# Statistically valide BER measurements

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#### eagle65

##### Member
I am trying to understand this particular concept and wondering if anyone can shed some more light if possible. What I am ultimately looking for are BER measurements that I plan to take where I can state that the values collected are within a certain statistical confidence level?

The article below (link provided) provides some explanation on BER and how to use an equation to determine the number of bits required to be transmitted in order to tie the BER measurement to a confidence level.

1) Why do we have to first run into an error before we can determine the number of bits required to be tested tied to a certain confidence level?

2) Why do they have to reduce the signal to noise ratio in order to reduce the amount of bits required which translates to less amount of time required to conduct a test?

I am interested in measuring a system that has no more than 9600 bps such as a P25 system. A lot of the articles I have found are focused more on the 10^9 bps and so on.

http://www.lightwaveonline.com/arti...ing-those-ber-testing-mysteries-53908512.html

#### slicerwizard

##### Member
1) Why do we have to first run into an error before we can determine the number of bits required to be tested tied to a certain confidence level?
Hm, I don't think that's what it says. It's more like how many error-free bits do you have to receive to reach a certain confidence level.

2) Why do they have to reduce the signal to noise ratio in order to reduce the amount of bits required which translates to less amount of time required to conduct a test?
Because you don't always have the luxury of tying up a channel for hours doing BER testing.

#### eagle65

##### Member

1) I guess what I am not understanding is the one and two bit errors as in the paragraph below.

Figure 2 illustrates the relationship between the number of bits that must be transmitted (normalized to the BER) versus CL for zero, one, and two bit errors. Results for commonly used CLs of 90%, 95%, and 99% are tabulated in Table. To use the graph in Figure 2, select the desired CL and draw a vertical line up from that point on the horizontal axis until it intersects the curve for the number of errors detected during the test.

2) I should have asked how and not why necessarily. How does reducing the SNR reduce the amount of bits that need to be transmitted?

#### jonwienke

Every statistical measurement becomes more accurate as the population of data values becomes larger.

The standard deviation of the data gives one an indication of how large the population must be to infer a particular degree of accuracy to a measurement or a prediction based on your population.

The trick is to collect enough data to be meaningful, without collecting so much you waste time and resources doing the collection.

#### eagle65

##### Member
Thank you, I understand the goal but I am having a hard time understanding the process of how you go about testing enough bits to state that the BER that you collected is within a certain confidence level.

So if I want to test a 9600 bps system, how many bits do I need to collect to say that the BER that I measured is within say a 95% confidence level?

#### RFI-EMI-GUY

##### Member
Thank you, I understand the goal but I am having a hard time understanding the process of how you go about testing enough bits to state that the BER that you collected is within a certain confidence level.

So if I want to test a 9600 bps system, how many bits do I need to collect to say that the BER that I measured is within say a 95% confidence level?
If this is for a P25 Land Mobile system, I would suggest TIA TSB-88D for some guidance in measuring proof of performance.

Sent from my SM-T350 using Tapatalk

#### RFI-EMI-GUY

##### Member

1) I guess what I am not understanding is the one and two bit errors as in the paragraph below.

Figure 2 illustrates the relationship between the number of bits that must be transmitted (normalized to the BER) versus CL for zero, one, and two bit errors. Results for commonly used CLs of 90%, 95%, and 99% are tabulated in Table. To use the graph in Figure 2, select the desired CL and draw a vertical line up from that point on the horizontal axis until it intersects the curve for the number of errors detected during the test.

2) I should have asked how and not why necessarily. How does reducing the SNR reduce the amount of bits that need to be transmitted?
2) for a fixed microwave link, there is a relationship between received signal level and fade margin. The test is to simulate the fade margin threshold and resulting BER which is published for the radio.

Sent from my SM-T350 using Tapatalk

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