The 3G standard originally had three types of wideband code division multiple access (WCDMA), CDMA2000, and time division synchronous code division multiple access (TD-SCDMA). In 2007, WiMAX was among them, making 4 of the 3G standards, plus 2G GSM. / CDMA, quasi 4G Long Term Evolution (LTE) standards, etc. There are currently more than seven wireless standards in use and to be used. Supporting such multiple standards is a burden for many equipment vendors, with the type of equipment. The increase, development and maintenance are all problems. If these standards can be integrated on one Hardware platform, it will undoubtedly greatly reduce the R&D and equipment costs of equipment manufacturers. In this context, multi-mode base stations have emerged.
As the name implies, a multimode base station is a base station that supports multiple wireless standards at the same time. Since multi-mode base stations can reduce hardware costs and enable operators to quickly introduce new services, manufacturers are concentrating on multi-mode base stations to take the lead in future competition.
A "bottleneck" for the development of multimode base stations lies in the baseband-radio interface. This interface is actually a data transmission link between the baseband unit and the radio unit, and of course, auxiliary information such as signaling and timing signals. Because of the different data formats and rates of various standards, how to integrate them together is a problem.
At present, the two most commonly used baseband radio frequency interface protocols in the world are the Common Public Radio Interface (CPRI) and the Open Base Architecture Protocol (OBSAI) interface. By studying these two protocols, the unique frame structure and layered design concept of the OBSAI protocol are discovered. Ideal for transmitting baseband data for multiple wireless standards simultaneously [1]. Although CPRI can transmit multiple standard baseband data at the same time, because its single data frame is large, it needs to be split into many small blocks, and the baseband data rates of different wireless standards are different, and the block size is different. It is relatively cumbersome, so you need to use the OBSAI protocol to build the baseband RF interface of the multimode base station.
1 Introduction to the agreementOBSAI is a standardization organization initiated by well-known companies such as Nokia, ZTE, and Samsung. It is dedicated to building a common interface between baseband units and RF units. Devices from different vendors can be interconnected through this interface.
The smallest unit of the OBSAI protocol is a message, which contains four parts of the destination address, data type, timestamp, and payload, for a total of 19 bytes. The length and meaning of each part of the message are shown in Table 1.
The OBSAI protocol divides the baseband radio interface into four layers, from top to bottom, the application layer, the transport layer, the link layer, and the physical layer. The application direction of the sending direction is responsible for inserting the baseband data and signaling data into the Message; the transport layer is responsible for crossing, summing, and multiplexing the respective Message code streams, and finally synthesizing one code stream; the link layer is responsible for inserting a regular special on the code stream. The code is used to mark the link quality; the physical layer is responsible for 8b/10b codec, serial-to-parallel conversion and serial transmission. The receiving direction is the inverse process of transmission. Each layer transmits its own extracted data to the upper layer, and finally extracts the baseband data and signaling data. There is no summation processing when receiving, but there may be when sending, CDMA/WCDMA has summation, and WiMAX does not. The structure of the entire protocol is shown in Figure 1.
Both the baseband unit and the radio frequency unit contain the complete 4 layers because they extract the baseband data, while the intermediate transmission unit does not need to extract the data, only the bottom 3 layers are sufficient.
OBSAI specifies three fiber speeds, namely 768 Mbit/s, 1 536 Mbit/s, and 3 072 Mbit/s, which are usually represented by 1x, 2x, and 4x. Two 1x streams can be cross-multiplexed by Message. It becomes a 2x code stream, and two 2x code streams can also be converted into a 4x code stream in the same way. Since the 1x code stream rate is low and the processing is convenient, the code streams for 2x and 4x are usually demultiplexed into several. 1x code stream, after processing, the corresponding code stream is multiplexed into 2x, 4x code streams.
The optical module has several fixed rate grades, such as 1.25 Gbit/s, 2.5 Gbit/s, 3.125 Gbit/s, etc., so the three rates of OBSAI can only be selected from these models. Since it is not exactly matched, there is a certain amount of bandwidth waste, resulting in low transmission efficiency of OBSAI. However, several rate grades of CPRI match the rate of the optical module, so the transmission efficiency is high. In actual development, OBSAI can also be operated at the CPRI rate level by adjusting parameters, thereby improving transmission efficiency. The internal structure does not change during development.
2 Basic principlesAt present, the baseband processing unit of the industry baseband-radio remote module (BBU-RRU) separates the baseband-radio connection form of the base station. One is that the baseband unit itself connects the radio frequency unit through the optical port, and the other is that the multiple baseband units themselves The data is sent to the intersection unit for aggregation and then connected to the RF unit through the optical port. If you need to transfer multiple standard data on a single fiber, you can only use the latter, because only then can you mix the various baseband data together. The two connection forms are shown in Figure 2. The fiber in the figure can also be a high speed cable.
From the perspective of resource sharing and convenient wiring, multi-mode base stations are likely to adopt the latter connection form. Therefore, this paper takes this connection form as an example to explain the baseband radio interface principle based on the OBSAI protocol.
A baseband unit supports several sectors. The data rate and format of each sector are the same, but the rate and format of the baseband data of different wireless standards are different. If they are to use the same transmission link, it is impossible to follow Their original format transmissions must be packaged in the same container to share a single transmission path. The Message in OBSAI can be used as a container to store different standard baseband data. As a separate entity, each Message can be arbitrarily cross-forwarded and truly out of the wireless standard.
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