Radio remote unit RRU(Radio Remote Unit) brings a new type of distributed network coverage mode. The large-capacity Hong Fengwo base station is centrally placed in the available central computer room, and the baseband part is centrally processed. The RF module in the base station is pulled to the remote RF unit through optical fiber and placed at the site determined by the network planning, which saves a lot of computer rooms required by the conventional solution. At the same time, by using large-capacity Hong Jizhan to support a large number of optical fibers, the conversion between capacity and coverage can be realized.
The working principle of RRU is that the baseband signal is transmitted to the antenna feed through frequency conversion, filtering, RF filtering and linear power amplifier. In the uplink, the received uplink signal of the mobile terminal is filtered, amplified with low noise, further amplified, filtered and down-converted, and then analog-to-digital conversion and digital intermediate frequency processing are completed.
There are two connection interfaces between the RRU and the base station interface: cpri (Public Wireless Interface) and obasi (Open Base Station Architecture Initiative). Among them, CPRI organization members include: Ericsson, Huawei, NEC, Nortel and Siemens. Members of OBSAI include: Nokia, ZTE, LGE, Samsung and Hyundai.
In terms of signal coverage, RRU can be extracted from NodeB by different scrambling methods at the same frequency. It can also be derived from RNC at the same frequency by different scrambling methods. These two covering methods are conventional. In addition, for a base station with three sectors but equipped with redundant channel boards and redundant baseband processing equipment, the redundant baseband processing equipment can be set as the fourth cell by using baseband pool sharing technology.
Both RRU and digital optical fiber repeater can transmit baseband signals by using the existing mature Ethernet digital optical fiber transmission technology, and interface with CPRI and OBSAI which meet the standards. In use, the RRU and the remote unit of the digital optical fiber repeater can be replaced with each other.
Both of them can be used as signal sources of indoor distribution system, and which one to choose depends on the number of carrier frequencies and indoor traffic demand in Hong Jizhan. If Hong Jizhan has many carrier frequencies and rich capacity, it is more suitable to use digital fiber repeater to zoom out, and at the same time, it can reduce the sector scrambling code. If the indoor traffic demand is large, RRU should be selected as the signal source. If the demand for business volume is large, such as large office buildings and conference centers, the joint networking of digital fiber repeater, RRU and Hong Jizhan should be considered.
In terms of coverage distance, both of them can be used as far-end systems of base stations, digital fiber repeater can be used as far-end of carrier pool, and RRU can be used as far-end of baseband pool. The distance of carrier pool depends on the coverage radius of the cell and the transmission speed of light on the optical fiber. Digital signal propagates in optical fiber, and its dynamic range is also larger than that of analog signal, so that the remote machine can achieve greater signal coverage. At the same time, the digital signal will not be attenuated with the attenuation of the optical signal, so its transmission distance will be further increased. After calculation, the longest distance can reach more than 40km, and the RRU used as baseband pool is basically not limited by distance and can be further expanded.
In networking mode, RRU can be used independently as a remote unit, while digital optical fiber repeater consists of a near-end machine and a far-end machine. In practical application, one near-end machine and one or more far-end machines can be connected in parallel or in series, and the networking modes can be diversified, such as daisy chain, ring and tree.
In the use of scrambling code, the scrambling code of the RF signal of the digital fiber repeater is always the same as that of the donor base station, and the digital fiber repeater does not increase the hardware capacity and orthogonal code capacity of the base station channel board, so it will not increase the scrambling code if it is widely used in the sector. RRU radio remote unit is a new sector, which is composed of the remaining channel boards and baseband processing equipment of the base station, and is pulled out through the optical fiber system. Some people call it baseband pool technology, while others call it remote microcellular technology. In short, it has hardware capacity and new scrambling and synchronization codes. Because RRU has the performance of base station, if it is widely used in the sector of Hong Jizhan, it will inevitably increase a large number of scrambling codes and neighbor list, and will produce pilot pollution and increase soft handover. As shown in figure 6. This is a problem that must be paid attention to in network optimization.
In terms of transmission delay, the transmission delay of digital optical fiber repeater is relatively large because there are two frequency conversion processes. However, RRU directly transmits baseband signals, and the delay is not obvious.
In noise floor lifting, digital optical fiber repeater only uses ADC and DAC, which may only introduce more quantization noise, thus improving uplink noise. RRU transmits pure baseband signals, so there is no need to consider the problem of background noise.
From the cost point of view, the adoption of RRU technology can save a lot of computer rooms needed in the conventional network construction mode and save the investment of baseband units. RRU is small in size and light in weight, and can be applied to situations where the computer room conditions in urban areas are not ideal or the computer room is scarce, but the application premise is that optical fiber is needed for transmission. But in terms of price, RRU is more expensive than repeater 1/3 or so. The cost advantage of digital optical fiber repeater is not obvious for a tow-by-tow system, but it is more obvious.