Broadband wireless technology based on the IEEE 802.16e standard has become the mainstream of WiMAX technology, and access to wireless networks has become a part of many people's lives. In order to meet the increasing transmission rate and high-speed mobility requirements of the IEEE, after the introduction of 802.16a, 802.16d, and 802.16e, the IEEE is about to propose the next-generation advanced air interface technology standard, 802.16m. In December 2006, the IEEE initiated the development of the IEEE 802.16m standard, and many world-renowned manufacturers will participate.
Technical characteristics of the WiMAX physical layer [1]:(1) Orthogonal frequency division multiplexing is used in the physical layer to achieve efficient spectrum utilization.
(2) Duplex mode: Supports time division duplex (TDD), frequency division duplex (FDD), and also supports half-duplex frequency division duplex (HFDD). FDD requires paired frequencies, TDD does not need to be, and flexible uplink and downlink bandwidth dynamic allocation can be achieved. The half-duplex frequency division duplexing method reduces the requirements of the terminal transceiver, thereby reducing the requirements for the terminal transceiver.
(3) It can support mobile and fixed conditions with a moving speed of up to 120 km/h.
(4) The bandwidth is flexible, and the bandwidth of the system ranges from 1.25 MHz to 20 MHz. WiMAX specifies several series of bandwidths: a multiple of 1.25 MHz and a multiple of 1.75 MHz. The 1.25 MHz multiple series includes: 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, 20 MHz, etc. The 1.75 MHz multiple series includes: 1.75 MHz, 3.5 MHz, 7 MHz, 14 MHz, and so on.
(5) Use advanced multi-antenna technology to increase system capacity and coverage.
(6) Adopt hybrid automatic repeat (HARQ) technology. The hybrid automatic retransmission operation incorporates the function of Forward Error Correction (FEC) so that each packet transmission operation can contribute to the final correct decoding. There are two main categories: chasing consolidation and incremental redundancy.
(7) Adopt adaptive modulation codec (AMC) technology. According to the quality of the received signal, the AMC adjusts the modulation, coding mode and coding rate of the packet at any time, so that the system can use the highest possible data transmission rate 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 coverage.
From the technical requirements of advanced international mobile communications and next-generation mobile networks, the future mobile communication transmission rate needs to reach hundreds of megabits per second or even gigabits per second. The highest physical layer rate in current IEEE 802.16e is 75. Mb/s, in order to achieve high data rates while ensuring communication quality, in the future standard evolution, the key technologies of the physical layer must be effectively evolved.
1 OFDM and OFDMA technologyOrthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) technologies have been introduced in 802.16d/16e. In the future evolution of physical layer technology, OFDM and OFDMA are still the main key technologies. one. Orthogonal Time Division Multiplexing (OTDM) is another multiplexing technology that has received much attention recently and may become one of the future physical layer multiplexing technologies.
1.1 Orthogonal Frequency Division Multiplexing
The main idea of ​​OFDM [2] is to divide the channel into several orthogonal subchannels, convert high-speed data signals into parallel low-speed sub-data streams, and modulate them onto each sub-channel for transmission. The orthogonal signals are separated by the relevant techniques through the receiving end, and the inter-subchannel interference (ICI) can be reduced under certain conditions. The signal bandwidth on each subchannel is less than the associated bandwidth of the channel, so each subchannel can be considered a flat fading channel, eliminating intersymbol interference (ISI). Since the bandwidth of each subchannel is only a small fraction of the original channel bandwidth, channel equalization becomes relatively easy.
OFDM technology is getting more and more attention because OFDM has many unique advantages:
The spectrum utilization is very high.
Strong resistance to multipath interference and frequency selective fading.
The dynamic subcarrier allocation technique enables the system to reach the maximum bit rate.
Through joint coding of each subcarrier, it can have strong anti-fading capability.
Discrete Fourier Transform (DFT)-based OFDM has a fast algorithm, and OFDM uses Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) to implement modulation and demodulation, which is easy to implement with digital signal processor (DSP). .
In addition to the above advantages, OFDM has three more obvious disadvantages:
Sensitive to frequency offset and phase noise.
The peak-to-average power ratio (PAPR) is large, resulting in lower power efficiency of the transmitter amplifier.
Adaptive modulation techniques increase system complexity.
OFDM as a modulation scheme to ensure high spectral efficiency has been adopted by some specifications and systems. OFDM will become one of the optimal modulation schemes for downlink in the new generation of wireless communication systems, and will be combined with traditional multiple access technology to become an alternative to the multiple access technology of the new generation wireless communication system.
1.2 Orthogonal Frequency Division Multiplexing Multiple Access
In an OFDMA system, the user only uses a part of all subcarriers. If the timing deviation and frequency deviation of the users in the same frame are sufficiently small, there will be no intra-cell interference in the system, which is more than the code division system. Advantage.
Since OFDMA can combine frequency hopping technology with OFDM technology, it can form a more flexible multiple access scheme. In addition, because OFDMA can flexibly adapt to bandwidth requirements, it can be combined with dynamic channel allocation technology to support high-speed data transmission. .
In the future evolution of physical layer technology, OFDMA will continue to be retained as a very important key technology.
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