FDD and TDD are all weak, look at the most bull ndd!.

Say FDD and TDD two brothers in order to Zhengchong noisy, suddenly jumped out of a stone ndd, claiming that he is the world's most cattle "DD", FDD and TDD two brothers immediately panic, eclipsed, secretly inquire about the ndd of the details. Today we're going to uncover the bottom of the ndd and see where he is.

Start with the duplex approach.

FDD (Frequency Division Duplex): Two symmetrical frequency channels are used to transmit and receive signals respectively.

TDD (Time Division Duplex): the transmit and receive signals are carried out at different slots in the same frequency channel.

It is said that FDD is Full-duplex and TDD is Half-duplex. In a sense, neither of the two is Full-duplex, because it is not possible to transmit and receive signals simultaneously under the same frequency channel.

Today we introduce the NDD, can realize the transmitter and receive signal at the same frequency at the same time transmission, is a true full-duplex. However, I prefer to call it "no division of Labor (no division duplex,ndd)".

Haven't we ever thought of such a cow's full-duplex way? Of course not. Because the transmitter in the wireless system will generate strong self interference to receive the signal, if press ndd that way, the system cannot work at all. In Full-duplex mode like NDD, if the transmit signal and the receiving signal are not orthogonal, the transmitter end produces a jamming signal billions of times times stronger than the useful signal received (greater than 100dB).

So, how did ndd do it? The core technology of NDD is to eliminate this 100dB of self interference.

Let's take a look at how the transmit signal generates a powerful self interference to the receiving signal.

From the above figure, due to the leakage of the duplexer, antenna reflection, multipath reflection and other factors, the emission signal doping into the receiving signal, resulting in a strong self interference.

How do you eliminate these distractions? Fortunately, because the transmit signal is known, then the emission signal can be used as a reference to eliminate the self interference. However, this reference signal can only be obtained from the digital baseband domain. When the digital signal is converted into analog signal, it is difficult to obtain the reference because of the influence of linear distortion and non-linear distortion. Therefore, any self interference cancellation technology must consider the non-linear distortion of the transmitting signal if it wants to succeed.

In addition, in order to avoid receiving saturation, it is necessary to consider the resolution limit of the receiving End mode/number converter, so the self interference signal strength of the input modulus/number converter must be guaranteed to be less than a certain value.

By solving these problems, we can effectively decompose the jamming signal and eliminate it.

The problem is said to have been settled by Ndd's owner.

Sachin Katti and his team from Stanford University broke through the challenges and set up a Kumu networks start-up in 2012 and are now deploying the technology for the small cell.

How did they break through? Kumu gives some simple introduction to the principle.

In fact, Kumu's self interference cancellation technology is not the first, but Kumu circuit algorithm is the most powerful, can eliminate 110dB interference signal.

Thanks to the powerful self interference cancellation technology, making true full-duplex communication possible, wireless spectrum efficiency and delay will be greatly improved, if it can be applied perfectly, this is a subversive innovation.

1) compared with FDD/TDD, the spectral efficiency will be increased by one times.

2 Compare TDD to shorten time delay significantly.

Because TDD is a time division duplex, can not send and receive data at the same times, NDD effectively solve the problem, send the data, receive feedback immediately, reduce time delay.

In addition, when the packet is transmitted, it is not necessary to wait for the packet to arrive to send the next packet, especially when retransmission, greatly reducing the delay.

This technology can be widely used in microwave backhaul, WiFi access, mobile devices and LTE access, the most efficient application is as "Self-backhaul Small cell" deployed in the current network.

Self-backhaul Small cell deployment only needs to be installed on the base station side of the board, and other hardware parts of the current network need not change.

At present, Kumu Networks has received a lot of financing, but to truly achieve landing, but also face some challenges.

1 circuit board parts design.

Self-interference cancellation circuit design needs to support broadband (>100mhz), MIMO (>32 antenna), and requires small size, low power consumption, the cost can not be too high.

2 optimization design of physical layer and MAC layer. such as coding, modulation, synchronization, detection, interception, conflict avoidance, ACK, etc., especially for MIMO physical layer optimization.

3 The optimization of the control surface of the dynamic switching between Full-duplex and Half-duplex, and the optimization of the existing frame structure and control signaling.