COFDM video wireless communication system adopts TDD-COFDM technology to achieve long range video and digital serial data transmitting. But how does the system work during application? What is the COFDM technology working principle for data sending? And what other application of COFDM technology in our daily life? The following article will make these points clear to you.
In our last blog What are the differences between COFDM and OFDM? We make it clear that COFDM is a encoded OFDM technology: Adding channel coding( error correction and interleaving) before OFDM modulation to improve the communication system reliability.
Typical wireless digital communication system
The above picture is a schematic diagram of a typical wireless digital communication system. Now I will focus on the "channel" because too many books simply describe the "channel" as "the transmission channel of the signal, including the wireless and wired channel. " In fact, I the channel is the basis of communication system design.
For example, if we are designing a 1km UAV video link and a 100km fixed-wing drone video transmitter, we first need to know that the wireless channel will have different effects on signal and this effects can be divided into two types large size and small size.
The large size effect is the signal fading caused by path loss, diffraction, shadows, vegetation, and so on. The small size effect is signal's dramatic changing due to the wireless signal multipath propagation and the transmitter or receiver's moving.
Therefore, some people think that "1Watt COFDM wireless transmission system can reach 10km, then 10Watts can reach 100km." They believe that increasing the transmission power can ensure the communication effect.
As we all know that scientists and engineers around the world have built different models based on suburban, sea-based, land-based and other communication scenarios.
For different range, the channel models for wireless signal transmission, length of error correction codes, and setting of guard intervals are different. Therefore, it is unacceptable and unprofessional to increase the transmission distance by simply increasing the RF power.
The first large-scale application of COFDM is the European digital video broadcasting-Terrestrial DVB-T (or digital TV for short), and its structure is as follows:
DVBT Tx Scheme
Broadcast transmission. The TV signal only needs to be transmitted downlink, and the receiver only needs to receive the signal. A program must meet the needs of multiple users at the same time.
Wide coverage. A signal tower can cover a large area to reduce the number of towers.
Signal transmission environment is complex. Signals may be blocked by buildings or trees during transmission, and multipath effects are complex.
Support mobile reception. Meeting the needs of car TV on public transportation.
Support HD programming. The channel needs to meet the broadband signal transmission requirements.
After the widespread use of DVB-T, new digital TV standards DVB-H, DVB-S and DVB-T2 have been introduced quickly. All of these standards adopt COFDM technology. In addition, Japan also launched its own digital TV standard ISDB-T, which is also based on COFDM technology.
Besides the above application of DVB-T, COFDM technology is also suitable for long range HD video transmitter for fixed wing drones and Multi rotors with the following features:
High Definition and High Bandwidth
The moving speed of the drone relative to the ground receiving end is generally within 120km/h, which is similar to the application scenario of the car TV.
Broadcasting type transmission, the increase of receiving devices on the ground do not lead to an increase in channel overhead and a smaller bandwidth.
The signal transmission environment of the drone is complicated, and the multipath effect of the long range hdmi fpv transmitter is obvious.
Although 802.11g/n also uses OFDM technology, however the channel characteristics of WIFI and fpv digital video transmitter are quite different.
The WLAN supports a short communication distance, but its channel conditions are good. Even after wall reflection, the channel delay is also short and the frequency selective fading is not obvious.
In the AP Station mode, adding a station will occupy bandwidth and slow down the network speed. In the application of the drone latency will be longer.
The frequency band of WIFI is getting more and more crowded, and there are many uncontrollable factors for long-distance communication.
Since the ground terminal needs to control the drone, it is necessary to add an upward data transmission link. The uploaded data is generally small, so it can be implemented by FH/DSSS. Therefore, the better UAV wireless link solution is as above.