CP-OFDM and its application in 5G

in Blog, 5G, 5G NR, CP-OFDM, OFDM

Now days, 5G new radio (NR) is becoming more popular because of its  high data rate, reduced latency, energy saving, cost reduction, higher system capacity, and massive device connectivity. The first phase of 5G specifications in Release-15 will be completed by March 2019 to accommodate the early commercial deployment.

New Radio (NR) and Long Term Evolution (LTE) are integral parts of 5G radio access, as shown in . LTE is expected to operate below 6 GHz frequencies, while NR is envisioned to operate from sub-1 GHz up to 100 GHz.Tight integration will enable aggregation of NR and LTE traffic.

(Source of figure: Reference [2]) 

5G NR has  unique features as compared to LTE, such as scalability  OFDM and the diverse use cases (human-centric and machine-centric) beyond mobile broadband services. The said scalability of  5g NR cab be achieved by radio waveforms and multiple access techniques.  The CP-OFDM and DFT-Spread (DFT-S) OFDM, are the right choices for 5G enhanced mobile broadband (eMBB). LTE uses Cyclic-Prefix Orthogonal Frequency Division Multiplexing (CP-OFDM) waveform for downlink transmissions and DFT-Spread OFDM for uplink transmissions.

On the road to 5G, the mobile communications community has witnessed abundant waveform proposals for NR. Most proposals, being multi-carrier waveforms, are either variations on CP-OFDM. The trend has been to tweak OFDM in any possible way – sub-carrier wise filtering or pulse shaping, filtering of groups of sub-carriers, allowing successive symbols to overlap in time, dropping cyclic-prefix, replacing cyclic-prefix with nulls or with another sequence. Various waveforms became strong contenders for 5G – numerous research publications showed CP-OFDM being outperformed. At one point, we felt that every multicarrier waveform was going to be part of 5G, except CP-OFDM.

In CP OFDM, in order to avoid the inter-symbol interference (ISI) or inter-band interference in OFDM last part of data of OFDM frame is appended in the starting of the OFDM frame and length of cyclic prefix should be greater than channel delay spread.  Moreover, the channel delay spread is frequency dependent and   CP length must be long enough to account for both interferences and CP length can be adaptive. 

In order to generate a CP OFDM we can write following program


nFFTSize = 64;

% for each symbol bits a1 to a52 are assigned to subcarrier 

% index [-26 to -1 1 to 26] 

subcarrierIndex = [-26:-1 1:26];

nBit = 2500; 

ip = rand(1,nBit) > 0.5; % generating 1's and 0's

% No bits PerSymbol

nDSC = 52;


%Symbols can be generated in following way 


nSym = ceil(nBit/nDSC);


% for generation of BPSK modulation

% bit0 is replaced by -1

% bit1is replaced by  1

ipMod = 2*ip - 1; 

ipMod = [ipMod zeros(1,nBitPerSymbol*nSymbol-nBit)];

ipMod = reshape(ipMod,nSymbol,nBitPerSymbol);


st = []; % empty vector


for ii = 1:nSym


inputiFFT = zeros(1,nFFTSize);


% assigning bits a1 to a52 to subcarriers [-26 to -1, 1 to 26]

inputiFFT(subcarrierIndex+nFFTSize/2+1) = ipMod(ii,:);


%  shift subcarriers at indices [-26 to -1] to fft input indices [38 to 63]

inputiFFT = fftshift(inputiFFT); 


  % Taking FFT, the term (nFFT/sqrt(nDSC)) is for normalizing the power of transmit symbol to 1 


outputiFFT =(nFFT/sqrt(nDSC))* ifft(inputiFFT,nFFTSize);


% adding cyclic prefix of  samples 

outputiFFT_with_CP = [outputiFFT(49:64) outputiFFT];


st = [st outputiFFT_with_CP]; 




close all

fsMHz = 20;

[Pcp,W] = pwelch(st,[],[],4096,20);    


xlabel('frequency, MHz')

ylabel('power spectral density')

title('Transmit spectrum of CP OFDM ');



[1] https://www.ericsson.com/en/blog/2017/5/in-the-race-to-5g-cp-ofdm-triumphs


[2] IEEE Communications Magazine article, Waveform and Numerology to Support 5G Services and Requirements, by A. Zaidi, R. Baldemair, H. Tullberg, H. Bjorkegren, L. Sundstrom, J. Medbo, C. Kilinc, I. Silva.


[3]  3GPP TS 36.101. “User Equipment (UE) Radio Transmission and Reception.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA). URL: http://www.3gpp.org.


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