7.3 3G Radio Network Antenna

7.3.1  Introduction

In 3G system (including WCDMA and cdma2000), which is as a new generation mobile communication system, multi-access mode has changed from TDMA/FDMA to CDMA/FDMA. However, as far as the wireless signal is concerned, it’s still facing the difficulties in making use of frequency resources efficiently and decreasing network interferences and transforming electrical signals with the utmost of efficiency.

A antenna is a communication system bridge between a user terminal and a base station control equipment, widely applying to cellular mobile communication system. As the communication technology is developing, antennas will be on progress consequentially. The mobile communication system in the seventies adopted omnidirectional antennas or angle reflector antennas, for the reason that a few carriers and base stations can meet the demands of few users in a mobile communication system of a city. As the economy goes forward, the amount of mobile terminals, whose demands can not be met by the old base stations, is boosting so rapidly. Especially as the development of digital cellular technology goes, new antennas are required to be configured to improve the multi-path fading, site assignment and multi-channel network in metropolis.

Plate type antennas are widely applying to 2G digital cellular system thanks to the features of low section, light framework, easy setting and good electrical specifications. From the middle 80’s to the late 90’s, vertical polarization (VP) antennas are usually adopted. A cell is usually divided into 3 sectors, each of which demands 3 antennas, so 9 antennas should be set in a cell. However, too many antennas will result in many problems, such as high setting cost. In addition, the optimum diversity reception gains are unable to be achieved with diversity reception antennas set, saying nothing of that the antennas are unable to be set in some base stations. In that case, the technology of dual polarized antennas emerges as the times require.

In 3G phase, as the wireless technology grows and the signal detection varies, the cellular network should be adjusted and optimized, which demands new base station antennas, such as self-adapting control antenna and intellectualized antenna.

7.3.2  3G Network Structure

3G system, a board-band CDMA system, inherits the features of narrowband CDMA system in the aspect of network structure. Thanks to code division multiple access, frequency reuse is not a big problem any longer. The network interference sources from itself, related to the amount of calling users at the same time. In a metropolis, three-sector base stations are commonly configured. In suburb, town and road, three-sector base stations or omni-directional base stations are configured as required and in arterial traffic, two-sector base stations are commonly configured.

For 3G (WCDMA), the channels per carrier are decided together by OVSF code and scrambling code, so capacity of the channels per carrier is great. You should set the number of the carriers and traffic channels in each base station based on the requirements of practical traffic distribution in engineering design.

When implementing multi-carrier, you should pay attention to the followings in designing radio network:

1)      Optimize hard handover to minimize the possibility of call drops.

2)      Avoid isolated multi-carrier base station and implement multi-carrier in central cells to avoid hard handover.

3)      Avoid heavy-traffic cells of being edge cells where hard handover occurs.

7.3.3  3G Radio Network Typical Antennas

You have several options to choose 3G typical antenna base on the following principles:

1)      Properly choosing half-power beam width and gain of antenna based on the number of base station sectors, traffic density and coverage requirements.

2)      Adopting duplexer to save antenna locations.

3)      Adopting dual polarization antennas in the dense urban areas.

The adjustment of antenna direction is the same as that in 2G engineering. The main lobe direction and angle of tilt of the directional antennas should be properly adjusted to the traffic distribution and communication quality requirements. When setting antennas, you should note that the isolation between antennas should meet the requirements of horizontal and vertical isolation to avoid interference. The setting height of antenna is up to the coverage. Therefore, it should be properly considered according to the coverage, interference, isolation and future development requirements.

The antennas used in 3G network are similar to those used in 2G network, whose requirements are as follows:

Sector antenna gain: 13-16 dBd

Omnidirectional antenna gain: 9-10 dBd

Sector antenna half-power beam width: 60-65 degrees or 90 degrees

Omnidirectional antenna deviation in roundness: < +/- 1 dB

Voltage standing wave ratio (VSWR): <1 .5="" o:p="">

Impedance: 50 ohm (unbalanced type)

Maximum input: >500W

Antenna diversity: Space diversity or polarization diversity is taken as standard configurations.

7.3.4  3G Smart Antenna (SA)

1. Principles of SA

By adopting SDMA that signals differ in the direction of transmission path, SA reduces the effects of time delay spread, Rayleigh fading, multi-path, channel interference, distinguishes the signals from the same frequency and timeslot, and combines with other multiple access technologies to maximize the use of frequency spectrum resources.

The SA in base stations is a kind of array antenna comprising multiple antenna cells. By adjusting the weight scope and phase of each signal and changing array pattern, it can cancel interferences and increase S/N. In addition, it can measure a user’s direction so that a beam is directed to the user.

The array antenna is composed of N antenna units, each of which has a corresponding weigher, totaling M groups of weighers and forming beams in M directions. M indicates the number of users, which can be greater than the number of antenna units. The dimension of array antenna and the number of antenna units decide the maximum gain and the minimum beam width, which means that the dimension of array antenna and antenna gain should keep balance with the antenna side lobe performance. By adjusting the signal phase and amplitude received from each antenna, SA combines them to a desired beam. This is called beam forming, which can form all kinds of beams, such as scanning beam, multi-beam, shaped beam and the beam with zero position controlled. According to the pattern, there are two types of SA: self-adapting pattern and shaped pattern.

The key technology of SA is to identify the signal angle of arrival (AOA) and the implement of digit-shaped. The algorithms to identify the signal AOA are MUSIC algorithm, ESPRIT algorithm, maximum likelihood algorithm, etc. The implementation of digit-shaped is to choose the optimum weight coefficient to get the optimum beam. For self-adapting algorithm, the first step is to set rules, which commonly are maximum likelihood, maximum S/N, minimum mean square error (MMSE), minimum square error. You should choose one of them according to the specific conditions. 

2. Applications of SA in 3G

The application example of SA in 2G network indicates that SA can efficiently prevent from interference. According to the 3G criteria, the SA application is required to improve the network capacity and performance, and take the technical factors, such as “converged beam”, “self-adapting beam forming” and “beam handover”, into consideration.

“Converged beam” is applied to special areas, aiming at enlarging coverage and increasing capacity. Such beam does not associate with a user, nor does it trace mobile users in the coverage. However, by increasing link scope and converging beams, it can reduce transmission power of mobile users and according increase capacity. If mobile user enters the area with great transmission attenuation, the converged beam will point to the mobile user and rest on him. If a mobile user enters the area with good coverage which converged beams become unnecessary, the mobile user will be in the charge of common pilot channel.

“Self-adapting beam forming”, applying to downlink, is in favor of link budget for individual user and a group of users to improve the system performance. In poor transmission conditions, such as cell edge (basement), the coverage is required to be spread to users with an aim to improve link scope.

“Beam handover system” can switch users between narrow beams to form narrow sectors without handover loss. Because the capacity in 3G system increases as the number of sectors increases, four 30-degree beams coverage can substitute one 120-degree one, resulting in increasing capacity by 2 to 4 times. Users are switched between beams without the requirement of any special auxiliary channel.

There are several options to apply SA in 3G. Beam handover SA is an option in starting phase. In network design, SA can reduce the external network interference (such as one frequency interference, adjacent frequency interference and other-system interference) and the internal network interference as well. The order of magnitude depends on the amount of beams.