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 directly 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.