# Coding, Modulation & the relationship between frequency and data rate

• Turksen
In summary, the conversation discusses the relationship between frequency and data rate in a communications system. The speaker is confused about the required frequency for a data rate of 10Mbit/s and asks if using on-off modulation with a frequency of 5MHz would suffice. They also inquire about using different modulation schemes or coding to improve data transmission. The responder suggests that the clock signal can be regenerated in certain cases and that signal transmission is a complex topic with various techniques that balance reliability and bandwidth. They encourage the speaker to think about different ways of using signal processing to transmit information rather than focusing on a specific frequency.
Turksen
Hi,

This is a general question rather than an explicit question, so apologies for not using the template.

I'm a bit confused by the relationship between frequency and data rate in a communications system and wondered if someone could help clarify please.

Say I want to achieve a data rate of 10Mbit/s - 10 millions bits transmitted across a link every second - and am using on-off modulation (say +5V = '1' bit and 0V = '0' bit), which by default is NRZ (non return to zero). What is the required frequency: 10 MHz, or could we say that each cycle can transmit 2 bits - so a frequency of 5 MHz would do?

Also, to then improve this could I use a different modulation scheme or form of coding? I've read that QPSK or QAM are more 'spectrally efficient' and can transmit more bits per symbol, but isn't the frequency determined by the BIT rate not SYMBOL rate?

Thanks for any tips!

something to think about: if you use a simple 5mhz or 10mhz "square-type" wave, that is, one where the signal pops up to 5v when there is a 1 and stays at 0v when there is a 0, what does your detector do when you transmit 10,000 successive 0's or 10,000 successive 1's ?

I appreciate, with no transitions, that the clock signal can't be regenerated & things aren't pleasant.

So say we have scrambled data, then what frequency would be sensible? Could you say that since a square wave is composed of many frequencies, we'll just transmit the fundamental (10 * 0.5 I think)? Or fundamental + first harmonic?

Thanks

Turksen said:
I appreciate, with no transitions, that the clock signal can't be regenerated & things aren't pleasant.

Well, yes, but that actually is NOT necessarily a problem. You could, for example, have a space-based transmission system that was slaved to a very precise 10Mhz (to use the example number from your post) frequency. Then the ground based system has only to lock onto where one edge is and phase-shift the signal accordingly and then it can easily detect 10,000 consecutive 1's or 0's because it knows exactly where they are and can then just look in the middle of where they are to see are they 1 or 0.

My point is that signal transmission is a highly complex topic and different coding techniques give different degrees of reliability, often traded off against bandwidth, so I'm not trying to NOT answer your question, I'm trying to get you to THINK about different ways a signal processing waveform might be used to transmit information. That is lots more interesting than just getting an answer of XXXhz.

I can provide some insights into the relationship between frequency and data rate in a communications system. Frequency and data rate are closely related, but they are not the same thing. Frequency refers to the number of cycles per second of a signal, while data rate refers to the number of bits transmitted per second.

In your example, if you want to achieve a data rate of 10Mbit/s, you would need a frequency of 10 MHz. This means that 10 million cycles of your signal would be transmitted every second, with each cycle representing one bit. It is not possible to transmit 2 bits per cycle, as the frequency of the signal is fixed and cannot be changed.

To improve the data rate, you can use different modulation schemes or coding techniques. These techniques allow for more efficient use of the available bandwidth and can increase the number of bits transmitted per symbol. However, the overall data rate is still limited by the frequency of the signal.

For example, QPSK (Quadrature Phase Shift Keying) and QAM (Quadrature Amplitude Modulation) are both more efficient than NRZ, as they can transmit multiple bits per symbol. However, the frequency of the signal remains the same, so the data rate will still be limited by the frequency.

In conclusion, the relationship between frequency and data rate is important to consider in a communications system. While different modulation schemes and coding techniques can improve efficiency, the overall data rate is still limited by the frequency of the signal. It is important to carefully balance these factors in order to achieve the desired data rate and ensure successful communication.

## What is coding and modulation?

Coding and modulation are techniques used in communication systems to convert data into a form that can be easily transmitted and decoded. Coding involves adding extra information to the data to protect it from errors, while modulation involves changing the characteristics of a carrier wave to carry the coded data.

## How does coding affect data transmission?

Coding can help improve the reliability of data transmission by adding redundancy to the data. This means that even if some bits of the data are altered or lost during transmission, the original message can still be recovered from the redundant bits. However, coding also adds extra data that needs to be transmitted, which can decrease the overall data rate.

## What is the relationship between frequency and data rate?

The relationship between frequency and data rate is known as the bandwidth of a communication channel. Generally, the higher the frequency, the higher the data rate that can be achieved. This is because higher frequencies can carry more information in a given amount of time. However, using higher frequencies also means that the signal can be more easily attenuated or disrupted, which can affect the reliability of the transmission.

## How do modulation techniques affect data rate?

Modulation techniques can affect data rate by changing the way data is transmitted and the amount of bandwidth required. For example, more complex modulation schemes can transmit more data in a given amount of time, but they also require more bandwidth. On the other hand, simpler modulation schemes may have lower data rates but can use less bandwidth, making them more suitable for certain applications.

## What are some common coding and modulation techniques used in communication systems?

Some common coding techniques include error-correcting codes like Reed-Solomon and convolutional codes. Modulation techniques include amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). More advanced techniques like quadrature amplitude modulation (QAM) and orthogonal frequency-division multiplexing (OFDM) are also commonly used in modern communication systems.

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