The industry will deliver 4G services as its next step in the quest for faster and better connectivity. The new networks that support these services promise higher data rates, increased network capacity, and better reliability for end users. These benefits can enable last-mile connectivity, citywide hot spots, and more robust mobile content without compromising network speed.

The antenna is the only structure in a mobile device that communicates directly with the wireless network. This makes the antenna performance pivotal to the experience of end users and how they perceive the device, network, and carrier. With so much depending on the antenna performance, device manufacturers and carriers alike must carefully select the right antenna solutions for 4G. 

Much debate surrounds the comparative benefits of WiMAX and Long-Term Evolution (LTE). Despite this ongoing debate, both 4G services rely on a multiple-input, multiple-output (MIMO) operation as a key enabler for enhanced performance. MIMO, which typically requires multiple antennas, is not a simple implementation because of space constraints, especially in small mobile devices. (Interestingly, a handset may not be that small at WiMAX frequencies, but is certainly small at AWS700 frequencies. USB dongles and ExpressCards are small for both WiMAX and LTE.)

Effective MIMO Systems

MIMO systems rely on recovering data broadcast over multiple independent channels existing between transmit and receive locations to increase data throughput or signal reliability. MIMO system implementation typically requires multiple antennas at both the receiver and transmitter locations. MIMO systems should generally exhibit the following characteristics:

• Low correlation coefficient: The correlation coefficient is a measure of the decorrelation or independence of the signals being broadcast, and it depends in part on differences in the radiation patterns for different antennas. Antennas with very different radiation patterns cover new spatial regions to achieve pattern diversity and signal independence. If the radiation patterns are exactly the same, the correlation is assumed to be 1. If they are completely different, the correlation is 0. Correlation coefficients closest to 0 are preferred in obtaining the greatest effectiveness of the MIMO system over the entire cell area. 

• High isolation/ low coupling: Coupling is a term describing how one antenna affects another antenna in close proximity. Closely coupled antennas exhibit low isolation from one another and begin to exhibit unwanted power transfer to each other and to the circuitry connected to each. High isolation and low coupling are therefore desirable characteristics from the standpoint of radiated power efficiency.

• High efficiency: Radiation efficiency is a measure of how much of the electrical power supplied to an antenna element is converted to electromagnetic power. A 100% efficient antenna would theoretically convert all input power into radiated power with no loss to resistive or dielectric elements. Efficiency affects battery life primarily when portable devices are used in the transmit mode.

Available space is at the root of antenna design challenges for MIMO systems. Designers must find the room in crowded devices to place extra antennas. Simply finding the space isn’t necessarily sufficient since relative antenna location is also a critical issue in meeting the operational requirements. These requirements include government-mandated biological radiation limits (specific absorption rate, or SAR), minimum radiation efficiency imposed by service carriers (AT&T, Verizon, etc.), and performance specifications for when devices are held in the hand or against the head. Because placing antennas in ideal locations with adequate size is often infeasible, antenna designers have developed a variety of different antenna solutions to address the problem, with an equally wide range of success.

WiMAX Solutions Today

Figure 1 shows data for six different WiMAX antennas as measured by Beceem Communications, a WiMAX chipset provider. Beceem collected the data using a RUSK channel sounder, which performs MIMO measurements based on the “switched array” principle. The equipment uses a chirp-like signal to sound the channel and records the complex channel transfer function. The maximum throughput can be estimated from the channel coefficients.