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1. Project Goals

 

The goal of the wideband testbed project is to support experimental research of multiple-input multiple-output (MIMO) wideband wireless communications. First specific goal is a 4 transmit antenna by 4 receive antenna (4x4) MIMO Orthogonal Frequency Division Modulation (OFDM) system that supports experiments on the ongoing 802.11n next generation wireless local area network (WLAN) standardization work.

 

2. Hardware Configuration

 

Figure 1. System Block Diagram of The Wideband Testbed

 

Figure 2.  A 2x2 bench test system using Electribit C8 channel emulator

The Wideband Testbed was acquired and configured as a 4x4 wideband system. Figure 1 shows the block diagram of the system. The testbed supports 802.11g compatible 20MHz RF bandwidth for each channel in the 2.4GHz ISM band and the entire 5GHz band. The testbed is based on software defined radio structure and consists of state of the art of COTS software radio digital signal processor board custom made high performance programmable radios. A Electricbit C8 channel simulator has been acquired for the testbed to support realistic bench testing. Currently only 4 channels are available in the channel simulator. Figure 2 shows a picture of the hardware setup of a 2x2 test system (unconnected).

 

Radio

A total of 16 dual converter radios (Figure 3) have been custom built to support two 4x4 systems. The radio operates in half-duplex mode and is completely programmable in terms of gain, transmit power, mode, and RF frequency. The radio does conversion between 2.4GHz and 70MHz IF signal.

 

 

Figure 3. 2.4GHz Wideband Radio

 

Laboratory tests show that these radios have fairly good RF performance. For example, Figure 4 is a screen shot when using an Agilent Vector Signal Analyzer 89600 to demodulate the received 802.11a packets from the IF output of the receive radio. It shows that the entire RF chain (including both transmit radio and receive radio) only causes about 3% EVM and supports 64QAM modulation very well.

Figure 4.  Test results using Agilent Vector Signal Analyzer 89600

 

The UnWiReD Lab is also interested in characterizing radio communications in the 5GHz band. Two 2.4GHz—5GHz radio converters have been acquired and tested to provide the possibility of testing in the entire 5GHz band. More will be available soon to support multi-antenna test.

Transmitter Digital Processing Unit

The 4-channel transmitter baseband digital processing unit(DPU) is based on industry standard VME bus structure. It mainly consists of a Pentek 4291 Quad DSP processing board and two Pentek 6229 digital up converter daughter boards. The baseband signal sampling rate is 20MHz.

Receiver Digital Processing Unit

The 4-channel receiver baseband DPU is also VME based. It consists of a Pentek 4292 Quad DSP board and two 6235 digital down converter daughter boards. Each down converter daughter board is equipped with a Xilinx Vertex II X3000 FPGA to offload intensive computation from the DSP. A 1 GByte memory card is available to store up to

10 seconds of acquired signal for offline processing. The receiver DPU supports streaming of data into the FPGAs and DSPs at more than 40MHz complex sampling rate on each channel and streaming all 4-channel’s data into one of the FPGA at 20MHz complex sampling rate.

3. Current Status

The hardware has been fully configured to support synchronized 4-channel transmission and 4-channel data acquisition. An offline processing system has been implemented in C/C++ to support testing of a variety of OFDM communications schemes simultaneously for fair comparisons. Implemented schemes include all 802.11a/g compatible modes, MIMO-OFDM with space-time orthogonal block code and convolutional code, MIMO-OFDM with space-time trellis code and MIMO-OFDM with spatial multiplexing and LDPC codes, etc. The offline processing software is well structured such that a new block could be tested within days if the C/C++ implementation (either binary or source code) is available. The system is ready for over-air field-testing. Figure 5 is a screenshot of an example test where multiple schemes were tested simultaneously in the UnWiReD lab.

 

Figure 5. Screenshot of the MIMO-OFDM test GUI

4. Test Results

A group of interference free auditoriums were identified on the UCLA campus for field-testing. A set of preliminary testing was conducted in these rooms. Below is some key information on the testing.

 

These preliminary testing demonstrates that high rate spatial multiplexing schemes, such as 120Mbps, are achievable with good coverage and packet error rate with 3 receive antennas. As an example,

Figure 6 shows the results of a test for a mode at rate 115.2Mbps.

Figure 6. Field test results for rate 115.2 Mbps

 

Table 1. parameters of test positions

Position

Tx/Rx Distance (feet)

Measured SNR per RX over 2 RX

Measured SNR per RX over 3 RX

LOS

RX attenuation in front of the antenna port (dB)

1

13

37.6

36.4

Y

20

2

21.5

32.4

30.8

Y

20

3

44.3

29.7

28.1

Y

20

4

50.9

25.8

24.7

Y

20

5

N/A

32.9

32.2

N (Next room)

0

 

One can see that light of sight (LOS) significantly degrades the performance when the receiver and transmitter are very close to each other.

 

5. Future Work