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Cable Modems
Published in Keshab K. Parhi, Takao Nishitani, Digital Signal Processing for Multimedia Systems, 2018
For a single channel the upstream PHYs proposed in MCNS and IEEE802.14 are quite simple when understood in the context of the cable system overview given in Section 7.3. The MAC layer of the modem will instruct the PHY to send a burst of data starting at a certain time. The PHY layer transmits the data on a headend specified channel using either QPSK or 16QAM depending on a predefined PHY profile set up by the headend for the channel. So that different users’ signals do not overlap in time there is a guard band between user transmissions. The guard band is used to let the user ramp up into transmission and ramp down after a transmission as any sudden change in signal energy from a user will cause energy to splatter into other channels. This is shown graphically in Fig. 7.25. Specifically, in the MCNS specification the guard time is specified between 5 and 255 symbols. The size of the guard band is determined by the accuracy of the estimate of packet time obtained by the methods described in Section 7.3. If you can estimate the start of a packet to an accuracy of 3.5 symbols then you have to pad the minimum guard time of 5 symbols by 3.5 symbols on each side to allow for a 3.5 symbol late packet followed by a 3.5 symbol early packet and still achieve 5 symbols in between the last and first symbols. So the guard time in this example would have to be greater than 12 symbols. The guard time of 5 was chosen as enough time for a burst to ramp down and not affect the next or previous burst.
Data Transmission
Published in Goff Hill, The Cable and Telecommunications Professionals' Reference, 2012
Stuart D. Walker, Emilio Hugues-Salas, Rouzbeh Razavi, Tahmina Ajmal
Two main forms of interference are adjacent-channel interference and cochannel interference. In adjacent-channel interference, signals in adjacent frequency bands have components outside their allocated ranges, and these components can interfere with a wanted signal. This can be avoided by introducing guard bands between the allocated frequency ranges. A guard band is a small frequency band used to separate two adjacent frequency bands. Cochannel interference, sometimes also referred to as narrowband interference, is due to other nearby systems (say, AM/FM broadcast) using the same transmission frequency. This can be minimized by dynamic channel allocation techniques.
Communication Techniques
Published in B K Bala, Energy Systems Modeling and Policy Analysis, 2022
Frequency-division multiplexing (FDM) is a form of signal multiplexing that assigns non-overlapping frequency ranges to each signal or “user” of the transmission medium. Each information signal is combined with a different high-frequency carrier signal. During transmission, each channel has access to a narrow range of frequencies all the time. A small frequency gap, known as a guard band, is left between channels to stop the signals from interfering with one another.
Curve Fitting Optimization based Improved Channel Estimation for VANET
Published in International Journal of Electronics, 2022
Devesh Shukla, Arun Prakash, Rajeev Tripathi
The 64 OFDM subcarriers are indexed from −32 to 31, comprising 48 data subcarriers, 4 pilot subcarriers, 11 virtual subcarriers and a dc subcarrier. Pilot symbols already known to receiver are inserted at −21, −7, 7, and 21 subcarrier positions for frequency and timing synchronisation. The guard band is created by keeping subcarriers 32 to −27 and 27 to 31 values to null. The remaining 48 subcarriers are used for data transmission. This entire allocation of subcarriers is composed as one OFDM symbol provided in Figure 2. Finally, the complete frame is composed by appending the signal with preambles and header. Preamble consists of 2 long training symbols and 10 short training symbols and is used for channel estimation. This complete frame is sent over the wireless channel where the signal gets distorted due to the channel behaviour explained later. The received signal is processed in a reverse manner with the support of estimator and equaliser. Estimation is an important aspect for making system adaptive and successful reception of data.
Energy efficient VLSI decoder chip with reduced PAPR in FECG monitoring
Published in International Journal of Electronics, 2020
D Preethi, R S Valarmathi, Harikumar R
As shown in Figure 1, the transmitter kernel of the MIMO OFDM transceiver section consists of a convolution encoder that combines the patterns of redundancy, to improve the signal-to-noise ratio for decoding at the receiver end. The complex values that are generated from interleaving are then mapped in accordance with the modulation type from QPSK mapper for transmit diversity and therefore require two antennas (Burg et al., 2005; Harvey, 2004). The space time diversity encoder is used for encoding the data which are then fed to the inverse fast Fourier transform (IFFT) to initiate the further processes of OFDM modulation. To remove burst error, interleaver is incorporated to improve the recital of error control codes. OFDM is preferred over other methods of modulation techniques as it completely eliminates the multipath effect and also removes the traces of inter symbol interference (ISI). As it is the data stream that is being also transmitted and processed, the information tends to overlap with the neighbouring data by the incurring of noise around the transmitter section (Soo-Woong Lee & Kumar, 2010). Hence, to avoid such interferences, guard band is introduced in between the symbols to avoid inter-carrier interference (ICI) (Huang & Zhu, 2010).
A Design of Bandwidth-Enhanced Conformal Antenna for Aircraft Applications
Published in IETE Journal of Research, 2023
The frequency modulated continuous wave technique uses Doppler shift to measure the velocity component of the target. In this technique, the total available band is split into three regions: transmission band, guard band and reception band. Achieving higher bandwidth provides more flexibility to have a larger bandwidth for each region. A larger bandwidth for transmission and a reception band help in increasing the cross-range resolution [3]. The interference between transmission and reception can be reduced by having a sufficient guard band.