introduction
The demand for high-performance, multi-functional handheld devices such as mobile phones, PDAs and smart phones is growing. One of the driving forces is the integration of multiple technologies into a device that can hold and hold the imported bag, which has and will continue to drive the development of the above requirements. The recent development trend of the handheld device market is that the above-mentioned devices have begun to have color screen display and camera functions , and Bluetooth wireless headphones are prevalent, which will eventually eliminate the connection between the mobile phone headset and the mobile phone host. The resolution of the display screen is getting higher and higher, the performance of the audio system is constantly improving, and the available memory capacity and CPU performance of the system are also constantly improving. The aforementioned evolving hardware and processing functions will drive the implementation of new applications, such as video telephony, VoIP on corporate and home WLANs , high-definition video streaming, and high-speed Internet, while also achieving ubiquitous mobile phone coverage.
Figure 1 More and more handheld devices have Bluetooth, WLAN , GSM , GPRS functions
Picture 2 Â Power spectrum distribution curve, 75% is outside the bandwidth
Picture 3 Â Transmit power of different wireless technologies
Design challenge
As we all know, due to the small size of these handheld devices and the limited capacity of the battery, each subsystem must be very small, and the power consumption must be very low. Adding new technology without increasing the size of the device is critical to every generation of product. In each generation of products, the life expectancy of batteries should be longer and longer. Another deep problem is that as multiple wireless technologies converge in handheld devices, a new variety of electromagnetic interference problems will emerge. In addition to the small form factor and low power consumption requirements, the design of the wireless subsystem must also enable each subsystem to coexist without harming each other. This article will discuss how to solve the above new requirements and challenges in order to design a new generation of high-performance, high-function wireless handheld devices.
For example, some new-generation handheld products will include GPS , WLAN, and Bluetooth, as well as 2.5G and 3G cellular technologies. These devices can easily operate in up to five to six frequency bands. The standards committee that set each of these standards does not specify receiver requirements, but assumes that the equipment is within the 10 cm spectrum of two to three other transmitters . In this way, in order to enable the above highly integrated handheld devices with multiple wireless technologies to work, the design team must calculate the transmission spectrum power level requirements and receiver adjacent channel suppression requirements to avoid self-interference in the handheld devices.
Wireless functions integrated in handheld devices
Before discussing engineering design details, we must first review why multiple wireless systems are integrated into a handheld device. Consumers are used to carrying mobile phones at any time, and can make and receive 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 technology that can truly achieve ubiquitous coverage. It will continue to promote automatic connection anytime, anywhere, and this connection is currently expanding from voice services to Internet data services.
In recent years, the WLAN technology market has continued to expand and penetrated into three key areas: homes, offices, and hot spots. Hotspots refer to places where users are highly concentrated, such as hotels or airports. The main advantage of WLAN is that it is very fast and has strong penetration in buildings, which makes it a natural choice in office environments. The service cost of WLAN is often not charged per minute, and the network capacity has reached tens of Mbps at present , and may even reach hundreds of Mbps in the future . In addition to consumers already accustomed to high-speed data services implemented by WLAN , WLAN is expected to realize high-speed video, video conferencing, audio and VoIP services.
Bluetooth also finds its place in the above devices. We can now see people wearing headsets and talking while walking. In the future, Bluetooth will become the key to the realization of this headset wireless technology. For low-speed devices that require low power consumption, such as wireless keyboards and mice, they do not require high bandwidth, and Bluetooth is also an ideal technology.
GPS , the global positioning system, is no longer a patent for aircraft. As the cost of the GPS subsystem is reduced, this technology has already appeared in handheld terminals, which can implement navigation equipment and have emergency location services.
WLAN design in handheld devices
All of the above has a wireless system design will be faced with many (mulTIdimensional) problems. Each system has its own transmission spectrum characteristics, and needs to customize the adjacent channel suppression function of the receiver. We will discuss this issue from the perspective of the WLAN subsystem, although the new generation system will be based on IEEE 802.11g (54Mbps) . For simplicity, we will specifically discuss IEEE 802.11b (11Mbps) .
The design philosophy of each wireless subsystem must follow the principle of "no damage" and the principle of "anti-interference and robustness". The first part of the "no damage" principle means that the emission spectrum mask must not significantly increase the lower noise limit in the bandpass of other wireless systems. Of course, the so-called other wireless systems are systems in the same handheld terminal device, so we can assume that isolation is at most between 20 dB antennas. As far as the lower limit of thermal noise is -114 dBm / MHz , the WLAN system preferably transmits no more than -94 dBm / MHz in the bandpass of other devices . In actual situations, the above requirements will add some margin and will not have a great impact on other systems. Another method is to limit the out-of-band transmit power so that the effect of sensitivity attenuation of other wireless systems will not exceed a given value.
The 802.11b standard determines that when the frequency exceeds the central frequency by more than 22 MHz , the emission spectrum should be reduced by 50 dB compared to the power in the passband . The maximum transmit power in the passband is usually 12 dBm / MHz , therefore, the above requirement will require -38 dBm / MHz outside the passband , which is far from "no damage". Regulatory agencies may impose further restrictions on out-of-band transmit power, but even these additional restrictions may not be sufficient to ensure adequate performance. This example clearly shows that the WLAN subsystem requires more specifications to ensure that it does not harm other wireless subsystems.
Similarly, the WLAN subsystem must also have "anti-interference robustness". In handheld terminal devices, this has a completely new meaning. As far as WLAN- capable notebook computers are concerned, the distance to other wireless transmitters is measured in meters or even tens of meters, while the distance in handheld terminals is measured in centimeters. The number of interference sources that must be tolerated varies from 0 dBm to 30 dBm depending on the transmission power , and varies from 0 to 1600 MHz from the WLAN system . Figure 3 gives some examples.
The 802.11b standard determines that the adjacent channel suppression must be at least 35 dB when the receiver's central frequency is 25 MHz . There is no further requirement in the adjacent channel suppression specification. For example, there is no relevant specification 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, this specification simply cannot guarantee that the receiver will work properly when the PCS is transmitted from the antenna 5 cm away and the transmission noise is 30 dBm . Additional design requirements must also be added to ensure that WLAN , Bluetooth and other wireless systems can work in the above-mentioned close-range environment.
A difficult problem is how Bluetooth and WLAN coexist in handheld devices. This problem is particularly difficult to solve in handheld terminal devices. This is because the isolation between Bluetooth and WLAN antennas is very limited, and it is easy to form mutual interference. The reason why Bluetooth and WLAN are difficult to coexist is because they both operate in the 2.4 GHz ISM band. Therefore, both systems are usually designed with channel preselection filters, occupying the entire 2.4 GHz band. This makes removing interfering signals particularly difficult. However, we have multiple solutions to this problem. One solution is to use time division multiplexing to avoid interference between systems. In addition, the adaptive frequency hopping in the Bluetooth 1.2 solution has been standardized. However, even with the above improvements, it is still difficult for WLAN and Bluetooth to work at the same time without using technologies such as transmit power control . Although the problem of coexistence of WLAN and Bluetooth is difficult to solve, we have found a solution, so users can achieve WLAN and Bluetooth operation at the same time , and major applications such as Bluetooth voice are not affected.
WLAN devices in devices such as smartphones, mobile phones, and PDAs are unique in terms of adjacent channel requirements, emission spectrum requirements, size requirements, and power requirements. Not every WLAN system can meet the performance requirements of the above environment. In addition, system designers must choose products that are specifically designed to meet the above requirements, otherwise the performance of the corresponding products will be difficult to satisfy. Fortunately, the above requirements are not impossible to achieve. These functions have been successfully implemented in today's most advanced WLAN systems and in reference designs for PDAs .
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