NI RF Hardware Overview - Robert W. Heath Jr.



National Instruments MIMO/OFDM Prototype

Hardware Description

May 25, 2004

By

Amit Gupta

Antonio Forenza

Robert W. Heath, Jr.

Wireless Networking and Communications Group (WNCG)

Department of Electrical and Computer Engineering

The University of Texas at Austin

1 University Station C0803

Austin, TX 78712-0240 USA

Phone: +1-512-425-1305

Fax: +1-512-471-6512

{amitg, forenza, rheath}@ece.utexas.edu}

Table of Contents

1 Hardware Overview 3

1.1 PXI Chassis 3

1.2 DC Power Supply 4

2 Description of the PXI Chassis 4

2.1 Transmitter 5

2.2 Receiver 5

2.3 Device Drivers 6

2.4 Clock Synchronization 6

2.5 Data Synchronization 6

3 Hardware Limitations 7

3.1 Phase Offset 7

3.2 Not a Real-time System 7

4 List of Additional Parts for the NI Hardware 7

4.1 Parts Purchased 7

4.2 Planned Purchases 8

Hardware Overview

The framework of the NI software/hardware used to implement the MIMO/OFDM system prototype is depicted in Figure 1. The hardware consists essentially of three devices: two PXI chassis and the DC power supply. Hereafter we provide brief description of these devices.

[pic]

Figure 1 NI software/hardware framework

1 PXI Chassis

In Figure 2 is a picture of the hardware. The hardware is housed in two18-slot PXI chassis. The two transmit units and the two receive units are placed in separate PXI chassis. Each chassis is connected to a PC through a fiber optic cable which connects the chassis’ MXI-3 card to a PCI card in the PC. The MXI-3 card allows the components in the chassis to be connected directly to the PCI bus of the PC; therefore, the PXI chassis must be turned on before the PCs are booted so that the PCs will correctly recognize the PXI hardware at boot up. More details on the transmit/receive PXI chassis are provided in the next section.

[pic]

Figure 2 Picture of NI hardware

2 DC Power Supply

We found that using low-noise amplifiers (LNA) at the receiver end of the system was the only way to provide us with a signal which was strong enough to be correctly decoded at the receiver. The amplifiers that we used were Minicircuits ZQL-2700MLNW amplifiers. On each of the receiver units before the signal reached the digitizer we employed a LNA. The LNAs need to be supplied 15 volts of DC power—which is why we have a DC power supply.

Description of the PXI Chassis

In Figure 3 is a picture of the PXI chassis. One slot is used to link the NI hardware to the software (LabVIEW). Moreover, three slots are employed for the transmitter unit and four for the receiver unit. Note, in Figure 3 transmit and receive slots are located in the same chassis only for demonstration purposes. However, in the latest version of the prototype one chassis contains the transmit units and the other one the receive units, as mentioned before.

[pic]

Figure 3 Picture of the PXI chassis

1 Transmitter

The transmit unit consists of two parts – an upconverter (PXI 5610) and an arbitrary waveform generator (PXI-5421.) When used together the unit is called the PXI-5670 Signal Generator. The arbitrary waveform generator (ARB) runs at a maximum sampling rate of 100 MSamples/s. When used in conjunction with the upconverter, the ARB takes the discrete I-Q waveform created in LabVIEW and then creates a continuous waveform at an IF of 25MHz which it sends to the upconverter. The upconverter then creates and transmits the desired RF signal. The upconverter can transmit on a maximum frequency of 2.7 GHz.

2 Receiver

The receiver also consists of two units. The first component is the downconverter (PXI-5600) and the second is the high speed digitizer (PXI-5620.) The downconverter also has a maximum frequency of 2.7 GHz and it downconverts the received signal to an IF of 15 MHz. It can receive a maximum bandwidth of 20 MHz. The digitizer operates at a maximum sampling frequency of 64MSamples/s. The digitizer is equipped with a digital downconverter chip (DDC) that can carry out digital downconversion from IF when the bandwidth of the signal is less than1.25Mhz. Otherwise if the bandwidth of the signal is greater than 1.25 MHz then the downconversion occurs in software, which is considerably slower.

3 Device Drivers

The transmit and receive units both consists of two pieces of hardware. There are two different ways of programming each unit. With the receiver, you can use the RF Signal Analyzer (RFSA) drivers to program the downconverter and digitizer together as a single entity or you can use the NI-Tuner drivers to program the downconverter and the NI-Scope drivers to program the Digitizer. When first using the hardware it is generally a better idea to begin by using the RFSA drivers since they are simple to use. As a tradeoff though, the ease of use comes at the cost of flexibility and robustness. Many of the systems parameters are automatically controlled by the drivers and as a result full control of all the hardware’s parameters is lost.

Using the two separate drivers gives the user much of that control back and parameters such as the sampling rate and IF frequency can now be changed. However, of course, the classic tradeoff is incurred as the hardware is a little more difficult to program. So far for my applications I have stuck to using the combined RFSA driver set. For the transmitter a similar driver breakdown applies. For the combined unit the RF Signal Generator drivers (RFSG) are used. Individually the ARB uses the NI-FGEN driver while the upconverter can be programmed using the same RFSG drivers as the combined unit.

4 Clock Synchronization

A 10 MHz reference clock is used to synchronize the upconverter/ARB at the transmitter sided to the downconverter/Digitizer at the receiver side. A connection on the front of the hardware devices synchronizes those components together. In order to carry out carrier synchronization, the clocks on the transmit unit and the receive units can also be synchronized together through two basic different configurations. The first is through the backplane of the PXI chassis—this of course only works of both the transmitter and receiver are in the same PXI chassis. The unit which is in slot 2 of the PXI chassis can be set to synchronize the 10 MHz reference clock which the chassis comes equipped with (actually, ONLY the unit in slot 2 of the PXI chassis can set the chassis reference clock). Usually it makes most sense to have the receive unit in slot 2 of the chassis and to synchronize according to the receivers clock. The transmit unit can then synchronize according to the PXI chassis reference clock.

The other setup is to use the external connections to synchronize the carrier frequencies of the transmitter and receiver. Connecting the 10 MHz clock out on the downconverter to the clock reference in input of the upconverter will also allow for similar synchronization. NI documentation tells us that this type of synchronization is more accurate the PXI backplane synchronization.

5 Data Synchronization

In order to carry out timing synchronization, a digital trigger is sent from transmitter to receiver to signify when signal transmission has begun. A trigger is sent externally using the PF1 connection on both the Digitizer and the ARB. You cannot send a trigger using the PXI backplane because the backplane is not connected all the way along all of the slots. The vertical bars on the front of the case in between the slot numbers demarcate the various PXI connections. The clock reference is one of the few signals which are available to all slots, but the trigger lines are not.

The operation of the data synchronization is also controlled in the software. There is a reference position input to the “niScope configure horizontal timing.VI” at the receiver which determines how the trigger operates. The input should be a decimal between 0 and 1 and it refers to the percentage of data which is acquired pre-trigger versus the percentage of data which is acquired post-trigger. Theoretically for the trigger to work as expected, the value should be set to 0 so that no pre-trigger data is acquired and that all data is post-trigger data. In reality, this does not happen. When the reference position is set to 0, some of the data is lost pre-trigger. Therefore it should be set to a very small value greater than 0 (we use 8.2x10^-6.) Depending on your application the only way to set this correctly is by trial and error.

Hardware Limitations

1 Phase Offset

When synchronizing the clocks of the transmitter and the receiver, the clock’s frequency will be locked, however the phase will not be. As a result there will be a constant phase shift between the transmitter and receiver. This phase shift will be random, but it will be constant for the whole acquisition period. For the OFDM application, this phase offset was not a problem as it is accounted for through channel estimation.

2 Not a Real-time System

One of the largest limitations of the current system is that it is not a real-time system. Data can only be acquired in chunks and not through a continuous acquisition. This is because the digitizer is not able to downconvert the signal from IF to baseband in hardware but instead does it through software. This downconversion is computationally intensive and requires substantial processing time. A baseband system which does not use the RF hardware should require less computation and may be able to implemented in a real-time system.

List of Additional Parts for the NI Hardware

1 Parts Purchased

1 Cables and Connectors

From SM Electronics



SMA HFLEX cables

6 - 24'' long, .141'' diameter, Part #HF141-24-0101

6 - 48'' long, .141'' diameter, Part #HF141-48-0101

6 - 72'' long, .086'' diameter, Part #HF086-72-0101

6 - 6'' long, .086'' diameter, Part #HF086-06-0101

SMA Formable Semi-rigid cables

4 - 6'' long, .086'' diameter, Part #HC086-06-0101

SMA Flexible 26.5Ghz cables

6 - 12'' long, Part #RF085H-0101-12

2 Adapters

SMA/SMA Straight 18Ghz  Adapters

4 - SMA Female/Female Au(Gold) Plating, Part #4951

4 - SMA Female/Female Bulkhead Au Plating, Part #5205/Au

SMA-TNC Adapters

6 - SMA/Female - TNC/Female adapter, Part #5018

3 Splitters

From

ZMSC-3-2 with an SMA connector, a 3-way power divider. Quantity 2

4 Antennas

There they have under their PCS and Dual Band Cellular Mobile Antenna

section they have Andrew Antenna Specialist Magnetic Mount Antennas with a

TNC connector, Item #97938

900mhz/1.8Ghz

Here is a direct link to the antenna



2.4Ghz Antennas



4.1.4 Amplifiers

Low Noise Amp



ZQL-2700MLNW (this is the one we currently have)

2 Planned Purchases

1 Amplifiers

From minicircuits

ZKL-2R7

2 Cables



SMA-SMA low loss double shielded coax cables (Model No. PE3138)

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Up-converter

Down-converter

DAC

ADC

Link to Computer

Transmitter

Receiver

LabVIEW VI

DC Power Supply

For Amps

Receiver

Chassis

Transmitter

Chassis

Analog

Digital

NI PXI-5600 Down-Converter

Carrier Frequency 250 kHz – 2.7 GHz

NI PXI-5620 High Speed Digitizer (64 million samples per second)

Analog

Digital

NI PXI-5610 Up-Converter

Carrier Frequency 250 kHz – 2.7GHz

NI PXI-5421 Arbitrary Waveform Generator (100 million samples per second)

LabVIEW VI

Transmit Antenna

Receive Antenna

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