Development Trends and Advantages of Wireless LAN Front-end Modules (Reposted)
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Today, WLAN is becoming the standard in traditional consumer products such as laptops and home networks, as well as emerging markets such as game consoles and cameras . With the built-in wireless IP voice (VoIP) phone and networking capabilities in media players, mobile phones and PDAs, WLAN standards (802.11a/b/g) are expected to further penetrate the consumer electronics field. The introduction of 802.11n will bring the overall market to a new high. The 300~600Mbps performance provided by this standard will finally realize the full home wireless media distribution function. This capability opens up the audio and video market, including TVs, set-top boxes and DVD players. These markets are extremely competitive and therefore very cost sensitive. For WLAN to enter these areas, it must provide wireless functionality in a way that is extremely low cost and has little impact on the final product yield. This article will discuss the advantages of WLAN radio frequency (RF) front-end modules (FEM) in meeting these requirements. Traditional 802.11a/b/g products Figure 1 shows the basic architecture of a WLAN FEM. RF engineers can implement the functions shown in the figure using 20 to 30 discrete components. However, compared with discrete solutions, WLAN FEM has the following advantages: 1. Higher performance Module solutions use custom die, wire bonding, and ultra-small stacking processes to achieve 0.5dB to 1.0dB lower implementation loss than discrete component solutions. On the other hand, designers using discrete designs have to use multiple packaged devices from different manufacturers, which results in packaging parasitics. This means that module implementations have higher output power and longer coverage than discrete designs, or the same output power as discrete designs, but with higher efficiency, which can extend battery life. 2. Smaller form factor A discrete design would require four times the area of a FEM solution to implement the functions shown in Figure 1. With a FEM, these complete functions can be implemented in a 4×5mm2 or even smaller form factor. This is especially important in the embedded market where space is a major concern.  | | Figure 1: RF front-end module architecture. | 3. Easy to design Since FEM is a fully matched module and generally only requires a power decoupling capacitor to complete the circuit, there is almost no need for adjustment and circuit board redesign. In addition, due to the high integration of the device, the terminal application circuit can be easily transferred to different form factors. In this way, in the computer industry, when the PCIE mini-Card design is completed, the same application circuit can be easily transferred to the form factors of mini-PCI and Cardbus. 4. Higher product yield The FEM is a fully tested solution, so there is no end-product yield loss due to RF issues. The main problem with WLAN FEMs has always been that they are more expensive than discrete solutions (about 10%). However, with the advent of newer generations of FEMs that are priced on par with discrete solutions, this disadvantage no longer exists. FEM suppliers have effectively reduced costs by integrating more functions into the core PA and switch chips, eliminating the need for stacking or passive components inside the FEM. Due to the advantages listed above and the same price, FEMs will continue to dominate the WLAN front-end field. 802.11n Market The 802.11n market has presented additional challenges to RF designers. As functionality has increased exponentially, designers have been forced to use two dual-band transmitters and receivers (as shown in Figure 2), and in some cases a third receive chain has been required. This increase in functionality has created two issues:  | | Figure 2: Schematic diagram of the 802.11n RF front-end module structure. | 1. Continuously reducing form factor The latest generation of WLAN cards, called PCI-Express mini-Cards, are half the size of mini-PCI cards, which were once the most common form factor for WLANs. Figure 3 illustrates the challenges facing WLAN designers. 2. Form Factor Power Budget Industry standard form factors such as PCI-Express mini-Card, mini-PCI, and Cardbus all define not only physical dimensions, but also signaling and power budgets. An 802.11n MIMO solution introduces more baseband processing, multiple transmit and receive radios at the same time, and multiple PAs to transmit. When all of these factors are taken into account, the overall power consumption approaches the overall power budget allowed by the form factor. 802.11n RF FEM provides a solution to the above two major problems. FEM can realize the functions required for a complete dual-band MIMO solution in an area 25% smaller than discrete solutions. As for power consumption, as mentioned earlier, FEM solution has less loss and higher efficiency in the transmit chain. Therefore, RF FEM can fully meet the requirements of the 802.11n market. In addition to the two main issues listed above, FEM can also solve some other problems in implementing 802.11n solutions:  | | Figure 3: Relationship between form factor and standard complexity. | 1. Gain matching Due to MIMO performance, it is most convenient for end product designers to ensure that the gain of the transmit links is within 2 to 3 dB of each other. In addition to helping MIMO performance, similar gain and performance also helps with end product calibration because if the channels are similar, only one channel needs to be calibrated, and the second channel becomes very simple to calibrate, which saves time and therefore costs. FEM solutions can solve this problem because: a. the module design company controls the complete module build and tests the final solution, verifying/screening through design and test to ensure matching; b. a complete integrated 802.11n FEM inherently guarantees that the two PA links are linked together. This is not the case with discrete designs, where the PAs are placed on the application board and may be derived from different manufacturing processes and do not have similar performance. The same is true for implementing an 802.11n solution using two 802.11a/b/g FEMs. 2. Receiving loss For similar reasons as gain matching, ensuring the receive chains are similar can provide performance and calibration benefits. Conclusion This article explains the benefits of using RF FEMs in WLAN applications, including the traditional 802.11b/g and 802.11a/b/g single-band and dual-band standards, as well as the newer and more complex 802.11n standard. FEMs are not only easy to design, can solve system implementation problems, but also can improve performance, so as their prices begin to match those of discrete solutions, FEMs will become the main RF solution in WLAN solutions.
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