FDD and TDD are both weak. Look at the best NDD!

FDD and TDD are both weak. Look at the best NDD!

When the FDD and TDD brothers had to fight for favor, they suddenly jumped out of a stone and declared that they were the best "DD" in the world ", the FDD and TDD brothers suddenly panicked and eclipsed, and secretly explored the details of the NDD. Today, we will reveal the bottom of the NDD to see where it is?

The first step is to start with the duplex mode.

FDD (frequency duplex): two symmetric frequency channels are used to transmit and receive signals respectively.

TDD (Time Division Duplex): the transmitting and receiving signals are carried out in different time slots of the same frequency channel.

Some people say that FDD is full duplex and TDD is half duplex. In a sense, neither of them is full duplex, because neither of them can transmit or receive signals simultaneously in the same frequency channel.

The NDD we introduced today can transmit and receive signals at the same frequency at the same time, which is a real full duplex. However, I prefer to call it "No Division of labor (No Division Duplex, NDD )".

Didn't we ever think of such a full duplex? Of course not. In wireless systems, transmitting signals can generate powerful self-interference to receive signals. If you play the same way as NDD, the system cannot work at all. In full-duplex mode like NDD, if the transmitting and receiving signals are not orthogonal, the interference signals produced by the transmitting end are several billion times stronger than the useful signals received (larger than DB ).

So how does NDD do it? The core technology of NDD is to eliminate the self-interference of 100dB.

First, let's take a look at how the transmit signal produces powerful self-interference to the received signal.

It can be seen that due to factors such as dual-device leakage, antenna reflection, and multi-path reflection, the transmitting signal is doped with the receiving signal, resulting in powerful self-interference.

How can we eliminate such interference? Fortunately, because the transmit signal is known, the transmit signal can be used as a reference to Eliminate Self-interference. However, this reference signal can only be obtained from the digital baseband domain. After a digital signal is converted to a simulated signal, it is difficult to get reference from it due to linear and non-linear distortion. Therefore, if any self-interference elimination technology is to succeed, the nonlinear distortion of the transmitted signal must be considered.

In addition, to avoid receiving saturation, the resolution limit of the input mode/number converter must be considered. Therefore, the self-interference signal strength of the input mode/number converter must be smaller than a fixed value.

After these problems are solved, the interference signal can be effectively decomposed and eliminated.

This problem is said to have been solved by the NDD owner.

Mongoin Katti from Stanford University and his team have broken through the above difficulties and established the Kumu Networks startup in 2012. He is currently deploying the technology for small cells.

How did they break through? Kumu provides some simple principles.

In fact, the self-interference elimination technology of Kumu is not the first, but the circuit algorithm of Kumu is currently the most powerful, able to eliminate the interference signal of 110dB.

Thanks to the powerful self-interference elimination technology, the real full-duplex communication is possible, and the wireless spectrum efficiency and latency will be greatly improved. If it can be used perfectly, this is undoubtedly a disruptive innovation.

1) Compared with FDD/TDD, the spectrum efficiency will be doubled.

2) compared with TDD, the latency is greatly reduced.

Because TDD is time-division duplex and cannot send and receive data at the same time, NDD effectively solves this problem. After data is sent, it immediately receives feedback to reduce latency.

In addition, when transmitting data packets, the next data packet is sent without waiting for the full arrival of the data packet, especially when retransmission, which greatly reduces the latency.

This technology can be widely used in microwave return, Wi-Fi access, mobile devices, and LTE access. Currently, the fastest application is to deploy it on the current network as the "Self-Backhaul Small cell.

Self-Backhaul Small cell deployment only requires the installation of boards on the base station side, while other hardware parts on the current network do not need to be changed.

At present, Kumu Networks has received a lot of financing, but there are still some challenges to implement it.

1) circuit board design.

The self-interference elimination circuit is designed to support broadband (> 100 MHZ), multi-MIMO (> 32 antennas), and requires a small size, low power consumption, and low cost.

2) Optimization Design of the physical layer and MAC layer. Such as coding, modulation, synchronization, detection, listening, conflict avoidance, ACK, and so on, especially for the MIMO physical layer optimization.

3) control plane Optimization for dynamic switching between full duplex and half duplex, as well as optimization for the existing frame structure and control signaling.

Author: hr_opt Source: Communication Technology