- About FLLs with band-edge filters
Using band-edge filters for carrier frequency recovery with an FLL is an interesting technique that has been studied by fred harris and others. Usually this technique is presented for root-raised cosine waveforms, and in this post I will limit myself to this case. The intuitive idea of a band-edge FLL is to use two filters to measure the power in the band edges of the signal (the portion of the spectrum where the RRC frequency response rolls off). If there is zero frequency error, the powers will be equal. If there is some frequency error, the signal will have more “mass” in one of the two filters, so the power difference can be used as an error discriminant to drive an FLL.
The band-edge FLL is presented briefly in Section 13.4.2 of fred harris’ Multirate Signal Processing for Communication Systems book. Additionally, fred also gave a talk at GRCon 2017 that was mainly focused on how band-edge filters can also be used for symbol timing recovery, but the talk also goes through the basics of using them for carrier frequency recovery. Some papers that are referenced in this talk are fred harris, Elettra Venosa, Xiaofei Chen, Chris Dick, Band Edge Filters Perform Non Data-Aided Carrier and Timing Synchronization of Software Defined Radio QAM Receivers and fred harris, Band Edge Filters: Characteristics and Performance in Carrier and Symbol Synchronization.
Recently I was looking into band-edge FLLs and noticed some problems with the implementation of the FLL Band-Edge block in GNU Radio. In this post I go through a self-contained analysis of some of the relevant math. The post is in part intended as background information for a pull request to get these problems fixed, but it can also be useful as a guideline for implementing a band-edge FLL outside of GNU Radio.
- Z-Sat VHF transmissions
Z-Sat is a microsatellite by Mitsubishi Heavy Industries that was launched in 2021. It is a demonstrator for multi-wavelength infrared Earth observation technologies. It carries an amateur radio payload that was coordinated by IARU and which consists of a BBS (bulletin board system) with a 145.875 MHz downlink and 435.480 MHz uplink. I have not been able to find more information about the amateur radio payload on this satellite.
Recently, Daniel Ekman SA2KNG asked me to analyze some transmissions by this satellite. Apparently it has recently begun to transmit a digital modulation, as shown in this SatNOGS observation, while it typically had transmitted CW telemetry in the past. The point where this started appears to be on 2025-06-20, as there is a SatNOGS observation of CW telemetry on that day followed by an observation of the digital modulation. In this post I analyze this digital modulation and explain what it is.
- 5G NR PDSCH
In my previous post in the 5G NR RAN series, I showed how to decode the PDCCH (physical downlink control channel), which is used to send control information from the gNB (base station) to the UEs (cellphones). In this series I am using as an example a short recording of the downlink of an srsRAN gNB. The PDCCH transmission that I decoded in the previous post was a DCI (downlink control information) containing the scheduling of the SIB1 PDSCH transmission. The PDSCH is the physical downlink shared channel, which is the channel where the gNB transmits data. The SIB1 is the system information block 1. It contains basic information about the cell, and it is decoded by the UE after decoding the MIB in the PBCH, as part of the cell attach procedure. In this post I will show how to decode this PDSCH SIB1 transmission.
- 5G NR PDCCH
This is a new post in my series about the 5G NR RAN. As in previous posts, I am analyzing a short recording of the downlink of an srsRAN gNB. There are no UEs connected to the cell during this recording, so there isn’t much interesting traffic, but the recording contains all the essential 5G signalling. In particular, there is a SIB1 transmission in the PDSCH, with its corresponding transmission in the PDCCH.
The PDCCH (physical downlink control channel) is used to transmit control information to the UEs in the form of DCI messages (downlink control information). The most common types of DCIs are those that specify the scheduling parameters of transmissions in the PDSCH (physical downlink shared channel), and the uplink grants for UEs in the PUSCH (physical uplink shared channel). The role that the 5G PDCCH plays is very similar to the role that it plays in LTE, so my post about the LTE PDCCH can be good for more context. However, in 5G the channel coding and physical layer of the PDCCH is substantially different from LTE.
- Time-dependent delay in GNU Radio
Since some years ago, a Doppler correction block has been available in gr-satellites. This block uses a text file that defines a time series of frequency versus time, and applies the appropriate frequency shift to each sample by linearly interpolating the frequency corresponding to the time of that particular sample. It can be used both to correct Doppler in a satellite propagation channel and other similar channels, as well as to simulate Doppler.
For some time I have been wanting to implement a similar block that applies a time-dependent delay to a complex baseband signal. The delay versus time would be defined by a text file in the same way as for the Doppler correction block. With this block, the delay of a satellite propagation channel can be simulated. It can also be used to correct the delay-rate of a satellite channel, which causes waveforms to be expanded or compressed in time due to a changing time-of-flight. This effect is known as code Doppler in GNSS, and as symbol frequency offset in general digital communications.
For accurate simulation of a satellite propagation channel, both the Doppler block and the delay block are needed, since the Doppler block accounts for the effects of variable time-of-flight on the RF carrier, and the delay block accounts for the effects on the band-limited modulation of the signal, by delaying its complex baseband representation.
I have now added a Time-dependent Delay block to gr-satellites. In this post I give a few details about its implementation and usage.
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