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Global Positioning System (GPS) capabilities are rapidly becoming the main market drivers for consumer electronics applications, and are becoming an important differentiator for a wide range of next-generation consumer electronics devices. Accurate positioning capabilities are added to applications such as automotive, personal navigation devices and even cellular handsets. In addition, many advanced GPS services are under development, such as location-based advertising. In fact, the ability to locate users on a map is enough to push GPS into mainstream devices, but without the need to significantly increase overall bill of materials (BOM) costs and processor load.

However, to provide the GPS performance and accuracy they expect at the price consumers are willing to pay, developers must be familiar with some of the key technologies for implementing GPS functionality in consumer electronics, especially the new Galileo satellite network. . With the help of the auxiliary signals provided by Galileo satellites, personal navigation devices can acquire and lock positions more quickly and accurately than GPS-only devices, especially in urban environments where location-critical services are most needed but GPS accuracy is not sufficient. . In addition, with the advent of innovative technologies such as software baseband processing (similar to software-defined radios), manufacturers can introduce location technology to devices such as personal multimedia players and mobile phones without compromising cost and power consumption. in. All of these factors make Galileo's combination with GPS a compelling technology.

Galileo: Effectively make up for the shortcomings of the GPS network

The Galileo satellite is a parallel global positioning satellite network developed and implemented under the auspices of the European Union. The development of Galileo satellites is not intended to compete with GPS, but to work with them. Galileo satellites will transmit signals in multiple frequency bands, one of which is the same L1 band frequency as GPS, and the band frequency is spaced between GPS satellites in complementary orbits so that a specific location can be captured. The amount of semaphores will increase greatly, which has a great impact on the accuracy of receiving equipment in high-rise buildings. To get enough position-locking signals, at least four satellites are required. Galileo/GPS-based personal guidance devices can use satellites from both systems simultaneously, which means more satellite signals can be used. In fact, this high level of precision is sufficient for a personal navigation device to determine which side of the way the pedestrian is heading.

GPS has been around for more than 30 years. In 1978, the first exploration satellites were sent to space; in 1989, the first utility satellites were put into orbit. GPS achieved initial operational capability (IOC) in 1993 and achieved full operational capability (FOC) in 1995. GPS is managed by the US Department of Defense and was not originally designed specifically for the commercial market.

In effect, Galileo is an effective complement to GPS. Since Galileo has more signals available and is not controlled by a government agency (for example, it can stop service or change the accuracy of satellites without warning), it can provide higher accuracy than GPS (in commercial applications, Galileo's accuracy is +/-4m, while GPS's accuracy is +/-10m).

Currently, the Galileo test satellite GIOVE-A has been deployed and validated all important transmission mechanisms of the technology. As the deployment progresses, 27 Galilean satellites will be transported to orbit. Due to the long product development cycle, many OEMs have begun to consider the implementation of Galileo/GPS-based architecture, and let related products gradually enter the market, so that once the Galileo system is officially operational, consumers can immediately enjoy the advantage. Ideally, these devices currently only work with GPS, but when the Galileo satellite positioning system is built, it can be quickly upgraded to collect Galileo signals. Even if the OEM does not plan to upgrade the already-delivered equipment, it is now designed to support both systems' architectures to avoid time-to-market delays and missed opportunities when the Galileo system is built.

Today's GPS architecture consists of an antenna, a radio frequency (RF) receiver, a baseband processor, and an output bus interface that connects to the application processor (see Figure 1). Such legacy devices are neither limited by power constraints nor require much flexibility because they are developed for specific devices, such as in-vehicle GPS, so the performance of the receiver hardware can be highly optimized. Their wireless parts, whether hardware or software, have little or no configurability. They are often sold to manufacturers in modules, so OEMs don't have to know more about the details of RF design and testing.

Figure 1: Block diagram of a GPS architecture based on hardware baseband processing.
Figure 1: Block diagram of a GPS architecture based on hardware baseband processing.

Despite the significant cost savings in implementations for specific applications, the cost of maintaining two distinct RF subsystems, Galileo and GPS, far exceeds the affordability of the consumer market. More importantly, the space occupied by the two RF sections and the power consumed are doubled, and two bus interfaces are required for the application processor. In this case, integrating these RF components into a single sub-assembly can reduce overall cost, complexity, and power consumption.

In fact, because GPS and Galileo use the same frequency band (center frequency is 1.575 GHz), it is possible that two systems share a single RF portion. However, small differences in signal acquisition methods need to be implemented in a configurable manner. In particular, Galileo signals use a 4MHz bandwidth, while GPS uses a 2MHz bandwidth and implements a different set of coding schemes. From the baseband point of view, these modulation schemes can be demodulated using the correlator, so a baseband processor can be used and both Galileo and GPS signals can be demodulated simultaneously by independently configuring a flexible correlator module.

Leverage underutilized computing power

Traditional baseband processing is implemented in hardware. However, the Galileo signal scheme has not yet been finalized. If it is now implemented in hardware, the flexibility of the baseband (which can only be implemented in software) needs to be reconfigured to make the necessary modifications according to the final standard. In addition, hardware-based implementations are often inflexible and difficult to adapt to new signal processing algorithms used to improve performance and accuracy.

The key to implementing Galileo/GPS functionality in a cost-effective manner is to implement some of the baseband processing functions in software by leveraging the underutilized computing power of the existing architecture. For example, mobile phones have an application processor that handles all functions that are not related to communication. As people become more interested in multimedia services such as music and video playback, such processors have become more powerful. However, when these services are not in use, the application processor is often idle and will generally be powered down to reduce the power it consumes.

Figure 2: Block diagram of a Galileo/GPS system implemented using a dual hardware architecture and a software-based architecture.
Figure 2: Block diagram of a Galileo/GPS system implemented using a dual hardware architecture and a software-based architecture.

When the baseband processing can be implemented in software on the application processor, the consumer Galileo/GPS receiver may become a reality. In this way, software-based Galileo/GPS is equivalent to software-defined radio (SDR) because one receiver hardware already supports multiple satellite systems. In addition, as wireless communication technologies continue to converge, it is foreseeable that in the near future, consumer electronics devices will utilize versatile radio technology to support Bluetooth, WiFi, and Galileo/GPS using configurable software baseband.

Developers can choose to continue using hardware to implement baseband processing of GPS, while performing Galileo's baseband processing in software using underutilized main processor resources; or implementing both GPS and Galileo baseband processing in software. Both of these methods can reduce the cost of implementing location services in consumer applications, but implementing both baseband processes in software can completely eliminate the need for hardware baseband chips.

In particular, if baseband processing is implemented in software, the price of the Galileo/GPS system can be reduced by more than 50%. The software-based Galileo/GPS system is expected to quickly reach the price point of $1 at the time of delivery, and one of the factors contributing to this goal is the business model of the functional software itself: when the software development is completed, there will be no Any manufacturing cost, and software has always been bundled with hardware as a means of promoting hardware. Further cost savings can be achieved by integrating fixed baseband processing techniques such as correlation techniques and RF circuits (see Figure 3). In addition, Galileo can be added to software-based baseband equipment at any time without increasing overall hardware costs. Relatively speaking, if it is a hardware-based implementation, implementing Galileo's configuration will increase the retail cost of the device, but it will not immediately bring value to consumers.

Figure 3: Block diagram of the various functions that are divided into software and RF sections for processing (ie, transferring the associated functions from software to the wireless section for processing).
Figure 3: Block diagram of the various functions that are divided into software and RF sections for processing (ie, transferring the associated functions from software to the wireless section for processing).

The feasibility of software baseband processing can be determined by evaluating the worst-case loading. For Galileo/GPS, peak processing occurs during the initial signal acquisition or after the location is lost (for example, after driving through a long tunnel). When the position is locked, the amount of calculation of the baseband processing is greatly reduced, because once the system has mastered the position information, it is easier to maintain the position information.

Of course, the worst case processing should not overly occupy the computing power of the application processor, so as not to affect other functions. The initial software baseband implementation would consume up to 66% of the computing power available to mobile application processors (such as ARM9), but software vendors are expected to reduce this load to slightly more than 10 to 15% acceptable.

One way to achieve this goal is to use non-real-time technology. The ability to process data in real time as a signal stream requires interrupt-based processing capabilities, but this can result in high overhead and the complexity of managing real-time tasks in different applications. In addition, since the processor is continuously interrupted to process various signals, its power supply is not always off, thus greatly increasing the power consumption of the entire system.

Non-real-time processing uses a burst mode that collects many data samples at a time for processing. While this adds latency, this small amount of latency is negligible and does not affect accuracy or user experience. Since the data is more centralized, when the application processor is not busy processing higher priority tasks, the progress of the processing schedule can be programmed. It should be noted that, unlike the real-time processing, this processor will not be used for baseband processing after being periodically awakened; on the contrary, when the processor is woken up by a task, baseband processing will be performed, so that the processor You can sleep for a long time later.

Solving the problem of sensitivity

Sensitivity is a key performance and accuracy indicator for Galileo/GPS receivers (especially mobile phones). The signal level at the receiver of the signal acquisition requirement (in the A-GPS system) is between -130 and -155 dBm, which is approximately 19 to 34 dB lower than the noise level obtained by the RF front-end module. The correlator despreads a 2MHz bandwidth signal into a 50Hz data signal, providing a 43dB correlation gain that boosts the wanted signal above the noise level for processing. However, any other communication signal close to the useful signal frequency or harmonics in the useful frequency band can be a source of interference and further reduce the sensitivity of the receiver.

The most common and devastating source of interference comes from the personal navigation device itself. For example, if the handset is far away from the base station and transmits at maximum power, this means that there may be a 30 dBm signal at the 1800 MHz frequency in the same device, which further leads to a worst case sensitivity degradation of the Galileo/GPS signal.

There are several ways to overcome internal transmission interference. One is that since the transmitted signal is known, it can be subtracted from the Galileo/GPS signal. Another method is to use a filter to reduce the interference of the cellular phone by more than 70 dB to protect the incoming satellite signal.

However, if the GPS has 2MHz bandwidth and Galileo expands to 4MHz, the dual receiver architecture has two optimal filters. The modulation mode of GPS is BPSK, and the modulation mode of Galileo is BOC(1,1). Thus, both signals can occupy the same signal bandwidth, and then the correlator can distinguish the GPS signal from the Galileo signal, and vice versa.

The filter is also suitable for baseband processors. When the baseband is implemented in hardware, since the parameters of these filters are fixed, the degree of optimization of the wireless portion is limited. However, if the baseband filtering is implemented in software, these parameters can be changed to match the specific signal conditions. Furthermore, as filter algorithms evolve, these filters can be applied to existing architectures. Even with the vast differences in the architecture of various handsets, this flexibility allows a single dual wireless receiver architecture to be applied to different product lines.

Sensitivity may also be severely degraded by a poor crystal or VCXO reference clock. In general, the more stable the clock source, the higher the cost, but the faster the acquisition time. For example, a 0.5 ppm reference clock will bring the lock time to the order of 40 seconds. If the acquisition time is not a problem, then a 2.5ppm reference clock should suffice.

Many people mistakenly believe that the GSM reference clock can generate a stable Galileo/GPS reference clock, but it is not. The GSM reference clock is locked to the network and requires frequent frequency correction. Sometimes, these corrections are implemented by driving a DAC through the GSM baseband, which in turn drives a VCTCXO. A gradual change in the reference clock frequency will not cause the Galileo/GPS receiver to remain locked with the satellite signal, especially if the signal is weak, which will result in loss of positioning. Therefore, the safest method is to use a separate clock for the Galileo/GPS subsystem, but this will increase the cost of the overall device. Developers need to carefully consider the trade-off between performance and cost, and avoid developing a design that does not meet the minimum accuracy requirements early in the architecture design process.

All in all, the Galileo system can improve the availability and performance of global positioning services, and the added precision is a perfect complement to GPS. With software-based baseband processing, personal navigation devices, including mobile phones and portable media players, will be able to fully exploit the idle processing power of the application processor and implement the Galileo/GPS dual wireless subsystem in a cost-effective manner to improve consumers. Global navigation.

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