This paper introduces the development background of mobile communication and the development of mobile
communication. It introduces the application principle, network structure, main technology, the advantages and
disadvantages of the three generations of mobile communication system respectively, and introduces the current
third generation mobile communication system, including its technical support and research direction, analysis and
comparison of the European WCDMA system, the United States CDMA2000 system and China's TD-SCDMA system
technical characteristics. Finally, the development trend and prospect of future mobile communication system are
discussed.

With the development of economy, the improvement of people's living standard has been increasing and the
construction of buildings is becoming more demanding. These buildings are large in scale and of good quality and
have a strong shielding effect on mobile phone signals. In the middle of the large-scale buildings, underground
shopping malls, underground parking and other environments, the mobile communication signal is weak hence the
phone cannot be used normally and forming a mobile communication blind area and shadow area. In the middle
fl oor, due to the surrounding diff erent base station signal overlap, the ping-pong eff ect, frequent switching of mobile
phones and even dropped calls are seriously aff ecting the normal use of mobile phones. In the building's higher fl oors,
due to the height of the base station antenna, it has abnormal coverage and there is also mobile communication blind
spot. In addition, in some buildings, although the phone can answer normal call but the user density, base station
channel congestion and mobile phone line is diffi cult. In particular, the network coverage, capacity and quality of
mobile communication are the key factors for operators to gain competitive advantage. Network coverage, network
capacity and network quality fundamentally refl ects the mobile network service level and is the theme of all mobile
network optimization work. The indoor coverage system is produced under this background. According to the relevant
statistics in some areas of indoor traffi c in the total traffi c accounted for a higher proportion. Therefore, strengthening
the indoor coverage is of great signifi cance to improve the quality of mobile communication.

We know that today's society is the information society. The highly developed information society requires that
the communication network can provide a wide variety of telecommunication services. The amount of information
transmitted, exchanged and processed through the communication network will increase, which requires the modern
communication network to be digitalized, integrated, intelligent and personalized. The transmission system is an
important part of the communication network, directly restricting the development of the communication network.
One of the most important developments in the world to develop the information superhighway, with one of the
key is to set up a large capacity transmission fiber network, continuously improve the transmission line on the
signal rate, widening the transmission band. It is like expanding capacity to accommodate a large number of traffi c
in highways. At the same time the user wants the transmission network to have a worldwide interface standard, to
access to global village, in where every user can easily communicate anytime and anywhere. Synchronous Digital
Hierarchy (SDH) is a comprehensive information transmission network that integrates multiplexing, line transmission
and switching functions and is operated by a unifi ed network management system. It is a synchronous information
transmission network developed by Bell Communication Technology Research Institute’s optical network (SONET).
The International Telegraph and Telephone Advisory Committee (now ITU-T) received the SONET concept in 1988
and renamed it SDH, making it a general technology system that applies not only to fiber but also to microwave
and satellite transmission. It is applied for eff ective management of network, real-time service monitoring, dynamic
network maintenance, interoperability between different vendors and other functions, greatly improves network
resource utilization, reduce management and maintenance costs, flexible and reliable and efficient network
operation and maintenance. So it is in the center of attention in world's information fi eld of transmission technology
development and application.

This paper mainly studies the basic concept of baseband transmission of digital signal and the transmission process
of digital signal baseband transmission and how to design digital baseband transmission system with MATLAB
software simulation. This paper fi rst introduces the theoretical basis of this subject, including digital communication,
the composition of digital baseband transmission system and digital baseband signal transmission process. Then the
paper introduced the characteristics of digital baseband transmission system, including digital PAM signal power
density and common line pattern, and by comparing choosing the fi nal bipolar zero return code. Then we introduced
the MATLAB simulation software. The conditions of the best reception of the digital baseband signal are described
and how the waveform of the baseband signal is observed by an oscilloscope. Finally, according to the basic steps
of simulation process, the simulation process of digital baseband transmission system is realized by MATLAB
simulation tool, and the system is analyzed.

QQ is China's most widely used instant messaging tool, its security for user security and network security has an
important impact. Once a QQ account is hacked, the harm and the impact is terrible, the user loses a lot of friend’s
information and contact information in a short time and is diffi cult to completely recover. This article will analyze
the security of QQ’s landing agreement, analyze its security, and point out the security vulnerabilities which exist,
and explain the possible attacks that QQ may suff er, and make suggestions for improvement and ways to enhance the
security of QQ communication. In addition, this article also conducts a corresponding study of QQ hacking, and puts
forward precautionary approaches.