Smartphone design tips (3): Cleverly solve 4G antenna design problems

In 2010, global mobile data consumption increased by 2.6 times. This is the third consecutive year that mobile data usage has increased by nearly 3 times. By 2015, global mobile data traffic is expected to increase by 26 times compared to 2010. One of the key factors leading to this dramatic growth is the rapid popularity of smartphones and tablets. Global mobile data users want their devices to be able to connect to the Internet at high speed anywhere in the world.

　　This expectation places a huge burden on network and device performance. In mobile data devices, the antenna is the only part that "reaches" the network, and optimizing antenna performance becomes increasingly important. However, the challenges of 4G antenna design in smartphones and tablets are daunting. Although there are multiple possible solutions to these challenges, each one has potential performance tradeoffs.
 

　　Smartphone design tips (1): Completely solve the problem of accidental touches on the touch screen

　　Smartphone design tips (2): separation and integration design considerations

　　4G Antenna Design Challenges

　　There are many factors that affect the antenna performance of a handheld mobile communication device. Although these factors are related, they can generally be divided into three categories: antenna size, mutual coupling between multiple antennas, and device usage model.

　　Antenna size Antenna size depends on three factors: operating bandwidth, operating frequency, and radiation efficiency. Today's bandwidth requirements are getting higher and higher, driven by FCC frequency allocations in the United States and operator roaming agreements around the world; different regions use different frequency bands. "Bandwidth and antenna size are directly related" and "efficiency and antenna size are directly related" - this generally means that a larger antenna can provide greater bandwidth and higher efficiency.

　　In addition to bandwidth, antenna size also depends on the operating frequency. In North America, operators Verizon Wireless and AT&T Mobility have chosen to promote LTE products that operate in the 700MHz band, which was part of the FCC UHF-TV reallocation band a few years ago. These new bands (17,704-746MHz and 13,746-786MHz) are lower than the traditional cellular bands used in North America (5,824-894MHz). This change is huge because the lower the frequency, the longer the wavelength, and thus the longer the antenna is needed to maintain the same radiation efficiency. In order to ensure radiation efficiency, the antenna size must be larger. However, device system designers also need to add larger displays and more functions, so the available antenna length and overall volume are greatly limited, thereby reducing antenna bandwidth and efficiency.

　　Antenna coupling Newer high-speed wireless protocols require the use of MIMO (Multiple Input Multiple Output) antennas. MIMO requires multiple antennas (usually two) to work at the same frequency at the same time. Therefore, multiple antennas need to be placed on the phone device, and these antennas must work at the same time and cannot affect each other. When two or more antennas are located very close to each other, a phenomenon called mutual coupling occurs.

　　For example, two antennas are placed in close proximity on a mobile platform. Some of the energy radiated from antenna 1 will be intercepted by antenna 2. The intercepted energy will be lost in the terminal of antenna 2 and cannot be used. This can be expressed as a loss in the system power added efficiency (PAE). According to the principle of interchangeability, this effect is the same in transmit and receive mode. The coupling amplitude is inversely proportional to the separation distance of the antennas. For mobile phone implementations, the distance between antennas operating in the same frequency band for MIMO and diversity applications can be 1/10 wavelength or less. For example, the free space wavelength at 750MHz is 400mm. When the separation is very small, such as much less than a wavelength, the coupling degree will be very high. The energy coupled between antennas is useless and only reduces data throughput and battery life.

　　The usage model of smartphones and tablets has changed significantly compared to traditional mobile phones. In addition to normal operation, these devices must also meet the regulatory requirements of electromagnetic wave energy absorption ratio (SAR) and hearing aid compatibility (HAC).

　　Another aspect of usage models is the type of content consumed. Video-intensive mobile applications such as massively multiplayer online role-playing games (MMORPGs) and real-time video streaming continue to drive data usage through the roof. According to ABI Research, data usage is expected to grow at a compound annual growth rate (CAGR) of 42% and 55% in Western Europe and North America, respectively, from 2009 to 2015. These and other applications are driving manufacturers to produce larger, higher-resolution displays. The increase in data usage is also quietly changing the way consumers hold these devices. For example, for gaming applications, users must hold the device with both hands, while other applications may not require the user to hold the device at all.

　　The increasing size of displays and changes in user grip make it increasingly difficult to find a good location for the antenna radiator that is not blocked by the display or the user's palm. In addition to these constraints, device manufacturers want to have fewer SKUs (minimum stock keeping units) in their product lines, and the development of platforms that can work anywhere in the world is the trend for such products.

　　Solution

　　To achieve global compatibility, a smartphone or tablet must be able to operate in a variety of frequency bands and protocols. Of course, it is not required to operate in all frequency bands and protocols at the same time, so an antenna system that can be adjusted to the target operating frequency band can be developed. This state-tuned antenna can be called a "smart antenna" or "adaptive antenna." The basic principle is to limit the instantaneous operating frequency to one or two narrowband frequency bands of interest to meet the protocol requirements of a specific region. In this way, the requirements for broadband operation are reduced, allowing the antenna to be packed into a more compact space without sacrificing radiation efficiency.

　　There are two basic approaches to antenna tuning: feed point matching and aperture tuning. However, there are many factors that influence the implementation decision of these methods, and there is currently no single solution that is suitable for every application.

　　Feed point matching is used in many antenna implementations, both tunable and non-tunable. The main function of the matching circuit is to match the antenna terminal impedance to the impedance of the rest of the radio system (usually 50Ω) over a wide range of operating conditions. Typical tunable matching implementations use shunt or series variable capacitors as part of the impedance matching circuit. Adjusting the capacitance can change the resonant frequency of the target circuit.

　　Depending on the required antenna size, the tuning range is compressed and a large range of capacitance changes are generally required to achieve frequency migration, thus usually requiring multiple tuning elements and/or a wide range of tuning values. Figure 1 shows an antenna feed point matching circuit using variable elements.

　　Figure 1: Fixed broadband antenna using a variable impedance matching circuit

　　Aperture Tuning Aperture tuning is achieved by changing the resonant structure of the radiating element. This is typically accomplished using a simple switch to select different load elements on the antenna structure. Switching the load elements affects the electrical length of the antenna, thereby changing the resonant frequency. Figure 2 is an AC circuit model of a variable state, aperture tuned antenna using a fixed impedance matching circuit.

　　Figure 2: Variable state antenna using fixed feed point matching circuit

　　Whether using feed point matching or aperture tuning, if the antenna is used for both transmission and reception, the tuning device must be able to withstand the maximum transmission power and maintain good performance characteristics.

　　Case Description

　　The following example illustrates the benefits of tuning methods in terms of antenna size reduction. Here, a 3D electromagnetic modeling program is used to analyze two different antenna configurations: a broadband design and a narrowband design that can be tuned over the same frequency range but uses four tuning states.

　　Figure 3a shows a 50x6x14mm 7-band antenna configuration and the associated radiation efficiency over the lower tri-band spectrum from 700MHz to 960MHz. Figure 3b shows a similar but smaller (50x6x7mm) antenna configuration. As can be seen, using a 4-state tuning circuit can produce almost the same efficiency and overall frequency coverage as the larger wideband antenna.

　　Figure 3: In the 700MHz to 960MHz range

　　a) Multi-band antenna

　　b) Comparison of the volume and radiation efficiency of tuned antennas (antenna dimensions in mm).

　　As can be clearly seen from the example in Figure 3, by tuning the antenna to a certain state, each state supports a specific set of frequency bands, the physical size of the antenna can be halved. When the antenna is operating, if you want to change the operating frequency band, you only need to change the state. However, the time required for this change must be consistent with the requirements of other functions in the radio system. The typical requirement is 10ms to 20ms or less.

　　Mutual coupling effect Mutual coupling effect occurs between adjacent antennas working at the same frequency, which can be mitigated by isolation technology. The most commonly used technology is to physically separate the antennas from each other. As the separation distance increases, the mutual coupling effect will decrease. However, for handheld devices, it is difficult to provide sufficient spacing to reduce the mutual coupling effect. In this case, system designers need to use other different antenna solutions to achieve the performance indicators required by the specification.

　　Another possible solution is to use the Isolated Mode Antenna Technology (iMAT) provided by SkyCross to generate two different modes from the same antenna structure. The iMAT antenna structure is placed at one end of the phone; the two feed points run different radiation modes. The two feed points are isolated from each other and do not suffer losses due to mutual coupling, so the efficiency of each mode is very high. In addition, the radiation patterns are different, resulting in a lower correlation coefficient. Figure 4 describes the implementation principle of the iMAT antenna, from which you can see the isolation between the two feed points on the same antenna structure.

　　Figure 4: iMAT antenna implementation principle

Combining state tuning and mode isolation#e#
　　Using the model

　　To mitigate the impact of various usage models, it is necessary to combine state tuning and mode isolation. Mode isolation allows a single antenna structure with multiple feed points to perform the functions of multiple MIMO antennas; while state tuning allows such a structure to be very small, but still operate very efficiently over a wide frequency range. Figure 5 shows the average measured efficiency of a variable state iMAT antenna structure covering multiple frequency bands with 6 tuning states. The iMAT structure can operate in balanced or unbalanced gain configurations and can provide higher performance in a smaller package than traditional antenna design techniques.

　　Figure 5: State-tuned iMAT architecture with two MIMO antenna ports covering all 3G/4G applications

　　For complex smartphone and tablet devices, implementing efficient antenna systems presents significant challenges. Emerging LTE and other 4G networks cover different frequency bands from 700MHz to 2700MHz. These new frequencies will be added to the traditional 3G bands to meet global mobile roaming and compatibility requirements.

　　Advanced wireless networks increase data throughput by using MIMO in user devices. In addition, data-intensive applications such as online gaming and video streaming are driving larger displays and a wide range of usage models. This also poses more challenges to system designers, such as finding enough space on the device to implement multi-band multi-antenna systems. Fortunately, advanced antenna design techniques such as state tuning and iMAT can help designers calmly meet the above challenges, flexibly realize stylish, feature-rich mobile devices, and provide true 4G network performance.