What are OFDM symbols?
Orthogonal frequency division multiplexing (OFDM) takes a digital information signal with bit rate Rb, maps n-bit words on to M = 2n symbols (each symbol being a complex number representing the amplitude and phase of an M-ary modulation scheme), splits the resulting symbol stream (rate Rs = Rb/n) into N parallel …
What is windowing in OFDM?
A very similar technique is known as Windowed-OFDM (W-OFDM) in which windowing is applied in a manner that the window overlaps with the complete OFDM symbol as well as the CP and the cyclic suffix. Moreover, the window amplitude is −3 dB at the start and end of the OFDM symbol.
Why OFDM is orthogonal?
In OFDM, the subcarrier frequencies are chosen so that the subcarriers are orthogonal to each other, meaning that crosstalk between the sub-channels is eliminated and inter-carrier guard bands are not required.
What does orthogonal mean in OFDM?
In a perfect OFDM signal, Orthogonality prevents interference between overlapping carriers. In FDM systems, any overlap in the spectrums of adjacent signals will result in interference. In OFDM systems, the subcarriers will interfere with each other only if there is a loss of orthogonality.
What are the main strengths of OFDM?
Immunity to selective fading: One of the main advantages of OFDM is that is more resistant to frequency selective fading than single carrier systems because it divides the overall channel into multiple narrowband signals that are affected individually as flat fading sub-channels.
What is OFDM PLC?
The PLC is a special narrow channel of 400 kHz wide that carries signaling and boot-strapping information (e.g. OFDM channel parameters and MAC management messages). This PLC can be easily recognized: it lies in the middle of a specially defined 6 MHz wide range containing 8 pilot subcarriers.
What is meant by cyclic prefix?
In telecommunications, the term cyclic prefix refers to the prefixing of a symbol, with a repetition of the end. It provides a guard interval to eliminate intersymbol interference from the previous symbol.
Why is OFDM better than CDMA?
OFDM CDMA comparison Both OFDM and CDMA have significant benefits. OFDM can combat multipath interference with greater robustness and less complexity. Equalisation can be undertaken on a carrier by carrier basis. OFDMA can achieve higher spectral efficiency with MIMO than CDMA using a RAKE receiver.
What are subcarriers in OFDM?
OFDMA subdivides a channel into smaller frequency allocations, called resource units (RUs). Both OFDM and OFDMA divided a channel into subcarriers through a mathematical function known as an inverse fast Fourier transform (IFFT). As shown in Figure 1, a 20 MHz 802.11n/ac channel consists of 64 subcarriers.
How to calculate the duration of an OFDM signal?
The total symbol duration thus becomes N +N CP N + N C P samples. For obvious reasons, the next OFDM symbol begins with a different amplitude and phase at the boundary of the current symbol, as shown in Figure below for N =64 N = 64 and N CP = 16 N C P = 16.
How is pulse shaping used in OFDM systems?
Nevertheless, many systems still employ pulse shaping in OFDM systems to smooth the edges at the OFDM symbol boundaries that reduces the out of band power to meet the regulatory requirements. Recall that the pulse shape employed in a single-carrier system is a Raised Cosine in frequency domain.
Is the raised cosine pulse shape in time domain?
Therefore, from time frequency duality, we already know that the Raised Cosine pulse shape in an OFDM system must be in time domain. We have already covered the topic of pulse shaping in depth here and here and hence briefly describe this operation in the current context.
How to calculate the number of samples in OFDM?
The total number of samples in this OFDM symbol is equal to N +N CP = N OFDM = 80 N + N C P = N OFDM = 80 as in Figure above and it should remain the same after the windowing operation. Here, we assume a full length cyclic suffix of length N CS = 16 N C S = 16 samples to understand the concept.