RDRN Hardware -- Digital Software Radio

Overview

Computers at the remote nodes communicate with the Mobile Access Point (MAP) and the rest of the network through wireless modems, which are designed, built, and tested here at ITTC. The wireless modem is a software radio which is capable of communicating at several Mbits/s in full-duplex mode. Different RF front-ends can be applied to the radio for different applications in different portions of the frequency spectrum, such as the Industrial, Scientific, Medical (ISM) band of 5.3GHz, 2.4GHz, and 900MHz. Other frequency bands in the 1.2GHz and 450MHz are also possible. The radio is envisioned to be capable of adapting to different operating environments by changing modulation scheme and data rate, and performing antenna beamforming in order to maximize frequency reuse.

A block diagram of the radio is shown below:

Radio Block Diagram

The radio utilizes 10BASE-T ethernet as the physical layer to communicate with the computer. Data arrives at the radio in ethernet packets, which are then encapsulated into HDLC frames by the radio's on-board processor. Each HDLC frame is "converted" to a serial data stream and passed to the transmitter section of the radio. When the radio on the other end receives the HDLC frame, it extracts the ethernet packet and passes it onto the computer.

The Third Generation

The radios currently being developed are the 3rd generation since the beginning of the project. It is significantly smaller than its predecessors, and operates on 12V DC. Here are the specifications:

Each radio can be divided into four sections:

Communication Controller Unit

The communication controller unit is a PowerPC MPC860. It contains a communication processor module (CPM) which processes ethernet, HDLC, and other serial communication protocols. Since the CPM is integrated into the processor, controller design is simplified. To further simplify hardware design, an MPC860 evaluation board is employed.
Another function of the MPC860 is to control the operation of the radio. It contains programming data for the modulator and demodulator unit, as well as frequency assignment of the uplink and downlink channels. The operation parameters of the radio, such as data rate, modulation scheme, frequency channel assignment, etc., can be modified by programming a different table into the MPC860 memory. The radio is designed to include the capability of changing operating parameters on the fly. This feature allows the radio to operate more efficiently under different conditions.

Modulator and Demodulator Unit

The modulator unit utilizes a 240MHz IF local-oscillator (LO) carrier. Current radio design allows binary and quadrature modulation (BPSK and QPSK). Baseband data is first differentially encoded, and then passed directly to the modulator for BPSK modulation, or split into in-phase (I) and quadrature (Q) data before reaching the modulator for QPSK operation. A 240MHz IF carrier is chosen for the modulator in order to avoid interference with the IF carrier at the demodulator.
The demodulator is a mixed analog and digital design. RF signal is first down-converted to IF at 70MHz. This IF frequency is widely accepted, which leads to more convenience in obtaining off-the-shelf components. The 70MHz IF signal is then sub-sampled by a high-speed analog-to-digital converter (ADC). Harris Semiconductor's digital quadrature tuner (DQT) and digital costas loop (DCL) chip set processes the digital data to bring the information signal back to baseband. The baseband data is finally passed to the PowerPC processor for final processing back to ethernet packets.

RF Front-End

The RF front-end up-converts the transmit IF signal to the proper frequency, and down-converts the receiving signal to the proper receive IF frequency. Each front-end is divided into transmit and receive section. Both of them achieve the desired frequency stability by using a phase-lock loop (PLL) and a very stable reference oscillator. As mentioned previously, the radio can operate in different frequency bands by using the appropriate front-end. At this point the radio is connected to a 5.3GHz front-end.

Antenna

The transmit signal from the RF front-end will further be amplified before hitting the antenna. The transmit antenna assembly contains a patch antenna with an integrated power amplifier at the back. This arrangement avoids signal loss between the antenna and the final power amplifier stage to ensure maximum power transfer.
The receive antenna is very similar to the transmit antenna, except that the power amplifier is replaced by a low-noise amplifier (LNA). The LNA is connected as close to the antenna as possible to minimize signal loss and to maximize signal-to-noise ratio (SNR) at the input and output of the LNA.
A MS PowerPoint slide is provided here with pictures and performance data of the antennas used in the RDRN radio.

Radio Performance Testing

There is a section dedicated to radio performance testing. Please refer to the page Field Tests on the left column.


Last Update: December 16, 1999 05:00pm (GMT-0600)