What Is WiMAX 2?

Global Interoperability for Microwave Access (WiMAX). Another name for WiMAX is 802.16.

WiMAX (World Interoperability for Microwave Access)

The entities of the network are as follows: Access network ASN: As a logical entity, it manages the IEEE802.16 air interface and provides wireless access for WiMAX users. Connected Service Network CSN: CSN is a set of network functions. WiMAX users provide IP connectivity. It consists of a router, an AAA proxy or server, a user database, and an Internet gateway device. As a new network entity of the new WiMAX system, it can also use some existing network equipment to implement the CSN function. Network access provider NAP and network service provider NSP: Support multiple NSPs to share one or more ASN networks in the same NAP, and support one NSP to connect with multiple ASNs managed by multiple NAPs. WiMAX application The core value of the WiMAX technology system lies in metropolitan coverage and large bandwidth transmission. WiMAX is mainly applicable to areas and cities that are not covered by wired networks. , Especially suitable for wireless high-speed access of metropolitan area networks. There are two types of WiMAX applications: when there is no other cable coverage method such as DSL and when the existing DSL, mobile network, etc. have insufficient transmission bandwidth. There are three main types of services: fixed wireless access: as a supplement to wired access methods such as DSL, access to WiMAX networks at fixed locations. Seamless wireless access: It is suitable for highly mobile people such as business people, and industries with strong mobile office needs such as transportation and logistics. It can access WiMAX networks at different locations. Roaming mobile access: With the development of WiMAX, 802.16E will have better mobile network access characteristics; it can achieve seamless roaming on foot or in the car. Prospects of WiMAX In the future, if WiMAX is to obtain better applications, it is necessary to solve the problem of frequency allocation, the cost performance of systems and terminals, and killer applications. The price of WiMAX terminals is currently relatively expensive. In the future, the price of user terminals needs to be less than 100 US dollars to be able to start the market. At the same time, the frequency used by WiMAX is different in different countries, and in some countries it is an open public frequency band. The effective allocation and management of WiMAX frequencies will become an important factor affecting the development of WiMAX. At present, what is most lacking in the development of WiMAX is the market driver. In general application modes, users do not need such high bandwidth. Therefore, the development of applications that match the WiMAX bandwidth will become a major influencing factor for its healthy development. The primary motivation of WiMAX certification is to hope that companies can access the Internet anytime, anywhere through lower prices, higher-level performance, and faster innovation, and expand the market for broadband wireless networking, thereby driving the prospects of hardware manufacturers and service providers. Service providers and enterprises enjoy the interoperability between devices; hardware suppliers can reduce differences in product specifications and reduce production costs. The arrival of WiMAX means that the network is more popular. Not only can you access the Internet at home or office, you can also surf the Internet at any time on the road ^[1]

The air interface defined by the IEEE 802.16 standard consists of the physical layer and the MAC layer, as shown in Figure 1. The MAC layer is independent of the physical layer and can support a variety of different physical layer specifications to adapt to various application environments.

The physical layer consists of the Transmission Convergence Sublayer (TCL) and the Physical Media Dependent Sublayer (PMD). Generally speaking, the physical layer mainly refers to PMD. The TCL segments the received MAC layer data into TCL protocol data units (PDUs). PMD specifically performs a series of processing processes such as channel coding, modulation and demodulation. The MAC layer uses a hierarchical structure and is divided into three sub-layers: a specific service convergence sub-layer (CS), a common part sub-layer (CPS), and a security sub-layer.

1) The CS sublayer is responsible for interfacing with higher layers and converging different services from the upper layers. It converts and maps external network data received through the service access point (SAP) into MAC business data units and passes them to the SAP at the MAC layer. The protocol provides multiple CS specifications as interfaces to various external protocols, which can realize transparent transmission of protocol data such as ATM and IP.

2) The CPS sublayer implements the main MAC functions, including system access, bandwidth allocation, connection establishment, and connection maintenance. It receives data from various CS layers through the MAC layer SAP and classifies it into specific MAC connections, and implements QoS control on the data transmitted and scheduled on the physical layer.

3) The main function of the security sublayer is to provide authentication, key exchange, and encryption and decryption processing. This sublayer supports 128-bit, 192-bit, and 256-bit encryption systems, and uses digital certificate authentication methods to ensure the secure transmission of information. ^[2]

(1) Wide application frequency

802.16 technology can be applied to a very wide frequency band, including 10-66GHz frequency band, <11GHz frequency band and <11GHz license-free frequency band.

(2) Flexible modulation

In the 802.16 standard, three physical layer implementations are defined: single carrier, OFDM, and OFDMA.

(3) Perfect QoS mechanism

In the 802.16 standard, a more complete QoS mechanism is defined at the MAC layer. The MAC layer can set different QoS parameters for each connection, including indicators such as rate and delay.

OFDM and OFDMA

The basic idea of OFDM technology is to divide the available bandwidth of the channel into several mutually orthogonal subcarriers, and carry out data transmission independently on each subcarrier, so as to achieve low-speed parallel transmission of high-speed serial data streams. It evolves from the traditional frequency division multiplexing (FDM) technology. The difference is that OFDM uses DFT (Discrete Fourier Transform) and IDFT instead of traditional band-pass filters to achieve the division between subcarriers. The subcarriers can partially overlap, but still maintain orthogonality, thus greatly improving the system's spectrum utilization. In addition, the low-speed parallel transmission of data enhances the ability of OFDM to resist multipath interference and frequency selective fading. Combining frequency division multiple access (FDMA) on the basis of OFDM technology, the available subcarrier resources within the channel bandwidth are allocated to different users for use, which is OFDMA ^[3] .

Multi-antenna technology

Multi-antenna technology can double the channel capacity without increasing the system bandwidth, thereby achieving higher data transmission rates and larger coverage areas, or improving signal transmission quality. The 802.16 standard supports multiple antenna technologies including multiple input multiple output and adaptive antenna systems.

Adaptive modulation and coding

The time-varying and fading characteristics of the wireless channel determine that the channel capacity is a time-varying random variable. To maximize the use of the channel capacity, only the transmission rate will change accordingly, which means that the coding and modulation method should have adaptive characteristics. Adaptive modulation and coding (AMC) technology is to dynamically adjust the coding and modulation modes according to the channel conditions to improve the transmission rate or system throughput. The basic method is to use high-order modulation and high coding rate (such as 64QAM, 5/6 code rate) when the channel conditions are good to achieve higher peak rates according to the measurement results of the channel quality; while the channel conditions are poor When using low-order modulation and low coding rate (such as QPSK, 1/2 code rate) to ensure transmission performance. By changing the modulation and coding method instead of transmitting power to improve performance, it can also greatly reduce the additional interference introduced by the increase in transmission power.

Hybrid automatic repeat request

Hybrid automatic repeat request (H-ARQ) is a technology that combines automatic repeat request (ARQ) and forward error correction coding, which can be used to reduce the negative impact of channel and interference jitter on data transmission.

The basic working process of H-ARQ is as follows: one or more MAC layer data units to be sent are connected in series, and encoded according to the specific specifications of the physical layer to generate 4 H-ARQ sub-packets. The base station sends only one subpacket at a time. Because there is a great correlation between the four sub-packets, the receiving end can decode correctly without obtaining all the sub-packets. Therefore, after receiving the first subpacket, the terminal attempts to decode. If the decoding is successful, the terminal immediately sends an acknowledgement (ACK) message to the base station, preventing it from sending subsequent sub-packets. If the decoding fails, the terminal sends a NACK message, requesting the base station to send the next sub-packet, and so on. The terminal will decode all the received sub-packets each time to improve the decoding success rate. It can be seen that H-ARQ uses the simplest stop-and-wait retransmission mechanism to reduce control overhead and send and receive buffer space. At this time, if the OFDMA physical layer is used, the defect of low channel utilization of protocols such as stop can be skillfully overcome. Therefore, only the OFDMA physical layer is provided in the protocol to provide support for H-ARQ.

Power Control

802.16e stipulates that power control should be performed in both the uplink and the downlink to comprehensively improve the performance of the system. The total transmit power consists of a fixed part and a dynamically adjusted part.

Media access mechanism

The design of the media access control mechanism is a problem that must be considered in any wireless access system that uses a shared channel method. Unlike the Carrier Sense / Collision Avoidance (CSMA / CA) strategy of IEEE 802.11, the approach taken by IEEE 802.16 is to fragment time resources at the physical layer and distinguish between uplink and downlink by time. The frame length of each physical frame is fixed and consists of two parts, uplink and downlink. The switching points of uplink and downlink can be adaptively adjusted through the control of the MAC layer. In TDD mode, each frame consists of n time slots. The downlink is broadcast, and the uplink is sent from SS to BS. Downward comes first, and upward comes last. For broadband wireless access systems, this media access mechanism takes into account both flexibility and fairness, and each SS has the opportunity to send data, which prevents the phenomenon of long-term competition and failure to channel; second, each SS only Data is sent within its own sending period, which guarantees that there is only one data stream on the media at any time; again, this mechanism facilitates control of QoS, service priority, and bandwidth.

QoS guarantee mechanism

WiMAX is the first wireless access standard to provide QoS guarantee at the MAC layer. As we all know, the influence of factors such as multipath and fading on wireless channels will lead to higher bit error rates and packet loss rates, and it is difficult to guarantee the reliability and effectiveness of data transmission. In order to meet the higher requirements for high-speed multimedia services such as delay, bandwidth, and loss rate, the MAC layer of WiMAX defines a series of strict QoS control mechanisms, which can provide different quality services for different services in the radio access network part. At the same time, this service is connection-oriented.

Switch

When the mobile subscriber station (MS) leaves the coverage of the original BS in motion or other BSs can provide better quality services, a handover (HO) procedure needs to be performed. Through the network topology message broadcast by the BS, the MS can obtain the DCD / UCD information of the neighboring cell. The BS can also allocate a scanning cycle to the MS to scan and range the neighboring base stations, evaluate its physical layer channel quality, and find potential target BSs for possible handover determination. The actual handover process can be initiated by the MS or the BS. The handover is a hard handover. In addition, IEEE 802.16e defines two optional handover modes: macro diversity handover (MDHO) and fast BS handover (FBSS). MDHO allows the MS to communicate with multiple BSs at the same time to obtain diversity gain and improve link quality. In FBSS, the MS can implement fast handover between any two BSs in a BS set without performing a regular handover process ^[4] .