Structure of an OFDM Packet

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Structure of an OFDM Packet

📘 What does an OFDM signal look like?
📘 What are the roles of cyclic prefixes in an OFDM packet?
📘 The process of sending an OFDM packet practically
📘 MATLAB Codes
📚 Further Reading

What does an OFDM signal look like?

An OFDM signal contains a header (that is called a cyclic prefix). That message bits are placed. Then again, a cyclic prefix and other message bits are located. The length of the cyclic prefix is a fraction of the length of the message bits. The structure is called a packet containing cyclic prefixes and message bits.

Let's assume I need to transmit 16 bits as an OFDM symbol. If the length of the cyclic prefix is 4, then the original OFDM symbol, for example, is 1001100101010110. The OFDM symbol with the cyclic prefix will be 01101001100101010110, where the last 4 bits of the OFDM symbol are copied to the front of the original symbol.

Fig: OFDM Modulation and Demodulation

What are the roles of cyclic prefixes in an OFDM packet?

It mitigates the inter-symbol interference (ISI)
It works as training bits
It helps in equalization

The process of sending an OFDM packet practically

Firstly, we apply inverse fast Fourier transform (IFFT) on the OFDM packet's bits to define it in the time domain. Oppositely, on the receiver side, we apply a fast Fourier transform (FFT) to recover the original OFDM packet.

For example, if you are transmitting 100 OFDM symbols, each symbol contains 64 bits, and the number of subcarriers is 64, then there will be 64 subchannels in parallel.

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GMSK BER vs SNR (with Simulation)

GMSK Performance: Why the BER vs. SNR Curve Matters If you’ve ever wondered why GMSK (Gaussian Minimum Shift Keying) became the backbone of the global GSM revolution, the answer lies in its unique balance of spectral efficiency and power performance. But there is no free lunch in engineering. To get that sleek, narrow spectrum, you have to pay a price in Bit Error Rate (BER) . In this guide, we break down the theoretical BER vs. SNR performance of GMSK, the "magic" of the BT product, and how to optimize your link budget for real-world conditions. What is GMSK and Why Should You Care? GMSK is a derivative of MSK (Minimum Shift Keying) that uses a Gaussian pre-modulation filter. This filter "smooths" the phase transitions, significantly reducing sideband power. The result? A constant envelope signal that allows you to use high-efficiency Class-C amplifiers without distorting the signal. This is the secr...

Parametric Down-Conversion of Photons

Nonlinear Optics One incoming photon is converted into two lower-energy photons, typically in a nonlinear optical medium. Energy is conserved: E in = E 1 + E 2 E in = E 1 + E 2 . Parametric Down-Conversion A nonlinear optical process in which one pump photon is converted into two lower-energy photons, known as the signal and idler photons. 1 Energy Conservation The energy of the pump photon equals the sum of the energies of the signal and idler photons. \[ \boxed{\hbar\omega_p=\hbar\omega_s+\hbar\omega_i} \] \[ \boxed{\omega_p=\omega_s+\omega_i} \] ωₚ Pump angular frequency ...

The cutoff frequency (in GHz) for the dominant TE10 mode of an air-filled rectangular waveguide of ...

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The electric field of a monochromatic plane wave travelling in a lossless isotropic and homogenous medium is given by E(z, t) = ...

Q. The electric field of a monochromatic plane wave travelling in a lossless isotropic and homogenous medium is given by E(z, t) = E₀ [x̂ cos(ωt − kz) + ŷ sin(ωt − kz)] in a right-handed orthogonal co-ordinate system. Which of the following is the correct polarization of the electromagnetic wave? (A) Right-handed circularly polarized Correct answer under the IEEE convention. (B) Left-handed circularly polarized (C) Linearly polarized (D) Linearly polarized at −45° to x̂ Solution 1. Identify the electric-field components Let ψ = ωt − kz. The components along the x and y directions are: Eₓ = E₀ cos ψ Eᵧ = E₀ sin ψ Both components have equal amplitudes, the same frequency, and a phase difference of 90°. These are the conditions for circular polarization. ...

A wireless digital transmission scheme is using 16-QAM over an additive white Gaussian noise channel ...

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An S-band radar transmitting at 3 GHz radiates 200 kW of power. Determine the signal power density at a range of 100 ...

Q. An S-band radar transmitting at 3 GHz radiates 200 kW of power. Determine the signal power density at a range of 100 nautical miles if the effective area of the radar antenna is 9 m². Solution Given, Transmitted power: P t = 200 kW = 200,000 W Frequency: f = 3 GHz = 3 × 10⁹ Hz Range: R = 100 nautical miles Effective antenna area: A e = 9 m² Speed of light: c = 3 × 10⁸ m/s Step 1: Convert Nautical Miles to Meters One nautical mile equals 1852 meters. R = 100 × 1852 = 185200 m Step 2: Calculate the Wavelength The wavelength is calculated using the relationship between the speed of light and frequency: λ = c / f λ = (3 × 10⁸) / (3 × 10⁹) λ = 0.1 m Step 3: Calculate the Antenna Gain The relationship between effective aperture and antenna gain is: G = (4πAₑ) / λ² ...

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