Wireless/SDR Archives | Page 13 of 22

A quasi-static assumption implies that the channel stays the same for each block but varies from one block to the next

December 30, 2017 Wireless/SDR

A Time-Varying Wireless Channel

Today we will discuss three strategies that are usually adopted for handling a wireless channel that is varying with time and hence acting differently on different data symbols. For a channel impulse response $c(t)$, number of multipath $N_{MP}$, channel gains $\gamma_i(t)$ and delays $\tau_i(t)$ for the $i$-th path, respectively, we can write \begin{equation*} c_B(t) = \sum _{i=0}^{N_{MP} -1} \gamma_i(t) \cdot \delta(t-\tau_i(t)) \end{equation*} i.e., channel gains $\gamma_i(t)$ and channel delays $\tau_i(t)$ are varying with time albeit at different rates. With the movement in the channel, the taps in a frequency selective channel are changing according to the rotation rates of path

Diversity implies two or more independent replicas of the same information

November 2, 2021 Wireless/SDR

Multiple Antenna Techniques

When computing approaches the physical limits of clocking speeds, we turn towards multi-core architectures. When communication approaches the physical limits of transmission speeds, we turn towards multi-antenna systems. What exactly are the benefits that led to scientists and engineers choosing multiple antennas as the foundation of 4G and 5G PHY layers? While having spatial diversity was the original incentive for adding antennas at the base stations, it was discovered in mid 1990s that multiple antennas at Tx and/or Rx sides open up other possibilities not foreseen in single antenna systems. Let us now describe three main techniques in this context.

Average trajectory for squared eye diagrams for a binary PAM sequence of 400 symbols shaped with Raised Cosine pulse with excess bandwidths 0, 0.5 and 1

December 31, 2021 Wireless/SDR

Lock Detectors for Symbol Timing Synchronization

Similar to the carrier lock detectors, timing lock detectors can also be constructed based on some property of the modulated signal. These lock detectors operate in parallel to the timing locked loop and aid the Rx state machine in executing necessary tasks according to each scenario. The expressions for two such timing lock detectors are as follows. The output of a timing lock detector should be at its peak for the correct timing. Therefore, when the matched filter output, denoted by $z(mT_M)$ with $T_M$ being the symbol time, is at its peak, the second sample in a signal oversampled by

Bandwidth, power and DSP correspond to the traditional trio of raw materials, energy and knowledge

October 21, 2020 DSP / Wireless/SDR

Why Building an SDR Requires DSP Expertise

In an introduction to signals, we discussed the idea that the any activities around us, starting from subatomic particles to massive societal networks, are generating signals all the time. Since mathematics is the language of the universe and digital signals are nothing but quantized number sequences, it is fair to say that the workings of the universe can be mapped to an infinitely large set of signals. With these number sequences in hand, an electronic computer can process the signals and either extract the information about the surrounding real world phenomena or even better influence its target environment. We saw

Square-root Nyquist filters for three different excess bandwidths

September 16, 2017 DSP / Wireless/SDR

How to Design Nyquist and Square-Root Nyquist Pulse Shaping Filters

The radio spectrum is a very precious resource like real estate and must be utilized judiciously. Pulse shaping filters control the spectral leakage of the transmitted signal in a wireless channel due to the strict restrictions to comply with a spectral mask. This is even more important for the upcoming 5G wireless systems which are based on a variety of wireless transmission protocols (such as mobile networks, Internet of Things (IoT) and machine to machine communications) combined in one comprehensive standard. Even for wired channels, there is always a natural bandwidth of the medium (copper wire, coaxial cable, optical fiber)