introduction The demand for high-performance, multi-function handheld devices such as mobile phones, PDAs, and smart phones is growing. One of the motivations is that multiple technologies are integrated into a device that can hold and load pockets, which has and will continue to drive these needs. The recent trend in the handheld device market is that these devices are beginning to have color display and camera functions , and Bluetooth wireless headsets are popular, which will eventually eliminate the connection between mobile phone headsets and mobile phone hosts. The resolution of the display is getting higher and higher, the performance of the audio system is also improving, and the memory capacity and CPU performance available for the system are also increasing. Above the continuous development of hardware and processing capabilities will contribute to the achievement of new applications such as video telephony, VoIP on enterprise and home WLAN, high-definition video streaming and high-speed Internet, and will also provide ubiquitous mobile phone coverage. Figure 1 More and more handheld devices with Bluetooth, WLAN , GSM , GPRS capabilities Figure 2 Â Power spectrum distribution curve, 75% outside the bandwidth Figure 3 Â Transmit power of different wireless technologies Design challenge It is well known that due to the small size of these handheld devices and the limited capacity of the batteries, each subsystem must be small and consume very little power. Adding new technology without increasing the size of the device is critical to every generation. In each generation, it is expected that the battery life will be longer and longer. Another deep problem is that as multiple wireless technologies converge in handheld devices, new electromagnetic interference problems can arise. In addition to small form factor and low power requirements, the design of the wireless subsystem must also allow subsystems to coexist without damaging each other. This article will delve into how to address these new requirements and challenges to design a new generation of high-performance, high-performance wireless handheld devices. For example, some next-generation handheld products will include GPS , WLAN and Bluetooth, as well as 2.5G and 3G cellular technologies that can easily operate in up to five to six frequency bands. The standards committee that sets each of these standards does not specify the receiver requirements, but assumes that the device is within the 10 cm spectrum of two to three other transmitters . Thus, in order for the highly integrated handheld device with multiple wireless technologies to work, the design team must calculate the transmission spectrum power level requirements and the receiver adjacent channel rejection requirements to avoid self-interference by the handheld device. Wireless capabilities integrated in handheld devices Before discussing engineering design details, we must first review why multiple wireless systems are integrated into one handheld device. Because consumers are used to carrying mobile phones at any time, and can call and answer calls at any time. This habit and lifestyle will not change, so cellular technology and related improvements will continue. Among all the technologies discussed, cellular technology is a truly ubiquitous overlay technology that will continue to drive automatic connectivity anytime, anywhere, and is now expanding from voice services to Internet data services. In recent years, the WLAN technology market has expanded and penetrated into three key areas: homes, offices and hotspots. Hotspots are highly concentrated locations such as hotels or airports. The main advantage of WLAN is that it is very fast and has a strong penetration in the building, making it a natural choice for office environments. WLAN service costs are often not charged on a per-minute basis, and network capacity is currently tens of Mbps , and even hundreds of Mbps in the future . In addition to consumers' accustomed to high-speed data services implemented by WLAN , WLAN is expected to achieve high-speed video, video conferencing, audio and VoIP services. Bluetooth has also found its use in the above devices. We can now see people wearing headsets while walking and talking. In the future, Bluetooth will become the key to the realization of wireless technology for such headsets. For low-speed devices such as wireless keyboards and mice that require low power consumption, they do not require high bandwidth, and Bluetooth is an ideal technology. GPS or Global Positioning System is no longer a patent for aircraft. With the reduction of the cost of the GPS subsystem, the technology has appeared in the handheld terminal, and the navigation device and the emergency location service can be realized. WLAN design in handheld devices Product designs with all of the above wireless systems face mulTIdimensional problems. Each system has its own transmission spectrum characteristics and requires adjacent channel suppression for its own custom receiver. We will discuss this issue from the perspective of the WLAN subsystem, although the next generation system will be based on IEEE 802.11g (54Mbps) , for the sake of simplicity, we will specifically discuss IEEE 802.11b (11Mbps) . The design philosophy of each wireless subsystem must follow the "no damage" principle and the "anti-interference robustness" principle. The first part of the "no damage" principle means that the emission spectrum mask must not significantly increase the noise floor in other wireless system bandpasses. Of course, the so-called other wireless systems are systems in the same handheld terminal device, so we can assume isolation between up to 20 dB antennas. For a thermal noise lower limit of -114 dBm/MHz , the WLAN system preferably transmits no more than -94 dBm/MHz in other device bandpasses . In the actual situation, the above requirements will add some margin and will not have a big impact on other systems. Another method is to limit the out-of-band transmit power so that the effects of other wireless system sensitivity degradations do not exceed a given value. The 802.11b standard determines that when the frequency exceeds the central frequency above 22 MHz , the transmitted spectrum should be 50 dB lower than the power in the passband . The maximum transmit power in the passband is typically 12 dBm/MHz , so the above requirement would require -38 dBm/MHz outside the passband , which is far from "no damage". Regulators may impose further restrictions on out-of-band transmit power, but even these additional limitations may not be sufficient to ensure adequate performance. This example clearly shows that the WLAN subsystem requires more specifications to ensure it does not harm other wireless subsystems. Similarly, the WLAN subsystem must also have "anti-interference robustness." In handheld devices, this has a completely new meaning. In the case of a WLAN- enabled laptop, the distance to other wireless transmitters is measured in meters or even tens of meters, while the distance in the handheld terminal is measured in centimeters. The number of sources of interference that must be tolerated varies from 0 dBm to 30 dBm depending on the transmission power , depending on the distance from the WLAN system from 0 to 1600 MHz . Figure 3 shows some examples. The 802.11b standard determines that adjacent channel rejection must be at least 35 dB at a receiver center frequency of 25 MHz . There are no further requirements for the adjacent channel rejection specification. For example, there are no specifications for compression points. In addition, the 802.11b standard specifies that suppression should be tested against another 802.11b waveform. We can expect this, of course, it is also needed, but for some purposes, it is not enough. For example, the specification does not guarantee that the receiver will operate normally with a PCS transmitted from the antenna at 5 cm and a transmission noise of 30 dBm . Additional design requirements must also be added to ensure that WLAN , Bluetooth, and other wireless systems can work in these close-range environments. A difficult problem to solve is how Bluetooth and WLAN coexist in handheld devices. This problem is particularly difficult to solve in handheld terminal devices, because the isolation between Bluetooth and WLAN antennas is very limited and it is easy to form mutual interference. Bluetooth and WLAN are difficult to coexist because both are running in the 2.4 GHz ISM band. Therefore, both systems are designed with a channel preselection filter that occupies the entire 2.4 GHz band. This makes it particularly difficult to remove interfering signals. However, we have multiple solutions to this problem. One solution is to use time division multiplexing to avoid inter-system interference. In addition, adaptive frequency hopping in the Bluetooth 1.2 solution is standardized. However, even with the above improvements, WLAN and Bluetooth are still difficult to work simultaneously without using technologies such as transmit power control. . Although the problem of coexistence between WLAN and Bluetooth is difficult to solve, we have found a solution, so users can realize WLAN and Bluetooth operation at the same time , and the main applications such as Bluetooth voice are not affected. WLAN devices in devices such as smart phones, mobile phones, and PDAs are unique in terms of adjacent channel requirements, emission spectrum requirements, size requirements, and power consumption requirements. Not every WLAN system meets the performance requirements required for these environments. In addition, system designers must select products that are specifically designed to meet the above requirements, otherwise the performance of the corresponding product will be unsatisfactory. Fortunately, the above requirements are not achievable, and these features have been successfully implemented in today's state-of-the-art WLAN systems and in reference designs for PDAs . 
