3.4 Simulation Results
3.4.3 Detection Efficiency and Interference Rejection Performance:
mance: h=1/4, L=3
3.4.3.1 Detection Efficiency: Bit Error Rate as a function of SNR
With an AWGN channel only, simulated error rate performance is shown in Figure 3.7. The symbol and bit error rate data was collected by accumulating a minimum of 100 symbol errors, or for the FEC BER graph a minimum of 100 bit errors after FEC. The receiver low-pass filter is removed for this simulation.
A BER of 10−6 is achieved with an Eb
No of 14 dB. Furthermore, when the
Reed Solomon FEC is included then Eb
No is approximately 10.6 dB at a BER of
10−6. This is 5.1 dB better than the ETSI requirement of 15.7 dB.
The existing CPM modem product achieves a10−6 BER at an SNR of 14
dB or 11 dB Eb
No [42] assuming ideal timing synchronisation and no degrada-
tion due to fixed precision arithmetic.3 These results show that the new CPM
configuration has the potential to be 0.4 dB better in terms of detection effi- ciency. However, a low-pass adjacent channel rejection filter is required to meet the ACI rejection requirements. This filter also degrades clear channel performance as described in the next section.
3SNR is 3 dB higher thanEb
Figure 3.7: Simulated Bit Error Probability with and without Reed Solomon FEC, AWGN Channel, No ACI Reject Filter, h=1/4, L=3RC, 27 Msymbols/s
3.4. SIMULATION RESULTS 29
Figure 3.8: Simulated Bit Error Rate with Adjacent Channel Interference, h=1/4, L=3, M=4, 27 Msymbols/s
3.4.3.2 1st Adjacent Channel Interference
Figure 3.8 shows adjacent channel performance with carrier to interference (C/I) ratios of -5 dB and -1 dB. This adjacent channel interference is 1 dB stronger than specified in the ETSI specification. For the 5 dB adjacent channel interferer test, the bit error rate is very close to meeting the target of10−5at an
Eb
No of 18.7 dB. This is well below the10
−3Reed Solomon FEC threshold and so
with FEC present, the BER falls well below the ETSI specification limit of10−6.
The less stressful 1 dB ACI test shows a bit error rate of less than10−6at 16.7 dB Eb
No, clearly meeting the ETSI requirement.
The radio’s intermediate frequency (IF) amplifier stages contribute signifi- cantly to the receiver’s selectivity. Nevertheless, the final IF stage in this radio is 36 MHz wide which passes a significant amount of 1st adjacent channel sig- nal power. The front end of the digital portion of this receiver contains decima- tion stages and other low pass filters which provide adjacent channel rejection. These are modelled with a single low pass filter of 96 taps.
The low pass filter cutoff frequency has a significant impact on receiver BER performance. If the cutoff is set too low then the in-band signal is distorted too much and BER performance is degraded. On the other hand, if the filter cutoff is set too high, then too much adjacent channel power passes into the demodulator and BER performance is degraded. A cutoff frequency of 14 MHz is chosen to provide a compromise between clear channel performance and adjacent channel rejection.
Figure 3.9: Simulated Bit Error Rate Demonstrating Effect of ACI Reject Filter and Adjacent Channel Interference, h=1/4, L=3, M=4, 27 Msymbols/s
Figure 3.9 shows the effect of the low pass filter on clear channel perfor- mance. At a BER of10−6, Eb
No is 14.4 dB, a degradation of 0.4 dB due to the
low pass filter. Without the filter present the new CPM configuration was 0.4 dB better than the existing product. This means that with the filter present the new CPM configuration has the same level of detection efficiency as the exist- ing CPM product. This is 4.7 dB better than the minimum required in the ETSI standard.
3.4.3.3 Co-channel Interference
Co-channel interference is simulated at carrier to interference (C/I) ratios of 19 dB and 23 dB and the results are shown in Figure 3.10. In both cases the bit error rate is less than10−5at an Eb
No of 16 dB. The requirement is for a bit error
rate of less than10−5 at Eb
No of 18.7 dB and 15.7 dB. This is a clear pass to the
ETSI requirements.
3.4.4
Detection Efficiency and Interference Rejection Perfor-
mance: h=1/5, L=2
The h=1/5, L=2 CPM configuration is interesting because compared to h=1/4, L=3 it has a matched filter bank 1/4 the size and Viterbi trellis with 1/3 of the
3.4. SIMULATION RESULTS 31
Figure 3.10: Simulated Bit Error Rate with Co-Channel Interference, h=1/4, L=3, M=4, 27 Msymbols/s
Figure 3.11: Simulated Bit Error Rate with and without Adjacent Channel In- terference, h=1/5, L=2, M=4, 27 Msymbols/s
states. The simulated detection efficiency performance of this CPM configura- tion is shown Figure 3.11.
At a BER of10−6, Eb
No is approximately 14.8 dB with the low pass ACI reject
filter present. This is 0.4 dB worse than the h=1/4, L=3 CPM configuration. However, the ACI rejection performance is considerably worse. At an Eb
No
of 18.7 dB the BER is5∗10−5. This does not meet the stated requirement of a
BER less than10−5at 18.7 dB Eb No.
The ACI performance could be improved by reducing the low-pass ACI re- ject filter cutoff frequency. However, this also increases the amount of in-band signal removed at the band edge. The clear channel performance is already reduced by 0.6 dB due to the low-pass ACI reject filter, so further lowering the cutoff frequency will degrade the in-band performance even further.