Antenna challenges in smartphones and tablets with 4G rising
Global mobile data consumption grew by 2.6 times in 2010. This is the third year in a row that mobile data usage has nearly tripled. By 2015, global mobile data traffic is projected to grow to 26 times the 2010 volume [1]. One of the key factors in this dramatic growth is the rapid adoption of smartphones and tablets. Global mobile data users expect their devices to connect at high data rates anywhere in the world.
This expectation puts a significant burden on network and device performance. In the device, the antenna is the only element that "touches" the network. Optimizing the performance of the antenna in mobile data devices is becoming increasingly critical and can make the difference between a satisfied customer and a customer who is ready to jump to another network provider.
However, the challenges for 4G antenna implementation in smartphones and tablets are significant. There are several possible solutions to these challenges, each of which results in potential performance trade-offs.
4G Antenna Implementation Challenges
A number of factors affect antenna performance in a handheld mobile communications device. While these factors are related, they generally fall into one of three categories: antenna size, mutual coupling between multiple antennas, and device usage models.
Antenna Size
The size of an antenna is dependent on three factors: bandwidth of operation, frequency of operation, and required radiation efficiency. Bandwidth requirements are increasing since they are driven by FCC frequency allocations in the United States and carrier roaming agreements around the world. Different regions use different frequency bands.
A description of the relationship between antenna size, bandwidth, and efficiency was developed in several seminal papers by (1)Harold Wheeler [“Effects of Antenna Size on Gain, Bandwidth, and Efficiency”, IEEE Trans AP-23,4] and (2)L. J. Chu [“Physical Limitations of Omni Directional Antennas”, Appl. Phys. Dec 1948], and later by (3)Roger Harrington [“Effects of Antenna Size on Gain, Bandwidth, and Efficiency”, Nat. Bur. Stand. 1960]. Simply stated, these limitations are: "Bandwidth and antenna size are inversely related" and "efficiency and antenna size are
directly related." This means that in general, a larger antenna will have larger bandwidth and efficiency.
In addition to bandwidth, the size of an antenna is also driven by the frequency of operation. In North America, Verizon Wireless and AT&T Mobility have opted to roll out their LTE offerings in the 700 MHz frequency band that was part of the FCC UHF-TV band reallocation a few years ago. These new frequency bands (Band 17, 704-746 MHz and Band 13, 746-786 MHz) are lower than the legacy cellular frequency band used in North America (Band 5, 824-894 MHz). This change is significant because lower frequencies have longer wavelengths and therefore require longer antennas to maintain radiation efficiency. These factors require the antenna size to increase in order to maintain radiation efficiency. However, device designers are adding larger displays and more features so the available antenna length and overall volume is restricted thus reducing the antenna bandwidth and efficiency.
Mutual Coupling Between Multiple Antennas
Newer, high-speed wireless protocols require MIMO (multiple-input, multiple-output) antennas. MIMO requires more than one antenna (usually two) to operate at the same frequency at the same time. Consequently, there are a larger number of antennas that need to be placed on the phone to function simultaneously without interacting with each other. When two or more antennas are in close proximity, they interact through a phenomenon know as mutual coupling.
Mutual Coupling Between Antennas
Newer, high-speed wireless protocols require MIMO (multiple-input, multiple-output) antennas. MIMO requires more than one antenna (usually two) to operate at the same frequency at the same time. Consequently, there are a larger number of antennas that need to be placed on the phone to function simultaneously without interacting with each other. When two or more antennas are in close proximity, they interact through a phenomenon know as mutual coupling.
Consider two closely spaced antennas on a mobile platform; a portion of the energy radiated from antenna 1 is intercepted by antenna 2. The intercepted energy is lost in the terminals of antenna 2 and cannot be used and is observed as a loss in system PAE (power-added efficiency). Due to reciprocity, the effect is the same in transmit and receive mode. The coupling magnitude is inversely proportional to the separation distance. For a handset implementation, the separation distance between antennas operating in the same band for MIMO and diversity can be 1/10 wavelength or less.
An example: free space wavelength at 750 MHz = 400 mm. When the separation is small, much less than a wavelength, the coupling is high. The energy coupled between antennas is wasted, decreasing data throughput and battery life.
Device Usage Models
Modern smartphones and tablets have a wide variation in usage models compared to conventional handsets. Not only does the device need to work as a phone when held to the head, it needs to work for a wide variety of handheld and body-worn positions. The device needs to perform well while still meeting specific absorption rate (SAR) and hearing aid compliance (HAC) regulatory requirements.
Another aspect of the usage model is the type of content being consumed. Video- intensive mobile applications like massively multi-player, online role-playing games (MMORPG) and real-time video streaming are driving an increase in data usage. According to ABI Research, from 2009 to 2015 data usage in Western Europe and North America is expected to increase at a compound annual growth rate (CAGR) of 42 percent and 55 percent respectively. These same applications are pushing manufacturers to incorporate larger, high-resolution displays on devices. Data usage is also driving the way consumers hold these mobile data devices. For gaming applications, it may be necessary to grip the ends of the device with both hands instead of holding it by the middle with one hand, yet other applications don’t require the device to be held at all.
Increased display area and variations in grip are making it increasingly more difficult to find a good location for the antenna radiator where it won’t be obstructed by the display or by the user’s hand. In addition to these constraints, device manufactures want fewer SKUs in their lineup, and there’s a growing trend toward platforms that work anywhere in the world.
How to Solve the Problem
For global operation, a modern smartphone or tablet must work in a variety of frequency bands and protocols. However, operation in all the frequency bands and protocols simultaneously is not required. It is possible to develop an antenna system that can be tuned to the desired band or bands of operation. This type of state-tuned antenna can be termed “smart antenna” or “adaptive antenna.” The fundamental principle is to limit the instantaneous operating frequency to one or two narrow bands of interest to satisfy the protocol requirements in a particular region. In this way, the need for wideband operation is reduced, allowing the antenna to fit into a more compact volume without sacrificing radiation efficiency.
Two basic methods are available to tune an antenna: feed point matching and aperture tuning. Several factors can affect the decision to implement feed point matching or aperture tuning. No single solution is appropriate for every application.
Feed Point Matching
Feed point matching is used in many antenna implementations whether tunable or not. The main function of the matching circuit is to match the antenna terminal impedance to the impedance of the rest of the radio systems, usually 50 ohms, over a wide range of operating conditions. A typical tunable match implementation uses parallel or series variable capacitors as part of the impedance matching circuit. Adjusting the capacitance will change the resonant frequency of the resulting circuit.
Depending on the desired antenna size compression and tuning range, a large variation in capacitance may be required to perform the frequency shifting. This commonly requires multiple tuning elements and/or a wide range of tuning values. Figure 1 illustrates feed point matching with variable elements.
Figure 1 – Fixed Broad Band Antenna with variable impedance match
Aperture Tuning
Aperture tuning is accomplished by changing the resonant structure of the radiating element. A typical implementation can use a simple switch to select different load elements on the antenna structure. Switching load elements changes the electrical length of the antenna resulting in a change in resonant frequency. Figure 2 shows an AC circuit model for a variable state, aperture-tuned antenna with a fixed impedance match.
Figure 2 – Variable State Antenna with fixed feed point matching
For either feed point matching or aperture tuning, if the antenna is being used to both transmit and receive, the tuning device must be able to withstand the maximum transmitter power while maintaining favorable performance characteristics.
An Example
The following example illustrates the benefits of tuning with respect to antenna volumetric size reduction. Two different antenna configurations were analyzed using a 3D electromagnetic modeling program: one a broadband design, and the other, a narrower band design capable of tuning over the same frequency range but using four tuning states.
Figure 3a illustrates a 50 x 6 x 14 mm seven-band antenna configuration and its associated radiation efficiency over just the lower three-band spectral region from 700-960 MHz.
A similar, but smaller (50 x 6 x 7mm) antenna configuration is shown in Figure 3b, illustrating that tuning using only four states can produce nearly the same efficiency and total frequency coverage as the larger broadband antenna.
It is clear from the example in Figures 3a and 3b that a physical volume reduction of one half can be achieved by tuning the antenna to one of several states, each supporting a certain set of frequency bands. The antenna during operation would therefore only be required to change state when the operating band is changed. The time required for this change must be compatible with other functions within the radio system. A typical requirement might be 10-20 microseconds or less.
Mutual Coupling
Mutual coupling effects between adjacent antennas operating at the same frequency at the same time can be mitigated using isolation techniques. The most common technique is to physically separate the antennas from each another. The mutual coupling effect drops as the distance increases. However, for handheld devices, it’s not always possible to find locations to give adequate separation to mitigate the effects of mutual coupling. In this case, device designers need a different antenna solution 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. 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. Figure 4 illustrates implementation of an iMAT antenna showing the isolation between two feed points on the same antenna structure.
Usage Models
To mitigate the effects of various usage models, it may be necessary to combine the benefits of state tuning and mode isolation. Mode isolation allows a single antenna structure with multiple feed points to perform the function of multiple MIMO antennas, while state tuning allows this structure to be very small and still operate across a wide range of frequencies with very high efficiency. Figure 5 shows the measured average efficiency for a variable state iMAT antenna structure covering multiple frequency bands in six tuning states. The iMAT structure is capable of operating in a balanced or imbalanced gain configuration and provides better performance in a smaller package than conventional antenna design techniques.
Significant challenges must be overcome in the implementation of an effective antenna system in today’s complex smartphone and tablet devices. LTE and other 4G networks are rolling out in a variety of frequency bands from 700 MHz to 2700 MHz. These new frequencies will be added to legacy 3G frequency bands to accommodate global mobile roaming and compatibility.
Advanced wireless networks use MIMO in the user equipment to improve data throughput. In addition, data intensive applications like on-line gaming and video streaming are driving larger displays and a wide variety of usage models. These factors make it difficult to find adequate space to implement multi-band, multiple antenna systems.
Advanced design techniques like state tuning and iMAT can answer the challenge, allowing device designers the flexibility to implement sleek, feature-rich devices to deliver on the promise of true 4G network performance.
References
[1]Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update, 2010
About the Author
Paul Tornatta is Chief Technical Officer of SkyCross (www.skycross.com), a global developer and manufacturer of advanced antenna solutions for the mobile phone, home entertainment, and computing industries. SkyCross technology delivers robust device connectivity for 4G networks worldwide. Mr. Tornatta has more than 25 years of experience in the aerospace, wireless, telecommunications, and automotive industries. Prior to joining SkyCross, he served as vice president of the automotive business unit at Radiall Corp., a leading supplier of RF interconnect products. He also served in leadership positions at Larsen Antenna Technologies, Metricom, and Lockheed Martin. Mr. Tornatta earned a bachelor’s degree in electrical engineering from New Mexico State University. He has written articles on RF design and electromagnetics for a variety of RF trade publications. He is a member of IEEE and the SAE.
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