Design of MAC layer for broadband fixed wireless access network

First, the introduction
The broadband access network is regarded as the last kilometer of the information superhighway, which will become the key point of the future national information infrastructure development. At present, there are several broadband access modes, such as optical fiber access, XDSL, broadband fixed wireless access network. Compared with other broadband access methods, broadband fixed wireless network has many advantages, such as fast construction, low cost, easy to maintain and overcome certain geographical constraints, has become a very promising way of access.
Broadband Fixed wireless access network refers to the fixed wireless communication system, which is mainly used to transfer data between the user terminal and the core network [1]. The topological structure of the broadband fixed wireless access network is shown in Figure 1. The broadband fixed wireless access network is a point-to-point structure, which mainly includes base station (BS), user station (SS) and network management system, which can be relayed by relay station (RS) between base station and user station under special circumstances. A user station can be connected to a single user terminal (TE) or a User resident network (CPN) [2].



Broadband Fixed wireless access network data transmission direction is two-way, uplink refers to the data transmission direction from the user station to the base station, the rate of 5.12 mbit/s, downlink refers to the direction of data transmission from the base station to the user station, the rate of 8.86 mbit/s. The duplex mode used in broadband fixed wireless access network is Frequency division multiplexing (FDD). Downlink Subgrade stations transmit data in broadcast mode, and each user station uses time Division Multiplexing (TDM) to share downlink resources. The uplink user station takes the burst mode data, each user station uses the time division multiple access (TDMA) way to share the uplink resources. The broadband fixed Wireless access network (3.5 GHz) is introduced in this paper to provide access to the circuit business and the data business.
The broadband fixed wireless access network is divided into physical layer and Mac layer, and the protocol model is shown in Figure 2. The physical layer provides transparent bit-stream transmission between the base station Mac and the user station Mac [3]. The following part of this paper will study the MAC layer function, focusing on the design of the MAC layer function of the technical scheme, and finally with Altera Company's FPGA chip implementation.



Second, MAC layer
The MAC layer of broadband fixed wireless access network consists of 2 sub layers, namely, the specific service aggregation sublayer and the common part child layer. The specific business aggregation sublayer is located on the common part of the child layer and utilizes the services provided by the common part of the child layer. The specific business aggregation sublayer receives the business data from the high-level and encapsulates the data into a business package so that the recipient-specific business aggregation sublayer can restore these business packages to high-level information. The common part of the sub layer is the core of MAC layer, including framing, bandwidth allocation, security, link establishment and maintenance. Framing is to encapsulate a business package of a particular business aggregation layer into a frame and pass it to the physical layer; Bandwidth allocation is how to allocate bandwidth resources of broadband fixed wireless access network to different user stations, security is to prevent illegal user station intrusion; Link establishment and maintenance is to establish communication link between user station and base station. and maintain this link.
The purpose of MAC layer design is to design the technical scheme of realizing the above functions, which is divided into the specific business aggregation sublayer design and the common part sub layer design.
Third, the specific business aggregation sub-layer design
The key of the specific business aggregation sublayer design is the design of the business package, and the different business data has different package format. Broadband Fixed Wireless access network provides access to data and circuit services, this paper designs the data business package and the circuit business package according to its business characteristic, and divides the specific business aggregation sublayer into 2 kinds, the data business aggregation sublayer and the circuit business aggregation sublayer, respectively, are used to realize the encapsulation process of these two kinds of business.
1. Data Business Package
The broadband fixed wireless access network receives the data business data from the high level is based on the IEEE802.3 frame format. The IEEE802.3 frame can not be bounded by frame itself, and the frame delimiter is required at the bottom. The main function of the data business package is to frame the IEEE802.3 frame. The data business package consists of 3 bytes of Baotou and net load. The header is "0x0ffff0", the function is to make the receiving-end data business aggregation sub layer can frame the IEEE802.3 frame from receiving the data stream. The net charge is a IEEE802.3 frame that is scrambling code. In order to prevent the IEEE802.3 frame data from the same as the header, the IEEE802.3 frame must be added to the scrambling code. The principle of scrambling code is to add a byte of "0x00" when the 3 consecutive half bytes are "0xF". The principle of data packet encapsulation and scrambling code is shown in Figure 3.



2. Circuit Business Package
High-level circuit business data received by broadband fixed wireless access network is based on E1 frame, whose frame structure conforms to ITU-T g.704. The circuit business Convergence Sublayer encapsulates the received E1 frame data into a circuit business package and restores the received circuit business package to the E1 frame at the receiving end of the circuit business aggregation sublayer. The Circuit business packet length is 18 bytes, including 2 bytes of headers and 16 bytes of net load. The header consists of a byte user station flag and a byte time slot flag. User Station logo is a broadband fixed wireless access network to distinguish between different user stations, the time slot is used to distinguish between the same user station different time slots. The net charge is composed of 16 consecutive frames of the time slot corresponding to the time slot sign. The circuit business package structure is shown in Figure 4. Cpeid is the user station mark, TSID is the time slot sign, F1 to F16 is the data in the time slot.