Broadband wireless technology based on IEEE 802.16e standard has become the mainstream of WiMAX technology, and access to wireless networks has become part of many people's lives. In order to meet the requirements of increasing transmission rates and high-speed mobility, IEEE has launched 802.16a, 802.16d, 802.16e, and IEEE will soon suggest the next generation of advanced vaccine technology standards - 802.16m. In December 2006 IEEE launched the formulation of IEEE 802.16m standards, many of the world famous manufacturers will participate.
The technical characteristics of the WiMAX physical layer [1]:
(1) In the physical layer, an orthogonal frequency division is used to achieve efficient spectral utilization.
(2) Duplex mode: Support time division duplex (TDD), frequency division duplex (FDD), and also supports half-duplex frequency division duplex (HFDD). FDD requires a pair of frequencies, and TDD is not required, and flexible upper and downlink bandwidth dynamic allocation can be achieved. Half-duplex frequency division duplex mode reduces the requirements of terminal transceivers, thereby reducing the requirements for terminal transceivers.
(3) The movement and fixed conditions can be supported, and the moving speed can be up to 120 km / h.
(4) The bandwidth is flexible, and the system has a bandwidth of 1.25 MHz to 20 MHz. WiMAX specifies several series of bandwidth: 1.25 MHz multiples, 1.75 MHz multiples series. Among them, 1.25 MHz multiples include: 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, 20 MHz, etc., 1.75 MHz multiples include: 1.75 MHz, 3.5 MHz, 7 MHz, 14 MHz, etc.
(5) Improve system capacity and coverage using advanced multi-antenna technology.
(6) Hybrid automatic retransmission (HARQ) technology. The function of the forward error correction (FEC) is combined in the hybrid automatic retransmission operation, so that the transmission operation of each packet package can contribute to the final correct decoding. Mainly divided into two categories: chase merger and incremental redundancy.
(7) Adaptive modulation codec (AMC) technology. According to the quality of the received signal, the AMC adjusts the modulation, encoding method, coding rate of the packet package, which allows the system to use as high data transmission rates on the basis of sufficient reliability.
(8) Using power control technology, the goal is to maximize spectral efficiency while meeting other system indicators.
(9) Adopt advanced channel coding technology to increase communication quality and expand the coverage.
From the technical needs of advanced international mobile communications and next-generation mobile networks, the transmission rate of future mobile communications is required to reach the 100 MMB per second or even the Giby Territory per second. The current IEEE 802.16e is the highest physical layer rate is 75. MB / S, in order to achieve high data rates while ensuring communication, in the future standard evolution, key technologies of the physical layer must be effectively evolved.
1 OFDM and OFDMA technology
In 802.16d / 16E introduced orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiplex (OFDMA) technology, OFDM and OFDMA remain main key technologies in the future physical layer technology evolution. one. Orthogonal time division multiplexing (OTDM) is another multiplexing technology that has received much attention, it is possible to become one of the future physical layer multiplexing technologies.
1.1 orthogonal frequency division multiplex
The main idea of OFDM [2] is to divide the channel into a plurality of orthogonal subchannels, convert high-speed data signals into parallel low-speed sub-data streams, modulate them on each sub-channel. The orthogonal signal is separated by the receiving end using the related technique, and the subchannel interference (ICI) can be reduced under certain conditions. The signal bandwidth on each subchannel is smaller than the associated bandwidth of the channel, so each subchannel can be regarded as a flat decline channel, thereby eliminating inter-symbol interference (ISI). Since the bandwidth of each sub-channel is only a small part of the original channel bandwidth, the channel equalization becomes relatively easy.
The reason why OFDM technology is more concerned, because OFDM has a lot of unique advantages:
The spectrum utilization is very high.
Anti-multi-diameter disturbance and frequency selective decline ability.
The use of dynamic subcarrier distribution technology enables the system to achieve the maximum bit rate.
With the joint encoding of each subcarrier, it can have a strong anti-fading ability.
Rapid algorithm based on discrete Fourier transform (DFT), OFDM uses fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) to achieve modulation and demodulation, easy to use Digital Signal Processor (DSP) .
In addition to the above advantages, OFDM has three more obvious disadvantages:
Sensitive to frequencies and phase noise.
The peak power ratio (PAPR) is large, resulting in low power efficiency of the transmitting amplifier.
Adaptive modulation technology has increased system complexity.
The modulation scheme that ensures high spectrum efficiency has been adopted by some specification and system. OFDM will become one of the best modulation schemes of downlink in a new generation of wireless communication systems, as well as traditional multi-access technology to become an alternative to a new generation of wireless communication system multiple access technology.
1.2 orthogonal frequency division multiple access
In the OFDMA system, the user only uses a portion of all subcarriers, and if the timing deviation and frequency deviation within the same frame are small enough, there is no interference in the cell, the composite system is more Advantage.
Since OFDMA can combine the frequency hopping technique and OFDM technology, it can constitute a more flexible multi-access scheme, and since OFDMA can flexibly adapt to bandwidth requirements, it can be used to support high-speed data transmission with dynamic channel allocation technology. .
In the future physical layer technology evolution, OFDMA will continue to reserve as a very important key technology. , Reading the full text, the technology area
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