 100 mm long; we'll ignore thickness for the moment.
The fundamental resonance of a platform with these dimensions is well above 1000 MHz. Therefore, designers must compromise bandwidth or efficiency to force the antenna to operate at lower frequencies. The carriers determine the required bandwidth in their frequency plan for a particular service.
The AWS700 auction frequency bands span about 100 MHz centered near 750 MHz. This limits the efficiency of the antenna if it must cover the entire 100 MHz or approximately 13% bandwidth:
Implementation losses will drive the maximum attainable efficiency to well below 50 percent as a starting point, and this also assumes only one antenna. Additional antennas operating within the same band or at nearby frequencies will interact through mutual coupling and further reduce the efficiency.
Requirement #3: Gain Balance Ratio – High as possible, approaching 1
This specification is intended to drive platform and antenna designers to develop solutions where both antennas in the MIMO configuration have equal gain performance. As the gain imbalance increases, the benefit of MIMO decreases.
With gain imbalance of 3dB or more, the benefit of MIMO is significantly reduced. To have two antennas with equal performance on a given platform, each of the antennas should have a similar counterpoise dimension.
The best efficiency for a single antenna occurs where the antenna excites the fundamental mode of the counterpoise. However, if both antennas excite the same mode, they will tend to interact though a phenomenon called mutual coupling. If both antennas terminate in loads, mutual coupling between the two antennas will have an adverse effect on the performance of both antennas.
For non-MIMO systems where diversity is used for interference cancellation, detuning one of the antennas mitigates this problem; therefore gain imbalance of 4dB or more is acceptable for many 3G diversity antenna implementations. This gain imbalance for 2×2 MIMO is unacceptable because it defeats the purpose of having two antennas.
Requirement #4: Correlation Coefficient – Low as possible, approaching 0
This parameter characterizes the independence of each communication path. The greatest capacity occurs when the correlation coefficient is 0, meaning the two communication paths are completely independent from each other.
The easiest way to make two antennas that operate at the same frequency at the same time independent from each other is to move them apart by a significant distance. A significant distance for most antenna systems is a quarter wavelength or more. In the AWS700 band, a wavelength at 750 MHz is 400 mm, so a quarter wavelength is 100 mm.
This suggests that antennas implemented on the ends of a 100 mm long smartphone might be far enough apart to have low correlation coefficient; however, this is only true if the antennas are truly independent of one another.
As we saw in the previous example, two antennas at opposite ends of the same handset (counterpoise) will tend to excite the same radiating mode and effectively have the same radiation pattern.
The Skycross Solution
Device designers need an antenna solution with different operating modes for two antenna ports to achieve the performance required by the specification. One possible solution is to excite two different modes from the same antenna structure using Isolated Mode Antenna Technology (IMAT) from SkyCross, Inc.
The IMAT antenna structure is placed on one end of the phone. Each of the two feed points launches a different radiating mode. The feed points are isolated from each other and do not suffer from the losses normally associated with mutual coupling, so the efficiency of each mode is high. In addition, the radiation patterns are different and produce a low correlation coefficient.
When properly implemented, LTE offers the potential for significantly higher data rates than we experience today. This capability opens the door for a wide range of high-content applications.
But, the industry can only realize these advantages by overcoming the design challenges of enabling the service on popular platforms such as the handset. The most challenging aspects of antenna design for LTE stem from the MIMO requirement, which demands two high performance antennas, and the small size of a typical handset in terms of wavelength for resonating at 700 MHz.
Solving this problem requires counterintuitive thinking and advanced antenna techniques. One possible solution is the application of Isolated Mode Antenna Technology (IMAT) from SkyCross.
IMAT enables a single antenna structure with multiple independent feed points to act like multiple high performance antennas. This technology is available today so carriers can plan 4G rollouts with confidence that devices for their networks will be ready.
Skycross Chief Technology Officer Paul Tornatta has more than 18 years of experience the aerospace, wireless, telecommunications and automotive industries. He earned a bachelor's degree in electrical engineering from New Mexico State University.