Chapter 9 is here!

The content is slightly new!

But it’s still a very “old” knowledge point!

Objectives of the WiMAX Forum:

1. Facilitate and promote global WiMAX deployment.

2. Make WiMAX service the platform of choice for broadband wireless and dominate the world market share.

3. Release a high-performance end-to-end IP network architecture to support fixed, portable and mobile users.

4. Ensure that WiMAX Forum certified products can win the trust of global service providers.

5. Develop WiMAX mode based on IEEE 802.16 and ETSI interoperability to serve the global market.

6. Expand user demand by providing novel and competitive application and service models.

7. Promote sound intellectual property policies.

The WiMAX Forum currently has 7 working groups, which are involved in promoting the products certified by the WiMAX Forum to many areas that the market needs to focus on. Certification Working Group (CWG), Technical Working Group (TWG), Spectrum Working Group (RWG), Market Working Group (MWG), Requirements Working Group (SPWG), Application Working Group (AWG), Network Working Group (NWG)!

The transmission of FDD uplink and downlink uses the duplex mode of two separate symmetrical frequency bands, and the system needs to be divided according to the symmetrical frequency bands.

Its technical characteristics are: paired frequencies are required, transmission and reception are performed on two separate symmetrical frequency bands, and a frequency interval of 190 MHz is required between the uplink and downlink frequency bands. When supporting symmetric services, the uplink and downlink spectrum can be fully utilized, but when asymmetric packet switching (Internet) works, the spectrum utilization rate is greatly reduced (due to the low uplink load, the spectrum utilization rate is reduced by about 40%).

2. Carrier bandwidth

IEEE 802.16 does not specify a specific carrier bandwidth, and the system can use a bandwidth from 1.25MHz to 20MHz.


(1) Orthogonal Frequency Division Multiplexing OFDM

OFDM is a multi-carrier digital modulation technique. Its basic idea is to divide a given channel into many orthogonal sub-channels in the frequency domain, use one sub-carrier for modulation on each sub-channel, and transmit each sub-carrier in parallel. In this way, although the total channel is non-flat and has frequency selectivity, each sub-channel is relatively flat, and narrow-band transmission is carried out on each sub-channel, and the signal bandwidth is smaller than the coherence bandwidth of the channel, so the signal waveform can be greatly eliminated. interference. The difference between the OFDM technology and the general multi-carrier transmission technology is that it allows the sub-carrier spectrum to partially overlap. As long as the sub-carriers are mutually orthogonal, the data signal can be separated from the aliased sub-carriers.

Advantages of OFDM:

① The spectrum utilization rate is very high.

② Strong ability to resist multipath interference and frequency selective fading.

③ The use of dynamic subcarrier allocation technology can enable the system to achieve the maximum bit rate.

④ Through the joint coding of each sub-carrier, it can have strong anti-fading ability.

⑤ OFDM based on discrete Fourier transform (DFT) has a fast algorithm, OFDM uses fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) to achieve modulation and demodulation, easy to use digital signal processor (DSP) accomplish.

Disadvantages of OFDM:

① Sensitive to frequency offset and phase noise.

② The peak-to-average power ratio (PAPR) is large, resulting in low power efficiency of the transmitter amplifier.

③ The adaptive modulation technology increases the system complexity.

This technology is now widely used, and the improvement of chip capabilities makes the application of adaptive algorithms more and more widely!

CS: This layer provides different functions according to the different services provided. For IEEE 802.16, the services that can be provided include digital audio/video broadcasting, digital telephony, asynchronous transfer mode ATM, Internet access, wireless relay and frame relay in the telephone network.

CPS: CPS is the core part of the MAC, and its main functions include system access, bandwidth allocation, connection establishment and connection maintenance.

PS: Provides confidentiality between the base station and the user station. It includes two parts: one is the encryption encapsulation protocol, which is responsible for the encryption of packet data transmitted over the air, and the other is the key management protocol, which is responsible for the key between the base station and the user station. safe distribution.

3. Security

In order to ensure the security of information transmission, 802.16 defines a security sub-layer in the MAC layer to implement key distribution management and data encryption. The security sublayer is located above the physical layer and belongs to the lowest sublayer of the MAC layer. Its encryption protocol is mainly based on the key management protocol in DOCSIS BPI+ cable modulation and transmission technology.

Editor: Huang Fei


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