SETTING UP A WIRELESS LINK USING ME1000 RF TRAINER KIT Introduction S Kumar Reddy Naru ME Signal Processing S. R. No - 05812 The aim of the project was to try and set up a point to point wireless link. The ME 1000 RF trainer kit has two blocks. One works as the transmitter and the other as the receiver. They include all the RF processing blocks, i.e. the transmitter includes the mixer (up converter), power amplifier, RF filter and the antenna. The receiver block has LNA, mixer (down converter), filter, IF amplifier and the IF filter. Both the blocks operate at a carrier frequency of 818 MHz. The ME 1000 was designed such that, the IF signal should be centered around 50 MHz and the maximum bandwidth allowed is 2 MHz. So, in order to use the kit, first we need to up convert the base band signal to 50 MHz and then use that as the IF signal. Same thing has to be done for decoding also. The IF signal output of the receiver unit is centered around 50 MHz. Hence to decode, we first need to down convert this output signal to base band and then decode. This down converting and decoding can be done using the Vector Signal Analyzer (VSA). The VSA has inbuilt software demodulators (FSK, PSK and some other standards). The IF signal was generated using Vector Signal Generator (VSG). The instrument was interfaced to a PC and VEE was used to program the VSG. Digital Modulation type and Parameter Selection: As it is a wireless link, the gain (or attenuation) from transmit antenna to receive antenna is dependent on the position of the antennas and also the surroundings. Because of this, the signal constellation undergoes variable rotation. Energy based schemes are simpler to implement. As the surroundings are stationary, the gain is time invariant, i.e. the channel is linear time invariant. First FSK was tried out using the following parameters.
2 FSK was used and the frequency deviation was 200 KHz. The symbol rate (same as bit rate for 2 FSK) was selected as 20 KHz. Frequency deviation of 200 KHz means that the two frequencies used for the 2 FSK were F F c c + 200KHz 200KHz For a symbol rate of 20 KHz, the minimum frequency deviation needed for orthogonality is 20 KHz only, in ideal cases. Large separation between frequencies will give good performance under non ideal conditions. Another main reason for selecting a large separation is because of the Vector Signal Analyzer. The parameters needed for decoding interact with each other and it is very difficult to decode large number of symbols at a time if the frequency deviation is small. This is because; the VSA takes chunks of data from the Oscilloscope and will do the decoding. It is not real time decoding. The length of those chunks depends on the parameters used for decoding. A large frequency deviation allows use to take larger chunks and hence large number of symbols can be decoded at a time. BPSK and QPSK modulations were also used later. IF Signal Generation: VEE was used to program the Vector Signal Generator. An inbuilt program was taken and was modified to generate the 2 FSK with the above parameters. First a complex signal was generated s1 = cos 2 pi s2 = cos 2 pi f Fs f Fs n + j sin 2 pi n j sin 2 pi f Fs f Fs n n The first signal generates the frequency Fc+f after carrier modulation and the second one generates Fc-f. The bits b were generated using random integer generator function randn, which generates 0, 1 each with probability 0.5. After that the complex signal was formed using the following operation. s = s1 b + s2 (1 b)
If the bit is one, s1 is transmitted and if it is zero, s2 is transmitted. Modulation with Fc was done inside VSG, using a carrier frequency of 50 MHz. The ME 1000 kit takes the power from the PC and the power gain of the amplifier is very low. Hence if we use the transmitter block, we will get very low power at the transmit antenna. But if we bypass the RF processing and directly give signal to the antenna, we can go up to 20 dbm. So for the experimentation part, the signal is modulated at 868 MHz, instead of 50 MHz in the VSA and is given to the antenna directly. Experiment Results 1) Receive Power Variation with distance between transmit and receive antennas The transmit power was fixed at 0 dbm and the distance between the transmit antenna and the receive antenna was varied. There is line of sight from transmit to receive antenna. The Signal power at the output of the receiver was found as a function of the separation. The experiment was done by giving a single tone (sinusoid) of frequency 50 MHz as the IF signal. It will be converted to 868 MHz by the transmitter and the receiver converts it back to 50 MHz. This 50 MHz signal power was measured at the output. The following figure indicates the power at different distances. The IF amplifier gain in the receive unit was set as 15 dbm. 4 Receive Power 2 0 Power in dbm -2-4 -6-8 -10-12 1 1.5 2 2.5 3 3.5 4 4.5 5 distance in multiples of 30cm Figure 1 Receive Power after down converting and amplification.
The small receive power at 2X than at 3X is because of the multi path effect. At 2X, the multi paths are adding destructively. 2) FSK Decoder Performance for different distances between antennas The decoding performance was studied for different distance of separations, for a transmit power of 0 dbm. The following figure indicates the decoder performance at 1.4m distance of separation. Figure 2 Receive Signal Characteristics for d=1.4m and transmit power of 0dbm. The subplot in the top left corner indicates the received signal constellation. The top right corner plot indicates the error performance. The x axis indicates the symbol number. The y axis is the percentage deviation from the ideal value. The plot in the bottom left corner is the signal spectrum. The figure in the bottom right cornet is the decoded bit stream. It also gives the percentage average rms error, which is 23%. The same format is used for all the figures in the report that indicate the decoder performance. The following figure indicates the performance for distance of separation 0.84m. It can be observed that the spread in the received signal constellation is small compared to the figure 2. This is because the SNR increases with the reduction in the distance of separation.
Figure 3 Receive Signal Characteristics for d=0.84m and transmit power of 0dbm. of separation. The average rms percentage error was plotted below as a function of the distance 25 % rms Error 20 Percentage Error 15 10 5 0 1 1.5 2 2.5 3 3.5 4 4.5 5 distance in multiples of 30cm Figure 4 Error in received Signal constellation Vs distance.
It indicates that, as the distance reduces the SNR increases and hence the error reduces. It can be reduced that there is no significant difference between 2x and 3x separation. This is because of the multi path effect as explained previously. Also, for small separations, the error is not falling rapidly, as if falls for large distance of separation. The reason is that at small separation, the signal power is large and the IF amplifier in the receive unit is saturating. Hence there is no increase in signal power. Only the harmonics power increases. 3) FSK Decoder Performance for different Transmit powers In this experiment, the distance was fixed and only the transmit power is varied. The distance of separation used was 1.18m. The following figure indicates the performance for transmit power of 0dbm. Figure 5 Receive Signal Characteristics for d=1.18m and transmit power of 0dbm. The percentage rms error is 8.6. The next figure indicates the performance for a transmit power of 16dbm.
Figure 6 Receive Signal Characteristics for d=1.18m and transmit power of 16dbm. Comparing the two figures, we can observe that the spread is small in case of 16dbm transmit power, as expected. The average rms percentage error was plotted below as a function of the transmit power. 9 % rms Error 8 7 % Error 6 5 4 3 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Transmit Power in multiples of 4dbm Figure 7 Error in received Signal constellation Vs Transmit power.
It can be observed that the fall in error is not that much at high transmit power. This is because of the saturation of the IF amplifier in the receive unit. Once the amplifier saturates, only the harmonics power increases, not the signal power. 4) Analog Demodulation (FM) of the FSK signal To see the frequency shift keying, we can use analog demodulation (FM). This should give us pulses corresponding to the frequencies. The experiment was done by keeping the antennas at a distance of 1.18m. The following figure shows the analog FM demodulator output, for a transmit power of 0 dbm. The figure in the top half shows the spectrum of the received signal. The bottom half shows the analog FM demodulated output. The switching of the frequencies between -200 KHz and +200 KHz around the center frequency of 50 MHz can be clearly observed. Figure 8 Analog Demodulated output for d=1.18m and transmit power=0dbm. dbm. The following figure shows the demodulated output at a transmit power of 15
Figure 9 Analog Demodulated output for d=1.18m and transmit power=15dbm. Comparing the two figures, we can observe that high transmit power, the spread in the demodulated output is small, i.e. the spread around the frequencies -200 KHz and +200 KHz is small. The following experiments are using phase based modulation schemes. BPSK and QPSK were used. No correction was used for the phase error that occurs. As the distance of separation is small and also, there is line of sight. So, the phase error will be small. 5) BPSK Demodulator performance BPSK signal with a symbol rate (same as bit rate) of 20 KSPS was used for this experiment. The signal was generated using the Vector Signal Generator. The Antennas were separated by 2.1m. The following figures shows the performance for a transmit power of 0 dbm and 15 dbm.
Figure 10 BPSK Receive signal characteristics for d=2.1m and transmit power=0 dbm. Figure 11 BPSK Receive signal characteristics for d=2.1m and transmit power=15 dbm.
It can be clearly seen that the spread in the signal constellation reduces with increase in the transmit power. The variation of the percentage rms error with transmit power is plotted below. 35 % rms Error 30 Percentage Error 25 20 15 10 0 0.5 1 1.5 2 2.5 3 3.5 4 Transmit Power in multiples of 5dbm Figure 12 Error in received Signal constellation Vs Transmit power. It can be observed that the error is slightly increasing at large transmit powers. The reason is the saturation of the IF amplifier in the receiver. Previously, for FSK it caused only saturation of error. But now, the error increases because the non linearities affect the phase very much. For FSK, it only saturates the signal power. As the distance of separation is large, the non linearity effect is not that significant. If the antennas were placed closely and the same transmit powers are used, the error increases significantly at large transmit powers. 5) QPSK Demodulator performance The symbol rate used was 20 KSPS. Hence the bit rate will be 40 KBPS. The Antennas were separated by 1.5m. The following figures shows the performance for a transmit power of 0 dbm and 15 dbm.
Figure 13 QPSK Receive signal characteristics for d=1.5m and transmit power=0 dbm. Figure 14 QPSK Receive signal characteristics for d=1.5m and transmit power=15 dbm.
The spread in the receive signal constellation is small at 15 dbm transmit power than at 0dbm. But the reduction is not significant because of the IF amplifier saturation, as the following figure indicates. 24 % rms Error 22 20 Percentage Error 18 16 14 12 10 0 0.5 1 1.5 2 2.5 3 3.5 4 Transmit Power in multiples of 5dbm Figure 15 Error in received Signal constellation Vs Transmit power. The error at 20dbm transmit power is much more than at 0dbm, because of the non linearities, as explained previously. References 1) Agilent InfiniiVision 6000 and 7000 Series Oscilloscopes Performance Guide Using 89600 Vector Signal Analyzer Software Application note 2) 89600 Vector Signal Analyzer Software Help 3) ME 1000 RF trainer - Quick Start Guide.