Communicating with a relativistic spacecraft can be problematic. This is because when a spacecraft travels close to the speed of light, communication becomes difficult. This can cause issues for crewed missions, but robotic missions are generally unaffected. 

Here are some examples of how communication can be affected: 

• Lag Communication will initially work, but there will be a lag due to the speed of light. 
• Messages not reaching spacecraft After a while, messages from Earth will no longer be able to reach the spacecraft.

In short, a spacecraft traveling at a significant fraction of the speed of light will experience time dilation, where its internal clocks will advance more slowly than mission clocks on Earth. Another consideration is that communications to and from the mission will be subject to Doppler Shift.

Astronauts use radio waves to communicate with Earth. Radio waves are electromagnetic waves that can travel through the vacuum of space

Astronauts also use radio waves to communicate with each other in space. They have devices in their helmets that convert sound waves from their voices into radio waves. These radio waves can then be transmitted to the ground or other astronauts in space. 

NASA uses the Deep Space Network (DSN) to communicate with spacecraft. The DSN is a collection of large radio antennas that can send and receive information. NASA also uses the DSN to send instructions to spacecraft. 

NASA is developing ways to communicate with infrared lasers. This type of transmission, called optical communications, will allow missions to have higher data rates.

Currently, NASA relies primarily on radio wavesfor communications, but the agency is developing ways to communicate with infrared lasers. This type of transmission — dubbed optical communications — will offer missions higher data rates than ever before

The Doppler effect, also known as Doppler shift, is the change in the frequency or wavelength of waves as observed by someone moving relative to the wave source. The Doppler effect can be observed in wireless communications, passing cars, and moving stars. 

In communications, the Doppler effect causes the received frequency of a source to differ from the sent frequency. This happens when there is motion that increases or decreases the distance between the source and the receiver. The shift in frequency depends on the relative velocity of the transmitter and receiver. 

The Doppler effect was theorized in 1842 by Austrian physicist Christian Doppler.

Space communication is the exchange of data between different points in space. Some examples of space communication include: 

• Satellite communication: Satellites create a channel between the transmitter and receiver at different locations on Earth. Satellites are used for telephone, radio, television, internet, and military applications. 
• Direct wave: Communication that travels directly to the receiving antenna. 
• Ground reflected wave: Radio waves that reflect off the ground before reaching the receiving antenna. 

There are two types of communication systems: 

• Radio frequency (RF): 
• Free space optical (FSO): Also known as laser communications. 

Most spacecraft communications systems are radio frequency based

Radio frequency (RF) is the most common technology used for space communication. NASA uses radio waves for communication, but is also developing ways to communicate with infrared lasers. This type of transmission is called optical communications

Someday, in the not-too-distant future, humans may send robotic probes to explore nearby star systems. These robot explorers will likely take the form of lightsails and wafercraft (a la Breakthrough Starshot) that will rely on directed energy (lasers) to accelerate to relativistic speeds – aka. a fraction of the speed of light. With that kind of velocity, lightsails and wafercraft could make the journey across interstellar space in a matter of decades instead of centuries (or longer!) Given time, these missions could serve as pathfinders for more ambitious exploration programs involving astronauts

Interstellar communication will require the use of interstellar probes. These probes will send back images and spectra of exoplanets, and may receive new requests and commands. 

Space communications rely on two things: a transmitter and a receiver. The transmitter encodes a message onto electromagnetic waves through modulation. These waves flow through space toward the receiver. 

The predominant mode of communication with spacecraft in deep space relies on radio signals transmitted to and from expansive antennas on Earth. The antennas also receive details about where the spacecraft are and how they are doing. 

The enormous distances of space create huge lag times for electronic communications. The signals have to make it from another planet’s surface back to Earth through a gauntlet of space radiation that degrades their clarity

There are a few problems with communication in space: 

• Latency The time it takes for a signal to travel from Earth to space and back. This can be a problem for real-time applications like voice or video calls. 
• Data quality The quality of data can deteriorate as transmissions travel over long distances or through the atmosphere. 
• Radiation Radiation from other missions, the Sun, or other celestial bodies can interfere with the quality of transmissions. 

Space is almost a perfect vacuum, so sound cannot travel. Astronauts communicate with each other in space using radio waves. Space suits are equipped with two-way radio communicators.

According to NASA, communications are limited by the speed of light, which is about 186,000 miles per second. This delay is negligible for spacecraft close to Earth, but it can become a problem for spacecraft farther away. 

Other factors that affect satellite communication include: 

Cosmic noise, Atmospheric noise, Rainfall noise, Ground noise, Interfering noise, Multipath fading(full article source google)

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