Water injection principle

Water injection is an optimal power allocation strategy, provided that channel CSI is obtained. According to the different channel conditions of the subchannels to allocate, they can be watered in time, frequency and space, and they can be jointly optimized for water injection.

The implementation of the principle of water injection, on the one hand due to the input signal is limited, the actual capacity must be less than the water injection capacity limit, on the other hand is to achieve the complexity of the problem

Personally think that OFDM has at least two advantages over traditional single-carrier communication:

1, software and hardware problems

In the case of single carrier, the rate is high and inter-code interference is large. Traditional time domain equalizers are difficult to implement. OFDM is divided into multiple subcarriers, and inter-code interference is no longer a problem. At the same time, FFT algorithm is used for OFDM hardware and software. Implementation is no longer a problem.

2, the problem of approaching channel capacity

OFDM is divided into multiple sub-carriers, each sub-band is approximately flat, so that it can approach the water injection algorithm through adaptive modulation (adaptive rate allocation) on each sub-band.

Although OFDM can be considered as inter-code-free crosstalk for each sub-channel, it can flood water between each sub-carrier (frequency domain water injection). However, the rate of each sub-carrier should be different after water injection. This means that multiple codes with different rates are needed. The complexity is high, and the code rate changes with the change of fading.

If single carrier is used, then we only need one code; time domain equalization is not easy, but it can be injected in the frequency domain; and with the change of fading, we only need to change the way of precoding (transmission equalization). From the complexity point of view, individuals feel that SC-FDE is better than OFDM.

When calculating the MIMO channel capacity C = log2 (I + Pi * into i), [into i is the singular squared value of the channel matrix H, and Pi is the same value as the SNR when the transmission power is Gaussian noise] using a pull-lange multiplication operator The allocated channel power Pi maximizes the channel capacity.

When the SNR is large, the power distribution of Pi is equal, and the efficiency of the water injection algorithm disappears. The white noise power is a constant, independent of frequency. If it is changed to color noise, then it will be related to the frequency. Now it is a problem of generating a color noise, which can be turned into a band-limited noise through a filter. Here, 1/λ is the power threshold determined by the environment (multi-path fading, SNR). Those steps indicate the state of the sub-channel. The lower the channel, the better.

The principle of water injection is to determine how to allocate power in each sub-channel by the state of 1/λ and sub-channels. (P* in the figure shows how much power is allocated to that ladder (ie sub-channel), ie, the difference between 1/λ and the ladder). The better the channel state is, the more power is allocated, the less difference is. When the state difference exceeds the threshold 1/λ, power is not allocated on this sub-channel.

In this way, it is like putting water in a pool. The height of the water surface is 1/λ, and the distribution of the bottom of the pool is composed of the states of all the sub-channels (that is, the steps in the figure). Therefore, it is called the water injection algorithm.

The above analysis is the principle of water injection in wireless communication, which involves channel power distribution. However, what we commonly call water injection may be more about the water injection principle of oil injection, water injection and water injection. Oilfield waterflooding is due to the continuous decline in oil layer pressure and the decline in well production as oil is extracted. In order to supplement and maintain reservoir pressures, we will adopt oilfield water injection methods to ensure high oil production and stable production. Water-injected pork and water-injected watermelon are all unlawful means for dealers. In order to obtain higher profits, they sacrifice the interests of consumers and even be healthy. Therefore, we strongly resist water injection of water-injected pork and water-filled watermelon. of. This is not only related to the interests of consumers. To a greater extent, it is a major issue that affects the health of the people and the quality of the people.

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