USB 2.0 interface

1. General introduction to USB

This text is taken from the wikipedia website:

Universal Serial Bus (USB) is a serial bus standard to interface devices to a host computer. USB was designed to allow many peripherals to be connected using a single standardized interface socket and to improve the plug-and-play capabilities by allowing hot swapping, that is, by allowing devices to be connected and disconnected without rebooting the computer or turning off the device. Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.

2. Type of transmissions

The maximal baudrate of a USB 2.0 (also called high speed USB) link is 64 Mbytes/s or 480 Mbit/s. This baudrate is shared by all USB devices connected to the same USB controler, and the baudrate needed for the control message has to be withdrawn from this value.

Several transfert mode can be used by an USB device to communicate with a computer:

• Command transfer: This transfer is used for command. Data integrity is ensured by the protocol
• Interrupt transfer: The USB controller asks the USB device if it has data to transmit. If so, the transmission time is guaranteed, as data integrity. This mode is used for mouse and keyboard for example.
• Block transfer: This mode is used to transfer large amount of data. No transmission time is guaranteed, but data integrity is.
• Isochronous transfer: This transfer is used for real time data transfert. The highest baudrate is available with this mode, but it does not support data integrity. This mode is used by SDR using an USB interface link.

3. Maximal baudrate of a radio interfaced to a PC with an USB 2.0 link

As mentionned, the USB 2.0 link can support at least a baudrate of 64 Mbytes/s. In practice, to ensure that the received data are not corrupted (missing samples for example), an effective baudrate of 20 Mbytes/s should be respected.

For radio sampling the received signals (real and imaginary parts) over 12 bits, 4 bytes are required by samples (2 bytes for the real part, and 2 bytes for the imaginary part). Hence, 5 Msamples can at most be transmitted in real time from a radio to a computer through a USB link.

As a consequence, radio using this technology are limited to work on signals with a most 5 MHz of bands. In practice, there is hence no hope to decode WiFi, UMTS, DVB signals with this kind of radio.