Design Note DN030
ACR Measurements on IEEE 802.15.4 Systems
By Espen Wium
Keywords
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1
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Selectivity
RF
IEEE 802.15.4
ACR
Measurements
CC2430
CC2420
CC2520
CC2530
Introduction
Selectivity is an important property of any
RF receiver, and describes its ability to
separate (and receive) the wanted signal
from unwanted signal sources transmitting
at other channels. ACR describes a
receiver’s ability to receive a wanted signal
in one channel when an interfering signal
is present one channel spacing away
(either up or down). In a typical set up, a
signal generator is transmitting a pseudo
random bit sequence (e.g. PRBS9)
correctly modulated on a carrier in the
wanted channel while another signal
generator is transmitting a different and
uncorrelated
PRBS
(e.g.
PRBS15)
correctly modulated on a carrier in one of
the neighboring channels. The wanted
signal is fixed at a certain level, e.g. 3 dB
above a reference sensitivity level, while
the level of the interferer is swept upwards
until reception of the wanted signal fails.
Failed reception is normally defined by a
certain bit error rate (BER) or packet error
rate (PER).
This design note discusses theoretical as
well as practical limitations encountered
when measuring ACR, with special
emphasis on IEEE 802.15.4 receivers.
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Design Note DN030
Table of Contents
KEYWORDS.............................................................................................................................. 1
1
INTRODUCTION ............................................................................................................. 1
2
ABBREVIATIONS ........................................................................................................... 2
3
ACR AS DEFINED BY IEEE 802.15.4 ............................................................................ 3
4
GENERATING WANTED AND INTERFERER SIGNALS .............................................. 3
4.1
FREQUENCY SPECTRA ............................................................................................... 4
4.2
THE W ANTED SIGNAL ................................................................................................ 5
4.3
THE INTERFERER SIGNAL ........................................................................................... 5
5
MAXIMUM ACR WHEN USING AN IDEAL MSK INTERFERER ................................... 6
6
ACR RESULTS USING DIFFERENT INTERFERER WAVEFORMS ............................ 7
7
THE ESG UND WAVEFORM COMPARED TO A REAL 802.15.4 DEVICE .................. 9
8
CONCLUSION .............................................................................................................. 10
9
REFERENCES .............................................................................................................. 11
10
GENERAL INFORMATION........................................................................................... 12
10.1
DOCUMENT HISTORY ............................................................................................... 12
2
Abbreviations
ACR
MSK
PRBS
BER
PER
O-QPSK
EVM
BW
PSD
RBW
Adjacent Channel Rejection
Minimum Shift Keying
Pseudo Random Bit Sequence
Bit Error Rate
Packet Error Rate
Offset Quadrature Phase Shift Keying
Error Vector Magnitude
Band Width
Power Spectral Density
Resolution Band Width
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Design Note DN030
3
ACR as Defined by IEEE 802.15.4
The IEEE 802.15.4 standard [1] imposes the following requirements on receiver selectivity:
Section 6.5.3.4 of IEEE 802.15.4, Receiver jamming resistance
The minimum jamming resistance levels are given in Table 1. The adjacent channel is one on
either side of the desired channel that is closest in frequency to the desired channel, and the
alternate channel is one more removed from the adjacent channel. For example, when
channel 13 is the desired channel, channel 12 and channel 14 are the adjacent channels, and
channel 11 and channel 15 are the alternate channels.
Adjacent channel rejection
0 dB
Alternate channel rejection
30 dB
Table 1. Minimum receiver jamming resistance requirements for 2450 MHz PHY
The adjacent channel rejection shall be measured as follows: the desired signal shall be a
compliant 2450 MHz IEEE 802.15.4 O-QPSK PHY signal, as defined by 6.5.2, of pseudorandom data. The desired signal is input to the receiver at a level 3 dB above the maximum
allowed receiver sensitivity given in 6.5.3.3.
In either the adjacent or the alternate channel, an IEEE 802.15.4 signal, as defined by 6.5.2, is
input at the relative level specified in Table 26. The test shall be performed for only one
interfering signal at a time. The receiver shall meet the error rate criteria defined in 6.1.7 under
these conditions.
6.5.2 Of IEEE 802.15.4 specifies an O-QPSK 2 Mchips/s signal with half-sine pulse shaping,
which is equivalent to 2 Mb/s MSK or 2-FSK with 500 kHz deviation. The maximum allowed
EVM is 35%.
4
Generating Wanted and Interferer Signals
Since MSK is a waveform supported by most vector signal generators, the most practical way
to generate the wanted signal is to convert the two 1 Mb/s I and Q bit streams of the O-QPSK
signal to a 2 Mb/s MSK bit stream and set the generator up to transmit this.
The test systems used by TI Norway to measure ACR on all 802.15.4 devices released so far
have all used an SMIQ from Rohde & Schwarz to generate the wanted signal and an
ESG4437B from Agilent to generate the interferer signal. The SMIQ transmits custom packets
(which are used to calculate PER), while the ESG transmits an uninterrupted MSK modulated
PRBS.
The SMIQ generating the wanted signal is set up to transmit GMSK with BT=2 (i.e. a slightly
shaped MSK signal) to emulate a more realistic transmitter.
The ESG transmitting the interferer signal is set up to transmit MSK using an option called
UND.
Both generators are capable of generating signals that more closely match an ideal 802.15.4
signal, but for reasons discussed in this design note we have chosen the above settings.
Using these settings, both signal sources transmit signals well within the limits specified in
IEEE 802.15.4.
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Design Note DN030
4.1
Frequency Spectra
The frequency spectra produced by the generators depend on their configured state as shown
in Figure 1. An ideal MSK signal plot is also included for reference.
Generated vs. Ideal MSK Spectra
0
E4437B Standard MSK
SMIQ MSK
Ideal MSK
-10
E4437B UND MSK
SMIQ GMSK BT=2
Relative Power (dB)
-20
-30
-40
-50
-60
-8
-6
-4
-2
0
2
4
6
8
Frequency offset in MHz
Figure 1. Frequency spectra
The trace marked E4437B Standard MSK shows the closest-to-ideal MSK signal that the ESG
generator can produce. Similarly, the SMIQ MSK trace shows the closest match for the SMIQ.
Comparing to the ideal MSK signal, it becomes evident that none of the generators are
capable of producing a perfect MSK signal, and that the error becomes greater further away
from the centre frequency. This is not unexpected, as all generators will have limited BW in
their baseband modules and mixers.
Figure 2 shows the same spectra as Figure 1, zoomed to a channel adjacent to the one being
transmitted in (5 MHz wide, 5 MHz away from the carrier).
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Design Note DN030
Generated vs. Ideal MSK Spectra
-25
E4437B Standard MSK
SMIQ MSK
Ideal MSK
-30
E4437B UND MSK
SMIQ GMSK BT=2
Relative Power (dB)
-35
-40
-45
-50
-55
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
Frequency offset in MHz
Figure 2. Spectral components one channel away from the carrier
4.2
The Wanted Signal
The wanted signal used in TI Norway’s sensitivity- and blocking measurements is shown in
Figure 1 as SMIQ GMSK BT=2, and is an MSK signal with a slight Gaussian filtering
(Bandwidth Time product of 2). There is only a very small difference between this signal and
the unfiltered (MSK) signal produced by the SMIQ. GMSK BT=2 is chosen as the preferred
wanted signal waveform as it is a little closer to a realistic IEEE 802.15.4 transmitter than the
unfiltered MSK signal.
4.3
The Interferer Signal
The interferer signal used in TI Norway’s blocking measurements is shown in Figure 1 as
E4437B UND MSK, and is an MSK signal with a 2.5 MHz Low Pass Base band
Reconstruction Filter. The difference between this signal and the ideal MSK signal is that the
spectral side lobes outside approximately +/- 3 MHz are greatly reduced for the “UND” signal.
This has the effect of concentrating the interferer power to the channel where the interference
is intended to take place and reduce the spectral components that would otherwise propagate
into the neighboring channels – most specifically the wanted channel. The result is that this
(interferer) signal enables us to measure a receiver’s ability to selectively receive a wanted
signal in one channel in the presence of a strong unwanted signal in an adjacent- or alternate
channel. If an ideal MSK signal had been used as the interferer waveform, the ACR would
quickly become limited by the noise level present in the wanted channel due to the high
spectral side lobes of the interferer.
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Design Note DN030
5
Maximum ACR when Using an Ideal MSK Interferer
As Figure 1 shows, an ideal MSK signal will place more of its transmitted power outside of the
intended 5 MHz channel than the filtered or shaped signals shown in the same diagram.
Integrated 5 MHz channel power
0
-5
-10
-15
-20
-25
-30
Integrated 5 MHz channel power
-35
-40
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
Figure 3. Ideal MSK PSD with 5 MHz RBW
Integrated 5 MHz channel power
-25
-26
-27
-28
-29
-30
-31
-32
-33
-34
-35
4.0
4.2
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
Figure 4. Integrated Adjacent Channel Power (ACP) is -31.2 dBc at 5 MHz offset
Figure 3 shows the same (simulated) ideal MSK signal as Figure 1, but with a higher RBW.
Each frequency point on the curve in Figure 3 is calculated by integrating the power in a 5
MHz band centered on it. Figure 4 shows that if you are transmitting a chosen power level
(e.g. 0 dBm) in one channel, then you are transmitting approximately 31 dB less than this
power level (i.e. -31 dBc) in the adjacent channels (centered on +/- 5 MHz).
This increases the noise floor in the affected channels in the same way as an interferer placed
in the wanted channel (a so called Co-Channel interferer) would, and consequently the CoChannel rejection of a receiver also tells us what the maximum ACR would be if we used an
ideal MSK interferer for the ACR measurement. An ideal MSK interferer source transmitting P
dBm in the adjacent channel will transmit (P-31) dBm in the wanted (Co-) channel; hence the
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Design Note DN030
maximum ACR will equal the Co-Channel rejection + 31dB. Co-Channel rejection is typically
around -3 dB which would give a maximum ACR of 28 dB, but at this point it should be
mentioned that measuring CCR in a DSSS system like an IEEE 802.15.4 receiver calls for
some additional clarification. A DSSS signal will be de-spread in the receiver to achieve the
available code gain, and whether or not you use a properly coded IEEE 802.15.4 signal as
interferer when measuring CCR will determine whether this code gain is included in the CCR
figure or not. The IEEE 802.15.4 code gain is 9dB (chip rate/bit rate =8), which means that if
you use an interferer signal that looks like noise to the receiver (such as a PRBS MSK signal),
the CCR result will be 9 dB better than if you use an interferer with a correctly coded 802.15.4
signal. When placed in the adjacent channel however, a correctly coded IEEE 802.15.4 signal
will not be de-spread (or achieve code gain) in the wanted channel. The above calculation of
the maximum ACR based on Adjacent Channel Power (of the interferer) and Co-Channel
Rejection (of the receiver) was therefore done using an interferer signal that behaves similarly
when used as a Co-channel and Adjacent Channel interferer. When specifying Co-Channel
Rejection however, one may argue that it would be more correct to use a real IEEE 802.15.4
signal as interferer even though this is not specified in [1]
28 dB is an easily attainable ACR, and a good receiver will achieve significantly better
performance than that if the interferer is limited to the adjacent channel. This is the reason
why TI Norway has chosen the E4437B UND MSK waveform for ACR measurements. If an
ACR measurement is limited by the S/N ratio in the wanted channel instead of the receiver’s
ability to filter out noise in the adjacent channel, we are effectively just measuring Co-Channel
rejection once more.
6
ACR Results Using Different Interferer Waveforms
In order to verify how the different wanted- and interferer waveforms impact the results of
selectivity measurements, the same CC2430 device has been measured using SMIQ and
ESG generators in different configurations. The wanted signal has always been generated by
an SMIQ while the interferer signal has been generated by an SMIQ or an ESG.
The results are shown in Figure 5 and Table 2.
A comparison of the two traces, Wanted SMIQ GMSK BT2, Interferer SMIQ MSK and Wanted
SMIQ MSK, Interferer SMIQ MSK shows that for the wanted signal, standard MSK or GMSK
with BT=2 give the same results.
The results also show that the measured ACR depends heavily on the waveform used by the
interferer, and correlate well with the levels of the spectral side lobes shown in Figure 1 and
Figure 2.
The lowest side lobes (UND MSK) give the best results (along with GMSK BT=1, not shown in
Figure 1 or Figure 2).
SMIQ MSK and SMIQ GMSK BT2 give similar ACR results, and they also have similar side
lobe levels (see Figure 2).
The poorest ACR results are obtained when using the ESG MSK waveform as interferer. This
is consistent with the ESG MSK signal having the highest side lobe levels in Figure 1 and
Figure 2.
As explained in section 5, the ACR of a good receiver should approach 28 dB when the
interferer approaches an ideal MSK signal. These results stop at 30 dB using an interferer that
has side lobes a few dB lower than the ideal MSK signal in the adjacent channel (see Figure
2).
The biggest differences are seen in the adjacent- and alternate channels. This suggests that
for channels further away, the signal generators’ BW limitations prevent them from
transmitting noise in the wanted channel.
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Design Note DN030
CC2430 Selectivity results using various wanted- and
interferer waveforms
70
60
50
40
Wanted SMIQ GMSK BT2,
Interferer SMIQ MSK
Wanted SMIQ MSK,
Interferer SMIQ MSK
Wanted SMIQ MSK,
Interferer SMIQ GMSK BT1
Wanted SMIQ MSK,
Interferer SMIQ GMSK BT2
Wanted SMIQ MSK,
Interferer ESG UND
Wanted SMIQ GMSK BT2,
Interferer ESG MSK
30
20
10
0
-10
2420
2425
2430
2435
2440
2445
2450
2455
2460
Figure 5. Different selectivity results from the same device
Frequency
[MHz]
2420
2425
2430
2435
2440
2445
2450
2455
2460
Wanted
SMIQ
GMSK
BT2,
Interferer
SMIQ
MSK
57
54
51
32
-3
34
51
52
59
Wanted
SMIQ MSK,
Interferer
SMIQ MSK
57
54
50
32
-2
34
52
52
59
Wanted
SMIQ MSK,
Interferer
SMIQ GMSK
BT1
57
54
50
35
-3
42
53
52
59
Wanted
SMIQ MSK,
Interferer
SMIQ GMSK
BT2
57
54
50
32
-3
34
51
52
59
Wanted
SMIQ MSK,
Interferer
ESG UND
56
55
51
35
-3
43
53
52
58
Wanted
SMIQ
GMSK
BT2,
Interferer
ESG MSK
56
54
44
30
-3
31
44
50
58
Table 2. ACR results in dB
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Design Note DN030
7
The ESG UND waveform compared to a real 802.15.4 device
As explained in 4.3, we have chosen to use a filtered waveform when generating the
interfering signal in our ACR measurements. But how relevant are these results with respect
to operation in an environment with real 802.15.4 devices? Obviously, all devices will not have
the same TX spectrum, and some will interfere more in neighboring channels than our test
signal does. To give an example of a real device with a TX spectrum similar to the ESG UND
waveform, Figure 6 shows the spectra of a CC2530, the ESG UND waveform and an ideal
MSK signal – all transmitting a pseudo random sequence.
Generated vs. Ideal MSK Spectra
0
Ideal MSK
E4437B UND MSK
-10
CC2530
Relative Power (dB)
-20
-30
-40
-50
-60
-8
-6
-4
-2
0
2
4
6
8
Frequency offset in MHz
Figure 6: CC2530 TX spectrum compared to ESG UND MSK and ideal MSK
Frequency
[MHz]
2420
2425
2430
2435
2440
2445
2450
2455
2460
Wanted SMIQ
MSK, Interferer
CC2530
56
56
54
47
-11
48
55
57
57
Wanted SMIQ MSK,
Interferer ESG UND
57.8
57.0
56.5
48.5
-2.2
47.7
56.6
57.5
58.2
Table 3: CC2530 ACR results in dB when using the CC2530 as interferer
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Design Note DN030
The resulting ACR performance is shown in Table 3 (Note that the results in Table 3 are
measured on CC2530 while the results in Table 2 have been measured on CC2430). Two
important observations can be made when looking at these results.
One is that the Co-channel rejection is 8.8 dB worse when using an 802.15.4 interferer instead
of pseudo random MSK data. The main reason is that the 9 dB code gain of the DSSS
modulation is lost, as discussed in chapter 5.
The other observation is that the difference in Adjacent Channel Rejection when using a
CC2530 device as the interferer compared to when using the filtered waveform of the ESG
UND signal as the interferer, is very small. Compared to an interferer resembling an ideal
MSK signal, the improvement is 17dB. The CC2530 has a significant improvement in ACR
over CC2430, but this would have been hard to measure without a filtered interferer.
These results are only relevant in environments with devices based on the CC2530 or similar
chips. For ZigBee devices that employ a TX spectrum closer to an unfiltered half sine shaped
O-QPSK signal, the interference in the adjacent and alternate channels will be significantly
higher. This will result in lower throughput in areas densely populated by 802.15.4 devices.
The CC2530 results thus show that shaping the TX spectrum to achieve a lower adjacent
channel power will create less interference to other networks and allow a better utilization of
the 2.4GHz ISM band if you need several networks in a limited space and frequency area.
8
Conclusion
As explained in this Design Note, TI Norway has found that the waveforms used when
measuring ACR can greatly influence the results. In order to get consistent results that can
differentiate between receivers with ACR of more than 30 dB in an environment with real
802.15.4 interfering devices, the waveform referred to as ESG E4437B UND MSK in this
document has been chosen as the interferer waveform.
The wanted signal is SMIQ GMSK with BT=2.
Both these waveforms are well within the limits specified for an IEEE 802.15.4 transmitter.
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Design Note DN030
9
[1]
References
IEEE Std 802.15.4™-2003, 1 October 2003
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Design Note DN030
10 General Information
10.1 Document History
Revision
SWRA255
SWRA255A
Date
2009.01.29
2013.01.24
Description/Changes
Initial release.
Added results when using CC2530 as interferer
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