ZTE´s Wide Coverage Solutions

In urban areas, the service traffic is always very high. Hence the distance between BTS sites is set to about 500 m to 1000 m and the balance between investment and return is reasonable; but in the rural areas, the condition is different. The returns are few because of lower traffic, and the most important thing the operator this is to consider how to improve the ratio between cost and returns. Here, the Wide coverage solution from ZTE that we are going to discuss is a good consideration for your network and finance. ZTE’s ZXG10-BTS product family is shown in Figure 1.

It is proved that adopting wide coverage solution to build new BTSs in rural areas of the local network is cost-efficient and rational.

1 Solution for Downlink

1.1 Improving Output Power of BTS (Base Transceiver Station)
At present, the transmitting power of the macro cell BTS used in the mobile network is typically 20 W-40 W, and the coverage radius is relatively small. If we continue to adopt this kind of BTS for wide coverage, the integrated investment cannot be greatly lowered even with second-party equipment.

  In order to broaden the coverage, improving BTS’ transmitting power is undoubtedly the best choice for wide coverage. Wide-coverage BTS can greatly cut back the number of BTS and the investment for supplementary equipment.

  In view of the features of wide coverage and low traffic, ZTE Corporation puts forward the solution of 80 W output BTS. In suburbs and rural areas, 80 W BTS can usually cover 15 km radius or wider. Theoretically, BTS output power is 40 W-80 W with the downlink power increased by 3 dB, and the uplink with the Tower Mounted Amplifier (TMA) can bring about a gain of 3-12 dB. This can completely fulfill the balance of uplink/downlink traffic, and increase the coverage radius of 40 W BTS by 20% or so.

  ZTE provides two solutions for setting up a 80 W PA (Power Amplifier). Solution I: Accomplishing 80 W (peak value 100 W in theory) power output through co-channel overlay via two 40 W PAs. Solution II: Accomplishing 80 W power output with a single PA.

  Figure 2 and Figure 3 show the principle block diagram of Solution I and board layout of Solution II respectively.

  The two solutions have their own features: Solution I can accomplish 6 Transceiver (TRX)/rack flexibly when the capacity increases and the coverage gets narrower, and the 80 W power output can be accomplished in a parallel mode with good redundancy. Solution II boasts a simpler structure and more flexible configuration.

1.2 Constructing Directional Coverage BTS
Since the gain of directional antenna is typically 4-7 dB greater than that of the omnidirectional antenna, the coverage radius of the macro cell directional BTS is far greater than that of omnidirectional BTS. Therefore, the directional BTS should be preferably chosen.

1.3 Combiner Duplexer and Antenna
After the BTS with high transmitting power and high static sensitivity is used, the high-gain antenna, low-loss feeder, low insertion loss combiner or duplexer can be adopted to improve the Equivalent Isotropic Radiated Power (EIRP) (Input power of BTS antenna plus the gain of BTS antenna) of the BTS, thereby further broadening the coverage and taking care of the antenna height.

1.4 Combiner and Duplexer
The combiner comes in two types: 3 dB electric bridge combiner and resonant cavity combiner. The 3 dB electric bridge combiner permits the carrier frequency to hop in a broader band, and supports RF frequency hopping with simple hardware structure. With more combiners, the insertion loss will grow much faster.

  The resonant cavity combiner features less insertion loss, but complicated hardware structure. The transmitting frequency is fixed at the port of each combiner, and only the base band frequency hoping is supported.

  In view of the support of Radio Frequency (RF) Frequency Hopping (FH) as well as the size and complexity of the combiner, the broadband combiner is widely used in the GSM network. Generally, 3.5-4.5 dB insertion loss occurs for 2-in-1 combiner, 7-8 dB for 4-in-1 combiner, and less than 1 dB for the duplexer.

  The combiner part and diplexer part are integrated in one physical entity in ZXG10-BTS. The wide coverage and low traffic features make O1, O2 and S111 the dominant types of BTS. As for one TRX per cell, a 2-in-1 combiner should be configured. Through the jumper, only its duplexer is used. When it is expanded to two TRXes, its combiner part can be used or it can be changed into
dual-duplexer. As for two TRXes per cell, a dual-duplexer should be used directly to reduce 3 dB of insertion loss and expand the coverage radius by 20%. As for 2-4 TRXes per cell, two 2-in-1 combiners instead of one 4-in-1 combiners can be adopted to reduce the insertion loss of the downlink signals by 3 dB, thereby achieving the cell coverage with two unipolarization antennas.

Figure 4 and Figure 5 respectively show the diagram of one and two TRXes per cell solutions.

1.5 Antenna Part
Selecting the antenna with different gains for GSM is a decisive factor of coverage range. The gain of high-gain antenna is up to 18.5 dBi, which is 3-7.5 dB higher than that of general directional antenna or omnidirectional antenna, broadening the coverage area by 20% or more. Moreover, the antenna designed for belt-like areas (such as expressways and valleys) has entered commercial services. Its half power angle is narrower and the gain is up to 21dBi/400. In order to meet the requirements of wide coverage under other conditions, high-gain antenna should be chosen carefully, with the half power angle of 650 or 900.

  The diversity receiver of the antenna can bring about a gain of 3 dB. In urban or rural areas, the space diversity has a better effect than the polarization diversity. Therefore, we use two unipolarization antennas per cell to achieve the diversity effect and the diversity gain.

2 Solutions for Uplink
In order to raise the level of the radio uplink signals, TMA (Tower Mounted Amplifier) and diversity receiving technologies can be used to make up the deficiency of uplink due to low transmitting power of MS（Mobile Station）, achieve a balance between uplink and downlink, and broaden the coverage of BTS significantly.

2.1 TMA
TMA is used to amplify the uplink signal, and make up for the deficiency of the uplink when balancing the budge of the uplink/downlink. When the 80 W PA and high-gain antenna are used, the uplink is usually inadequate and has to be made up by a TMA. A TMA typically brings about a gain of 3-12 dB .

2.2 Improving the Receiving Sensitivity of  BTS
The receiving sensitivity, i.e., static sensitivity of BTS, is a key factor reflecting the performance of BTS. Under the same conditions, higher receiving sensitivity indicates wider BTS coverage. The typical receiving sensitivity is -108 to -110 dBm. The GSM BTS of ZXG10 has a receiving sensitivity of up to -112 dB, raising the uplink signals by 2-4 db and the coverage radius by almost 10%-20%.

2.3 Feeder and Connector Part
The loss of the feeder connector is controlled typically below 3 dB. As for general macro-cell BTS, the feeder is divided into the main feeder part and the jumper part (upper segment and lower segment). As the working band of wide coverage is 900 MHz, the 7/8 feeder and 1/2 jumper are usually adopted. The loss of 7/8 feeder is about 4 dB/100 m, and the loss of 1/2 jumper is 10-12 dB/100 m. The loss of feeder can be eliminated by reducing the feeder length or employing thicker feeder.

Manuscript received: 2004-12-15