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GPS based positioning plays a critiical role in modern location based services. Assisted GPS (A-GPS) with assistance server were first come out by Bell Labs and later developed to enhance the positioning performance of a GPS receiver and satisfy US FCC's E911 mandate.

            

Figure 1. GPS System Archiecture: Space Segment, Control Segment and User Segment

1.  All GPS satellites transmit on L1 and L2 frequencies. •Each satellite uses different ranging codes: C/A code and P-code. •L1 band is for civilian use. 
2. The C/A code (coarse/acquisition code) is modulated onto the L1 carrier only, while the P-code (precise code) is modulated onto both the L1 and L2 carriers. 
3. The C/A code is less precise and less complex than the P-code and available to all users. •The P-code is intended for military uses and is added to both L1 and L2.

 

Figure 3. The Block Digrame of GPS Positioning

Figure 4. GPS Positioning Error Sources

Figure 5. The Block Diagram of A-GPS System

[IEEE International Conference on Communications (ICC) 2008]

Location based services (LBS) for mobile are the services supported by cellular networks for providing mobile users with various location sensitive applications such as E911, Friendfinder, personalized advertisement, etc. LBS accelerate the convergence of 3C (computer, communication and consumer electronics). One aspect of LBS market is the rapid growth of GPS market, which is predicted to reach $28.9 billion by 2010 by GPS World. It is believed that LBS is bringing huge revenue opportunities for wireless network operators and service providers. The driving force behind of the growth of LBS market includes regulator’s mandates, the development of more efficient location technologies and the expanding of LBS from network operator to third service provider.

In this tutorial, the state of art of mobile location based services (LBS) will be explored in terms of technologies, standards and implementations. There are five major parts in this proposed tutorial. Within the first part, an introduction to LBS is presented along with an overview of the growing LBS market. Two examples of LBS, E911 and telematics, are emphasized. In the second part, LBS from a network operator perspective is discussed with a survey of wireless location technologies, the exploration of location management in cellular network, and LBS standards activities. The architecture and operation of the network-dependent LBS control plane of cdma2000 and UMTS networks are reviewed, respectively. In the third part, the IP-based LBS user plane is discussed from a service provide perspective. An overview of the related standards by OMA and 3GPP2 is given and the principles of LBS user plane are illustrated from multiple application scenarios. Finally, the further works and standard activities for LBS are presented.

In summary, this tutorial is intended to provide a comprehensive overview of mobile LBS for a wide array of audiences, including LBS services providers, application developers, marketing managers and system researchers, etc. It includes not only the background information but also standards activities.

Broadcast multicast service (BCMCS) has increasingly been popular for delivering multimedia content to mobile users. Traditional digital broadcast air interfaces are designed with the tradeoff between maximum achievable rate and intended coverage in mind. The actual rates are usually limited by the maximum transmit power and the worst channel condition so that every user in coverage can reliably receive the services as well as contents of same quality. The users under good reception condition may have no advantage, even if their potential throughputs can be much higher. This happens often, especially on the mobile users whose reception conditions change all the time. And there are rising interests in upgrading existing digital broadcast systems with more services for new users and delivering more quality of service (QoS) options to users with advanced receivers while still guaranteeing existing users' services. Furthermore, recent advances in wideband speech coding, e.g., EVRC-WB, and scalable video coding, e.g., H.264/MPEG-4 AVC, suggest unequal error protection on content delieveries with providing graceful degradation of quality in the presence of increasing packet loss. It is possible for the users in good reception condition have more opportunities to enjoy high quality services while the user with low throughput can still decode the content of basic quality. Many technologies are under investigation for these goals, e.g., rateless coding, hierarchical modulation, multiple-input multiple-output (MIMO), selective retransmission and superposition precoding (SPC). Backward compatibility, implementation complexity and upgrading cost are among the major concerns in upgrading existing systems with additional services. Among those candidates, hierarchical modulation, also called layered modulation, is the most popular one, in which multiple data streams are multiplexed and modulated into one single symbol consisting of base-layer subsymbols and enhancement-layer subsymbols. It has been widely proven and included in various standards, such as DVB-T, MediaFLO, UMB (Ultra Mobile Broadband, a new 3.5^th generation mobile network standard developed by 3GPP2), etc., and is under study for DVB-H.

Figure 1. Enhanced hierarchical modulation example: QPSK/QPSK

In this contribution, the regular hierarchical modulation is firstly extended by allowing additional rotation on the enhancement layer signal constellation. The generalized hierarchical modulations are then studied and analyzed from four different perspectives, such as achievable capacity, modulation efficiency, demodulation robustness and peak-to-average-power ration (PAPR) when it is combined with the popular orthogonal frequency-division modulation (OFDM) transmission scheme. At first, the achievable capacities of hierarchical modulations over Gaussian broadcast channel are studied from an information-theoretical perspective. As an example, the capacity of a regular 16QAM is tore down into the equivalent capacities of a base layer and enhancement layer. It is shown that there is a capacity loss on the base layer due to the inter-layer interference (ILI) from the enhancement layer. And this capacity loss can be mitigated by properly rotating the enhancement signal constellation. From a signal-processing perspective, it is known that the capacity loss is also related to the Euclidean distance profile of the hierarchical modulation signal constellation. For example, in high signal-to-noise ration (SNR) region, the symbol error rate usually is dominated by the minimum Euclidean distance. Obviously, with properly rotating the enhancement layer signal constellation and maximizing the minimum Euclidean distance, the resulted symbol error rate will decrease. Additionally, for tracking Euclidean distance profile changes, several parameters like effective signal power, effective SNR and modulation efficiency are discussed too. After this, hierarchical modulations are analyzed from an implementation perspective with considering channel estimation errors, which includes both channel amplitude estimation errors and channel phase estimation errors. It is shown that the demodulation robustness of hierarchical modulations can also be controlled by changing the Euclidean distance profile. Finally hierarchical modulations are discussed from a transmit power efficiency perspective when it is combined with multicarrier transmission. With avoiding high back-offs and maximizing average output power, it shows that high RF transmitter power efficiency is achievable by properly rotating the enhancement layer signals. With the analyses from different aspects of hierarchical modulation, a in-depth understanding of it can be achieved.

Figure 2. Capacity tear-down of 16QAM, a hierarchical modulation perspective