Why 100 PRB in LTE 20 MHz?

4G LTE
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1

Hi All,

According to 3GPP specifications LTE channel bandwidth can be 1.4, 3, 5, 10, 15, 20 MHz.

All available spectrum is divided into Resource Blocks (RB).

For example, for Channel Bandwidth = 20 MHz we have 100 Resources Blocks.

My question is: where does this 100 PRB comes from (what is the calculation)?

Because we have:

1 PRB (in frequency domain) = 12 subcarriers
1 subcarrier = 15 KHz
12 subcarriers = 180 KHz
Bandwidth = 20 MHz = 20,000 KHz
20,000 / 180 = 111.111

Anyone can shed a light?

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2

10% is a guard band, it is not 20MHz, actually it is 18MHz

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3

Those 100PRBs are used for PDSCH channels only. You forgot to include the other channels and guard bands

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4

Because of the Guard Band (narrow frequency range that separates two ranges of wider frequency, in order to avoid interference which would result in decreased quality for transmission).

In LTE Guard Band is 10% of available bandwidth.

So, in the 20 MHz: 10% = 2.000 KHz.

And remaining is 18.000 KHz.

So: 18.000 / 180 = 100

Note: it do not apply for 1.4 MHz

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5

Relation between Bandwidth and Resource Block:

Bandwidth directly affects the throughput. Different BWs have different number of RBs.Here is the calculation how to find out the numbers of subcarriers and Resource Blocks.10% of total bandwidth is assumed to be used for guard band. Though 10 % guard band assumption is not valid for 1.4 MHz bandwidth. Let’s take an example of 20MHz.10% of 20 MHz is 2 MHz, used as guard band, thus effective bandwidth will be 18MHz. Number of subcarriers = 18 MHz/15KHz = 1200. Number of Resource Blocks =18 MHz/180KHz = 100. Same calculations can be done with other bandwidths to calculate the number of subcarriers and Resource Blocks.

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6

Why there are 2 symbols of pdcch for 1.4 MHz in LTE and not for others BW?

With CFI = 1

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As it’s not wider bandwidth so ue required 2 symbol to decode pdcch to get ul/dl grants
But I remember 3 can be tried as well in new release,

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Kindly explain how they are used in PDSCH. Thanks
5Mhz=20PRBs
10MHz=50PRBs
15MHz=75PRBs
20MHz=100 PRBs

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Hi Vinod,
Per 3GPP, there are conditions like if no. RBs>10 than CFI = CFI else if no. of RBs<10 than CFI=CFI+1…
So for 1.4 Mhz, there’s only 6 RB that’s we used CFI +1 else for other B.W 3MHz to 20MHz CFI is used same.
2nd
Min. allocation for 1 UE is about to 1CCE (9REG’s) that’s equal to 36 RE’s but for CFI=1 ( like 6 RBs) case can’t meet this req. so need to give CFP+1 so meet the at least minimum critera.

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Thanks for sharing, but it also to be consider as guard band 20% to 10 %
which is 100 PRB for 20Mhz.