Wireless Physical Layer Basics

This page provides an introduction to the basics of the wireless physical layer and links to various wireless physical layer standards. These standards include WLAN (802.11a, 802.11b, 802.11n), WiMAX (Fixed OFDM - 802.16d, Mobile OFDMA - 802.16e), GSM, GPRS, CDMA, TD-SCDMA, LTE, WirelessHART, and more.

According to the OSI model, the physical layer refers to any medium that carries information, whether in analog or digital form. This medium can be a copper wire, a twisted shielded cable, or even air. The wireless physical layer, specifically, is the layer that protects data to ensure it reliably reaches the receiver from the transmitter, even in a noisy channel environment.

The channel introduces various impairments, including fading, Additive White Gaussian Noise (AWGN), phase noise, frequency offset, and others.

Therefore, the role of the wireless physical layer is to incorporate features that facilitate the retrieval of data from the corrupted received packet or frame. The most common functions of the wireless physical layer are outlined below, along with a block diagram.

• It provides modulation and demodulation functionalities at the transmit and receive ends of the wireless communication chain.
• It provides Forward Error Correction (FEC) functionality at the receiver by incorporating a Convolutional encoder at the transmitter and a Viterbi decoder at the receiver. Other FEC techniques, such as Reed-Solomon coding, Turbo coding, and Low-Density Parity-Check (LDPC) codes, are also employed based on the channel conditions and Bit Error Rate (BER) requirements of the system.
• It incorporates an interleaver at the transmit end and a deinterleaver at the receive end to help recover data in case of selective fading conditions.
• It incorporates a scrambler at the transmit end and a descrambler at the receive end to help distribute energy across a larger bandwidth and avoid adjacent channel interference.

The figure above describes the basic wireless physical layer modules and their typical order.

Initially, data is fed to the randomizer, which randomizes the binary data and removes long streams of zeros or ones. This helps spread the power across a larger bandwidth, as mentioned earlier, and also aids in clock synchronization at the receiver.

The next module in the chain helps in error correction at the receiver by incorporating redundancy at the transmit side. This enhances the bit error rate of the system at the same Signal-to-Noise Ratio (SNR) compared to systems without FEC.

The third most important block, which ensures transmitted data reaches its destination in the presence of small-scale and large-scale fading environments, is the interleaver. It permutes the contiguous subcarriers, so that contiguous data from the FEC encoder will be mapped onto distributed subcarriers. The permutation equation is known to the deinterleaver module in the receiver, which helps to depermute the received sequence.

The interleaved data is fed as input to the data mapping or data modulation. There are various types of modulation, with digital modulation being the most common in next-generation wireless standards. The modulation techniques employed typically are Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), and Quadrature Amplitude Modulation (QAM).

After modulation, the baseband data is processed through a Digital-to-Analog Converter (DAC) and passed to the RF stage. In the RF part, up-conversion to the transmit frequency is performed, and the signal is amplified before being coupled with the antenna. Similarly, in the receiver, data is passed through a preamplifier (mostly a Low Noise Amplifier - LNA) and down-converted before being passed to an Analog-to-Digital Converter (ADC). The output of the ADC is then processed by the reverse modules of those used at the transmit end.