Frontier technologies of wireless communication or satellite communication related to China telecom industry.

With the development of mobile communication system, satellite mobile communication system plays an increasingly important role. Satellite universal mobile telecommunications system (SUMTS) will provide global coverage for UMTS users, so that users can communicate anywhere. In order to provide global coverage for future fixed and mobile communications, satellite systems are essential. The satellite part will play a very important role in the global information infrastructure (GII), and the European COST252 working group is formulating the relevant satellite personal communication standards. The data rate of 3G mobile system ranges from 144kb/s to 2Mb/s, and the upper limit of the satellite part is144 KB/s. In act project and Ka-band commercial system, the satellite part aims to provide higher data rate for fixed and mobile terminals. 3G global multi-satellite multi-beam system adopts code division multiple access technology, such as wideband CDMA satellite system (SW-CDMA) of European ESA, which is a combination of satellite wideband/time division multiple access technology and CDMA technology.

Second, the satellite system structure

Satellite system contributes to the growth of Internet applications based on TCP/IP, especially multimedia services that require high bandwidth and bandwidth flexibility on demand. Therefore, ATM, TCP/IP and satellite technology will be the basis of global system networking in the future.

Satellite is a part of network infrastructure, and its interoperability with terrestrial backbone network is very important, which is helpful to provide QoS and be compatible with different types of services.

1. System status

SUMTS-SUMTS network is connected to the ground network, providing a data rate of 2MB/s. ..

SATM (Satellite Asynchronous Transfer Mode)-There are two different views on the layered implementation of satellite ATM. One is to put ATM protocol on the non-ATM satellite protocol platform without changing the existing satellite protocol structure. Another view is that the satellite network adopts a complete ATM structure, in which the ATM layer of the satellite part is S-ATM (which is different from the ATM layer of the terrestrial fixed network), supporting traditional ATM services, TCP/IP applications and UDP/IP applications. The advantage of the former is that the satellite platform is transparent to the protocol standards of different user terminals; The satellite access protocol ends at the gateway station and will not be seen by the external network; There is no need to modify the existing satellite standards. The disadvantage is that it is difficult to provide the best performance for different protocols. Satellite ATM with this hierarchical structure is called ATM encapsulation on non-ATM. The advantage of the latter is that it is suitable for highly integrated satellite-ground ATM environment, but the disadvantage is that the protocol is complex and the existing satellite protocol and internet interface protocol need to be modified.

SIP (Satellite IP)-Using IP transmission, it can be directly connected to the IP backbone network, and it is also convenient to adopt new Internet standards, such as IPv6, RSVP and mobile IP. Satellite systems with satellite inter-satellite links (ISL) can use redundant paths and avoid network congestion. In LEO satellite network, IP routing is attractive, which supports multicast and networking with terrestrial IP networks, but it is not suitable for circuit-switched networks. Different commercial systems adopt different methods: Celestri and SkyBridge incorporate ATM into satellite switching; Teledesic uses a dedicated connectionless adaptive routing protocol to provide fast packet switching.

2. System requirements

Capacity SUMTS can provide a single user with a data rate of 144kb/s/s, and the data rate provided by Ka-band broadband satellite system for each user is as follows: Teledesic Global Satellite System Uplink 16kb/s? 2Mb/s, the downlink is 16kb/s? 64Mb/s； Spaceway uplink 16kb/s? 6Mb/s, the highest downlink 92 MB/s; Astrolink can reach 20Mb/s in uplink and 155mb/s in downlink.

Frequency band-the current frequency band of UMTS is 1885? 2025MHz and 2 1 10? 2200MHz, the satellite part is only 30MHz. The uplink and downlink of satellite mobile communication (MSS) work in L and S bands respectively, and the feedback link provides traditional narrowband services in C band. Provide broadband service, Ka band (20? 30GHz) and very high frequency (EHF) band (40? 50GHz).

3. Satellite constellation

At present, most satellite systems adopt geostationary orbit (GEO) satellite system. The performance of geosynchronous orbit is affected by the transmission delay, which is 0.5 seconds, that is, the propagation time from satellite to ground. This is very unfavorable for real-time business flow.

The new generation broadband system requires extremely low delay, which requires more satellites in the non-stationary orbit (NGEO) constellation. LEO satellite (altitude 500? 2000km) has a time delay of 10-40ms, but the coverage of LEO satellite is relatively small, and there is a great Doppler shift when transmitting. In order to keep real-time transmission uninterrupted, this requires frequent inter-satellite handover, which means that the handover between beams requires huge signaling overhead (one beam is equivalent to a cell in the terrestrial cellular system).

Medium orbit (MEO) satellite (height 2000? 20,000 kilometers) is located between GEO and LEO satellites. It can last for an hour before the user switches to the next satellite.

Other satellite constellations can also be used. For example, the apogee and perigee of HEO satellite system are far apart. Commercial Ellipso and Pentriad systems are HEO satellites, which can provide communication services when the satellites move slowly along apogee. However, these systems are limited to specific services.

Mobility governance mechanism-When switching calls between operators, due to the dynamic movement of satellites in NGEO constellation, adopting GSM for mobility governance will lead to great signaling overhead, which can be overcome by calculating the probability that users need FES switching when calling. In this mobility management mechanism, when the mobile terminal leaves the FES for a certain distance, it updates its location. The user's mobility is detected by a satellite-based positioning system. Terminals in the FES area can update their locations, and do not need to perform FES handover within a certain range. QoS of service provider (including FES handover probability, call loss rate, etc.). ) determines the size of the FES service area.

SATM-Many mobility problems are related to wireless ATM networks, such as virtual connection tree, which can be used in dynamic satellite FES networks. According to the original virtual connection tree algorithm, the mobile terminal can roam freely in a large area. This area is covered by several wireless access points and handover is performed using predefined virtual circuits. When a call is established, the mobile user accesses the virtual connection tree and establishes a lookup table in the middle exchange point of the connection tree.

In an S-ATM network, the root of the connection tree can be a GTW station or an ATM switch. A leaf node is an input part, that is, a beam or a group of beams. The virtual tree will be built and released dynamically according to the movement of the satellite. When a mobile user accesses a satellite station and initiates a call, his position can be accurately calculated and his next handover time can be accurately guessed. In the call establishment stage, the number and direction of user handover can be guessed according to the mobile multi-beam state. From this point of view, it has more advantages than the ground mobile system, because the access beam list can be defined in advance.

8. Agreement

S-ATM- broadband satellite network has two main protocols:

ATM protocol encapsulation and fast packet switching are used for user establishment and management of satellite part. According to the interface and gateway of the satellite, satellite protocols support different protocol standards. The existing protocol does not need to be modified, but it will increase the overhead of grouping and reduce the efficiency of the protocol.

A highly integrated scheme with ATM protocol stack is to replace the standard ATM layer with S-ATM layer, and only need to modify the cell header and function accordingly. MAC uses MF-TDMA or CDMA.

There are many similarities between the two protocols, both of which have fixed information units and can carry control data and user data through different network interfaces. When the network is connected, user data is established, maintained, published and transmitted with different high-level protocols. Future 2? Within five years, most of the standards in Ka band will adopt the new version of ATM protocol layer. S-ATM cell header contains necessary routing and control information. Different techniques such as partial packet dropping (PPD) can be used to detect erroneous cells in satellite switching.

PRMA- A typical packet reservation multiple access protocol (PRMA) is used in terrestrial cellular systems. It is based on slotted ALOHA access technology and TDMA transmission mode, combined with random access of slot reservation mechanism. By using the silent period in the call, multiple calls can be multiplexed on one channel. Therefore, the time slot allocated to the terminal is not fixed, but dynamically processed according to the currently active terminal. PRMA is superior to TDMA in managing voice and data streams and improving capacity.

An improved PRMA mechanism PRMA-HS can be used for real-time variable bit rate VBR service in voice service and available bit rate ABR service in data service. When the terminal waits to receive the reservation result, the terminal does not stop the competition. This mechanism can provide higher efficiency and is insensitive to time delay in LEO system. Therefore, PRMA-HS can be used as a unified MAC protocol solution for future mobile communication systems.

9. Air interface

Satellite propagation and satellite diversity are two main problems, because NGEO satellite constellation may be used for future mobile and satellite services. LEO, MEO, HEO and GEO systems have been tested in L-band. In EHF, some related tests show that there is shadowing effect on the direct route, and there is almost no echo on the suburban road. Compared with L-band, EHF band has fewer echoes and higher attenuation. In urban areas, the signal in shadowing effect is more obvious.

In the EHF frequency band, Lntz proposed a channel model, which has two types: the good channel obeys the Rice distribution and the poor channel obeys the Rayleigh distribution, corresponding to the cases without shadowing effect and with shadowing effect respectively. Considering the power limitation of uplink, the measures to reduce shadowing effect are: path diversity and satellite diversity.

Using an active antenna array, the beam service area can be dynamically changed by configuring satellite antennas to cover fixed beams or shapes and sizes. In both cases, the most important requirement is the continuous coverage of business areas.

The capacity of dynamic coverage system is greatly improved, and the satellite diversity probability is high (>: 90%), so it is very attractive for the design of future systems.

Thirdly, CDMA system.

In 3G, SUMTS adopts WCDMA, which is suitable for variable rate services, and CDMA technology is the basis of S-UMTS.

1.TCH code

TCH code is a binary nonlinear nonsystematic cyclic block code with length n=2m. It shows good performance in FEC and maximum likelihood decision decoding, and can be implemented in the decoder with DSP.

TCH sequence has good autocorrelation and cross-correlation characteristics, which is very important, because CDMA system not only relies on cross-correlation characteristics to reduce interference between users, but also relies on autocorrelation characteristics to synchronize. Therefore, the TCH code can use a simple correlation receiver to detect different users in CDMA.

2.CDMA receiver

CDMA uses nodes with time-varying structure and uses multi-user detection to reduce multipath fading. Due to multiple access interference (MAI), the performance of single user receiver in traditional CDMA communication system is not very good.

Although the optimal multiuser detection algorithm provides high capacity potential and performance improvement, it is very complicated to implement. Therefore, suboptimal schemes, such as decorrelation detection or multistage receiver, are proposed. A multi-user cancellation detection mechanism in SW-CDMA. The receiver has a hierarchical structure. For all interfering users, according to the needs of users, the parallel interference cancellation multi-user detector (PIC) performs selection-based (S-PIC) detection before making the final decision. The basic assumption of the receiver is that the output of the matched filter is divided into two different groups. Directly detect and eliminate the reliable signal in the whole received signal. No further processing delay is needed before the unreliable signal or copy is determined.

Therefore, the parallel interference cancellation method has better performance and lower complexity than the RAKE receiver.

Different from blind adaptive multiuser detection, it is necessary to analyze and estimate the performance of LEO and MEO satellite mobile communication systems in DS-CDMA satellite communication system using BPSK. The receiver is used for the uplink satellite link endpoint of the base station, and the communication system lacks synchronization among users using satellite channels with multipath fading. These mechanisms are based on blind adaptive multiuser detection proposed by Verli, Honig and Madhow. In the former mechanism, the blind receiver includes different detectors, while the latter vertically detects the whole received signal. It strikes a good balance between complexity and performance. Compared with the traditional single-user receiver, the multi-user detection system has a good effect on the near-far effect. It does not need training sequence, but only needs necessary user information (such as the number of active users, processing gain, etc. ).

Four. Concluding remarks

In order to provide global coverage for future mobile and personal communication systems, satellite systems are necessary. This paper introduces a new generation of satellite personal communication system COST252. The work of COST252 includes: the program entity of MF-TDMAMAC protocol; Routing algorithm (DT-DVTR) and inter-satellite link measurement in LEO system: resource management and DCA technology in geosynchronous orbit and low earth orbit constellation; Use the protocol of PRMA, etc. The next generation of mobile and fixed satellite services will use IP technology, which is a research direction in the future.