digital communications - LTE questions (subcarriers and cyclic prefix)

Assume we have transmission bandwidth of 10 MHz, in this case the total number of subcarriers (FFT size) present is 1024. According to LTE standards, 600 out of 1024 subcarriers are data subcarriers.

data subcarriers= total number of subcarriers- pilot/null subcarriers

Moreover, 0.07 % of 1024 are used as Cyclic Prefix extension

1) Is it correct that we have 1024-600 are therefore unused subcarriers?
(where 600 = total number of Resource Blocks (RB) * number of subcarriers/RB = 50*12)

2) An OFDM symbol is made up of data part and CP part. Is it correct to say that the number of QAM symbols for each OFDM symbol is then 600 (equal to number of data subcarriers) and the number of CP symbols are 72? How is the actual transmission over the subcarriers taking place. For example in downlink, does the transmitter allocate a certain number of subcarriers over which it transmits?

I hope I can get some feedback! Thanks

Answer

1. 
   

   The FFT size in LTE is defined as 2^(nextpow2(Nsubcarriers/2)+1) or the nearest power of two that is also higher than the number of subcarriers for a given bandwidth. For the 10 Mhz case, for example, this means that 424 FFT bins(1024-600) are zero padded apart including the DC bin. The usual practice is to zero pad the central 423+DC bins and use the rest for filling the data so that you get a continous spectrum of 10 MHz around DC. You can also fill the central 600 FFT bins with data and do a fftshift. See table 1 in this document.

   
   


2. 
   

   Yes. Each Resource Element(i.e.1 subcarrier and 1 OFDM system in the Time- Frequency Grid) has one QAM Symbol in shared channel.

   
   

Check out this visualization to get a better clarity about the LTE resource grid. The resource is allocated both in time and frequency domain and in the case of Multi User MIMO(MU-MIMO) transmission mode, spatial resource allocation of resource elements to different user equipment in the downlink also take place. To keep things simple, let's just focus on a single-antennae transmission. The following diagram will give you a better idea of the whole transmission process.