Introduction

As the mobile communications is embracing the LTE era, a larger backhaul bandwidth is required. The throughput requirement of a single eNodeB is estimated to reach 1 Gbit/s.

 

As a major method for backhaul transmission of base station services, microwave links are expected to deliver larger air interface bandwidth with higher efficiency.

One method to achieving this is improving spectral efficiency of existing spectrum resources. Another option is providing a wider range of spectrum resources.

To meet the mushrooming needs, Huawei provides the optimum solution in maximizing microwave air interface bandwidth in the ultra broadband era.

High-Order QAM—Maximizing Spectral Efficiency

Quadrature amplitude modulation (QAM) is a widely used modulation scheme in modern communication systems. In a QAM signal, each symbol represents an N-bit binary numeral capable of representing M states (M = 2^N). QAM with M states is called M-QAM. The most common forms are 4-QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM, and 256-QAM.

NOTE:

In a binary baseband signal that is not modulated, each symbol represents a binary numeral, which can be 0 or 1.

Compared with 4-QAM, QPSK only needs to use a single carrier wave to realize the same modulation effect. Therefore, in practice most equipment adopts QSPK instead of 4-QAM.

QAM conveys two quadrature signals by modulating the amplitudes of two carrier waves, which can be illustrated in a constellation diagram. In a constellation diagram, each constellation point represents a symbol. As the following figure shows, the more points in a constellation diagram, the more information (bits) that can be transmitted per symbol.

 

During the signal transmission, these points are susceptible to noise and may deviate from their original positions. If constellation points are to be added with the condition that the mean energy of the constellation remains the same, the points must be closer. This will result in a higher bit error rate requiring a higher signal-to-noise ratio (SNR) in order to make correct demodulation.

The following examples use a progression in vehicle load capacity to show how a higher-order QAM can carry more bits (or, cargo in the truck example).

At a fixed symbol transmission rate, the higher-order QAM is used to ensure more bits can be transmitted. The transmission capacity of the 2^N QAM scheme is N/(N-1) times of the 2^N-1 QAM scheme.

 

The greater the load, the better the road needs to be to ensure safe transport. As for QAM, a stable, interference-free transmission channel and an advanced demodulation technology are required.

 NOTE: Transmission channel quality is affected by many factors, such as multipath fading. Therefore, high-order QAM is often used together with adaptive modulation (AM).

Huawei RTN 300 and RTN 900 not only support 1024-QAM and 2048-QAM, but also take the lead in supporting 4096-QAM. The spectral efficiency of 4096-QAM is up to 9.6 bps/Hz, which is 45% higher than the transmission capacity of 256-QAM.

With 4096-QAM, RTN products can obtain a capacity over 500 Mbit/s in a single 56 MHz channel.

 

 

 

===Link archives (updated 2015-05-26)===

>>>Click here for the latest links

 

Amazing Microwaves	Demystifying MSTP	WDM Optical Transport
Huawei RTN: Birth and Evolution	Prelude●Overview and History	Speaking of 100G
How to Maximize BW of Air Interface (1)	MTU, MFL and Jumbo Frames
How to Maximize BW of Air Interface (2)	Key Technologies of EoS: GFP, VCAT and LCAS
How to Maximize BW of Air Interface (3)	PCM Ports, Boards, Services, and Application Scenarios

very informative

really good post.

Hello Zakiul, your feedback can encourage our team to provide more content :)

When you want to buy Huawei microwave equipment don't forget to buy the license of highest QAM, as the fastest solution for upgrading any microwave link is by increasing the QAM, it just needs 15 seconds
And don't forget to activate AM feature also to avoid outage in bad weather conditions.