China
Africa
Explore chapters and articles related to this topic
D/A and A/D Converters
Published in Jerry C. Whitaker, Microelectronics, 2018
PCM is a technique where an analog signal is sampled, quantized, and then encoded as a digital word. The PCM IC can include successive approximation techniques or other techniques to accomplish the PCM encoding. In addition, the PCM codec may employ nonlinear data compression techniques, such as companding, if it is necessary to minimize the number of bits in the output digital code. Companding is a logarithmic technique used to compress a code to fewer bits before transmission. The inverse logarithmic function is then used to expand the code to its original number of bits before converting it to the analog signal. Companding is typically used in telecommunications transmission systems to minimize data transmission rates without degrading the resolution of low-amplitude signals. Two standardized companding techniques are used extensively: A-law and μ-law. The A-law companding is used in Europe, whereas the μ-law is used predominantly in the United States and Japan. Linear PCM conversion is used in high-fidelity audio systems to preserve the integrity of the audio signal throughout the entire analog range.
Noise
Published in Geoff Lewis, Communications Technology Handbook, 2013
Companding. Companding is the compound term used to describe the compression and expansion of the dynamic range of a signal amplitude. This is achieved by using nonlinear amplitude-dependent amplifiers with characteristics similar to those shown in Fig. 24.4(a), before and after transmission. The compressing amplifier in this example has a slope of 0.5. The receiver expanding amplifier therefore must have a slope of 2, to give overall unity response. Figure 24.4(b) shows the overall response of the system and can be used to explain the noise reduction properties of the concept. Signals above the level OdBm (1 mW) are assumed to be either non-existent or unaffected by companding. During the compression stage, the dynamic range is halved by lifting the amplitudes of the lower level components. Assume that there is −30 dBm (1 μW) of noise present in the transmission channel. This would have swamped the original low level signals, but now is only just comparable with them. The expansion process at the receiver effectively depresses the low level signal components and the noise to restore the dynamic range to its original value, at the same time improving the signal to noise ratio.
D/A and A/D Converters
Published in Jerry D. Gibson, The Communications Handbook, 2018
logarithmic function is then used to expand the code to its original number of bits before converting it to the analog signal. Companding is typically used in telecommunications transmission systems to minimize data transmission rates without degrading the resolution of low-amplitude signals. Two standardized companding techniques are used extensively: A-law and m-law. The A-law companding is used in Europe, whereas the m-law is used predominantly in the United States and Japan. Linear PCM conversion is used in high-fidelity audio systems to preserve the integrity of the audio signal throughout the entire analog range.
On the selection of the best companding technique for PAPR reduction in OFDM systems
Published in Journal of Information and Telecommunication, 2019
Mohamed Mounir, Mohamed Bakry El_Mashade
Recent wireless communication applications require excessive high data rate, which make it more subspecies to frequency selectivity. Orthogonal frequency division multiplexing (OFDM) has been widely used in high-speed wireless communications, due to its capability to combat frequency selectivity. However, a serious disadvantage of OFDM is its high peak to average power ratio (PAPR), which significantly decreases the efficiency of power amplifier (PA) in order to meet the linearity requirements (in terms of bit error rate (BER) and out-of-band (OOB) radiation) of wireless communication standards. To enhance the efficiency of PA without degrading BER performance or increase OOB radiation, many PAPR reduction techniques are introduced in the literature (Zahra, Tarrad, & Mounir, 2014). Among them companding transforms have the lowest computational complexity regardless of the number of subcarriers which make companding transforms attractive techniques. Companding transforms are typically applied to speech signals to optimize the required number of bits per sample. Since OFDM and speech signals behave similarly in the sense that high peaks occur infrequently, the same companding transforms can also be used to reduce the OFDM signal's PAPR (Rahmatallah & Mohan, 2013). However, the major drawback of companding techniques is that they achieve PAPR reduction at the cost of increasing the BER. This degradation in BER performance is due to two factors: First, companding distorts the modulating data symbols at the transmitter from their original constellation; second, the channel noise is expanded at the receiver by the decompanding process (Rahmatallah, Bouaynaya, & Mohan, 2011a, December, 2011b, April). For any companding technique, there is the PAPR reduction gain value at which the BER reaches its minimum. This is called efficient PAPR reduction gain. Efficient PAPR reduction gain is not necessarily the lowest possible value of PAPR. Finding efficient PAPR reduction gain would mean to find optimal companding transform parameters. Thus, performance of specific transform depends heavily on its parameters (Huang, Lu, Chuang, & Zheng, 2001).