Application Note AN041

Size: px

Start display at page:

Download "Application Note AN041"

Arthur Casey
5 years ago
Views:

1 CC24 Coexistence By G. E. Jonsrud 1 KEYWORDS CC24 Coexistence ZigBee Bluetooth IEEE IEEE 82.11b WLAN 2 INTRODUCTION This application note describes the coexistence performance of the CC GHz transceiver when operating in an environment together with Bluetooth, WLAN or other ZigBee devices. The coexistence performance, which is described by parameters such as the adjacent and alternate channel rejection, is particularly important when selecting the right transceiver for a crowded radio environment. The CC24 exhibits excellent rejection of interfering transmitters. The CC24 is a true single-chip RF transceiver intended for low power wireless networks operating in the 2.4 GHz SRD band compliant to the ZigBee and IEEE standards. Application Note AN41 (Rev. 1.) SWRA94 Page 1 of 22

2 Table of Contents 1 KEYWORDS INTRODUCTION ABBREVIATIONS TEST DESCRIPTION TEST RESULTS ZIGBEE INTERFERER BLUETOOTH INTERFERER WLAN INTERFERER RANGE CONSIDERATIONS ZIGBEE VS. ZIGBEE BLUETOOTH VS. ZIGBEE WLAN VS. ZIGBEE ZIGBEE IN A WLAN ENVIRONMENT... 7 CONCLUSION REFERENCES GENERAL INFORMATION DOCUMENT HISTORY Application Note AN41 (Rev. 1.) SWRA94 Page 2 of 22

3 3 ABBREVIATIONS CC24EM CCEB CRC CSMA-CA DSSS IEEE PER PSDU SFD SRD WLAN CC Evaluation Module CC Evaluation Board (used with CC24 EM) Cyclic Redundancy Check Carrier Sense Multiple Access Collision Avoidance Direct Sequence Spread Spectrum Institute of Electrical and Electronics Engineers Packet Error Rate PHY Service Data Unit Start of Frame Delimiter Short Range Device Wireless Local Area Network Application Note AN41 (Rev. 1.) SWRA94 Page 3 of 22

4 4 TEST DESCRIPTION An Agilent N1 wireless connectivity test set was used to generate the interfering signal, either ZigBee / IEEE , Bluetooth / IEEE or WLAN / 82.11b modulated signals. Another signal generator, ANRITSU MG3681A with Rohde & Swartz AMIQ baseband generator, transmitted the desired signal, i.e. IEEE /ZigBee packets. The desired signal level was kept at -82 dbm while the interfering signal level was increased until the packet error rate (PER) reached just below 1 %. This is in accordance with the receiver jamming resistance testing described in [1]. The test setup used is shown in Figure 1. The test packet consisted of preamble, synchronisation word (SFD), length field and a byte pseudo random payload (PSDU). A computer program calculated PER in per cent as the sum of missing packets and CRC errors divided by the number of transmitted packets. RF GENERATOR RF GENERATOR Ext. 1 in Ext. 1 in RF Out RF Out COMBINER 1 2 Sum CCEB + CC24EM IBM PS/2 Figure 1: Test setup for jamming resistance. The ZigBee and Bluetooth modulated interferer was set to transmit continuously while the WLAN interferer was set to send short packets with a 9 % duty cycle. The ZigBee interferer spectrum is shown in Figure 2, the Bluetooth spectrum in Figure 3 and the WLAN spectrum in Figure 4. Figure 5 shows transmit power vs. time for the 9% duty cycle WLAN signal. Application Note AN41 (Rev. 1.) SWRA94 Page 4 of 22

5 Figure 2: ZigBee / IEEE spectrum. Figure 3: Bluetooth / IEEE spectrum. Application Note AN41 (Rev. 1.) SWRA94 Page 5 of 22

6 Figure 4: WLAN, IEEE 82.11b spectrum. Figure 5: WLAN signal with 9% duty cycle and 27 µs between packets. Application Note AN41 (Rev. 1.) SWRA94 Page 6 of 22

7 5 TEST RESULTS The plots included in this chapter show CC24 s rejection of the interfering signal in receive mode. The interferer frequency is shown in MHz on the x-axis and the interferer rejection along the y-axis. Rejection = interferer level desired signal level. The desired signal level is calibrated to -82 dbm at the input of CC24EM. 5.1 ZigBee interferer The characterisation of CC24 with a ZigBee interferer was performed with the interferer constantly turned on (1 % duty cycle), transmitting a pseudo random data pattern. In a real situation, where the interferer does not have a 1 % duty cycle, the interferer and desired signal will not always overlap in time. The frequency and level of the desired signal is kept constant for each plot. The ZigBee interfering signal is swept over all channels except the desired channel and the signal level is increased to the highest level where the PER is still <1 %. Figure 6 shows the rejection of a ZigBee interferer with the desired signal at 25 MHz. Figure 7, Figure 8 and Figure 9 show the rejection of a ZigBee interferer with the desired signal at 243, 2455 and 248 MHz respectively. The measured results show slightly poorer results for + 5MHz compared to the data sheet. The reason is that the side lobes of the interfering signal is only about db lower than the peak of the signal, as can be seen in Figure 2. Hence there is a significant level of in-band power when measuring adjacent channel rejection. The data sheet measurements reflect the true rejection performance by filtering the sidelobes of the interferer Rejection of ZigBee interferer: desired 25 MHz and ZigBee interferer swept in 5 MHz steps Figure 6: Rejection of ZigBee interferer, desired 25 MHz. Application Note AN41 (Rev. 1.) SWRA94 Page 7 of 22

8 Rejection of ZigBee interferer: desired 243 MHz and ZigBee interferer swept in 5 MHz steps Figure 7: Rejection of ZigBee interferer, desired 243 MHz Rejection of ZigBee interferer: desired 2455 MHz and ZigBee interferer swept in 5 MHz steps Figure 8: Rejection of ZigBee interferer, desired 2455 MHz. Application Note AN41 (Rev. 1.) SWRA94 Page 8 of 22

9 Rejection of ZigBee interferer: desired 248 MHz and ZigBee interferer swept in 5 MHz steps Figure 9: Rejection of ZigBee interferer, desired 248 MHz. 5.2 Bluetooth interferer The characterisation of CC24 with a Bluetooth interferer was performed with the interferer constantly turned on, transmitting a pseudo random data pattern. In a real situation the interferer will transmit packets and hop between 79 channels so that the interferer and desired signal will have little overlap in frequency and time. The frequency and level of the desired signal is kept constant for each plot. The Bluetooth interfering signal is swept across all channels in 1 MHz steps except the desired channel and +/- 1 MHz from the desired channel. The signal level is increased to the highest level where the PER is still <1 %. Figure 1 shows the rejection of a Bluetooth interferer with the desired signal at 25 MHz. Figure 11, Figure 12 and Figure 13 show the rejection of a Bluetooth interferer with the desired signal at 243, 2455 and 248 MHz respectively. Application Note AN41 (Rev. 1.) SWRA94 Page 9 of 22

10 Rejection of Bluetooth interferer: desired 25 MHz and Bluetooth interferer swept in 1 MHz steps Figure 1: Rejection of Bluetooth interferer, desired 25 MHz Rejection of Bluetooth interferer: desired 243 MHz and Bluetooth interferer swept in 1 MHz steps Figure 11: Rejection of Bluetooth interferer, desired 243 MHz. Application Note AN41 (Rev. 1.) SWRA94 Page 1 of 22

11 Rejection of Bluetooth interferer: desired 2455 MHz and Bluetooth interferer swept in 1 MHz steps Figure 12: Rejection of Bluetooth interferer, desired 2455 MHz Rejection of Bluetooth interferer: desired 248 MHz and Bluetooth interferer swept in 1 MHz steps Figure 13: Rejection of Bluetooth interferer, desired 248 MHz. Application Note AN41 (Rev. 1.) SWRA94 Page 11 of 22

12 5.3 WLAN interferer The characterisation of CC24 with a WLAN (IEEE 82.11b) interferer was performed with the interferer transmitting short packets with pseudo random data at a 9 % duty cycle as shown in Figure 5. The frequency and level of the desired signal is kept constant for each plot. The WLAN interfering signal is swept over channels 1 through 11 and also channel 14. The signal level is increased to the highest level where the PER is still <1 %. Figure 14 shows the rejection of a WLAN interferer with the desired signal at 25 MHz. Figure 15, Figure 16 and Figure 17 shows the rejection of a WLAN interferer with the desired signal at 243, 2455 and 248 MHz respectively. As can be seen in Figure 4, the WLAN spectrum is as wide as to overlap the ZigBee / IEEE channel when the two carriers are less than approximately 12 MHz away. This overlapping channel explains the relatively low rejection of the WLAN signal for carriers that are close in frequency. For WLAN interferers at +/- 12 MHz or further from the desired channel, the rejection is better than 35 db. It is better than 55 db for channels at +/- 22 MHz or larger offsets and for +/- 33 MHz and further, the rejection is better than db. 7 Rejection of WLAN interferer: desired 25 MHz and WLAN interferer swept across channels 1 through 11 and Figure 14: Rejection of WLAN interferer, desired 25 MHz. Application Note AN41 (Rev. 1.) SWRA94 Page 12 of 22

13 Rejection of WLAN interferer: desired 243 MHz and WLAN interferer swept across channels 1 through 11 and Figure 15: Rejection of WLAN interferer, desired 243 MHz. 7 Rejection of WLAN interferer: desired 2455 MHz and WLAN interferer swept across channels 1 through 11 and Figure 16: Rejection of WLAN interferer, desired 2455 MHz. Application Note AN41 (Rev. 1.) SWRA94 Page 13 of 22

14 Rejection of WLAN interferer: desired 248 MHz and WLAN interferer swept across channels 1 through 11 and Figure 17: Rejection of WLAN interferer, desired 248 MHz. Application Note AN41 (Rev. 1.) SWRA94 Page 14 of 22

15 6 RANGE CONSIDERATIONS In this section the test results are summarised and interpreted for practical applications. There are several techniques and methods available to system engineers to enhance coexistence between different systems. These techniques will be based on differences in frequency, time, space, modulation or coding or a combination of these. This application note focuses on frequency division. The interferer is set to transmit continuously or almost continuously to representing a worst case interferer situation. Friis equation for free space radiation, Equation 1, is used to estimate radio ranges. P r is the received signal level, P t is the transmitted signal level, and G t and G r is the transmitter and receiver antenna gains respectively. λ is the wavelength and d is the distance between the transmitter and the receiver. When the received signal level is equal to the sensitivity, the corresponding distance is the maximum possible communication distance between the receiver and the transmitter, i.e. the radio range. P r = PG G λ 2 t t r 2 2 ( 4π ) d Equation 1: Friis formula for free space propagation. The free space estimates are only reasonable for a few meters ([1] uses it up to 8 meter for their channel model) and in line of sight when applied indoors. For all calculations the antenna gains are assumed to be db. The interferer effectively degrades the sensitivity of the receiver. The rejection performance is measured with the desired signal level at -82 dbm. If the interfering signal level is higher than -82 dbm at the receiver input, the sensitivity is degraded. The received power is calculated using Friis formula. The rejection is characterised in the previous chapters. If the received power is -5 dbm, and the rejection 35 db, the result is -5 dbm 35 db = -85 dbm, i.e. less than -82 dbm. In this case, the interferer has no influence on the desired signal reception. If the received power is -21 dbm and rejection is 55 db, the result is -76 dbm. This is higher than -82 dbm and the effective sensitivity is degraded by 6 db. In the following sections the minimum distance between interfering transmitter and desired signal receiver is estimated. A 6 db degradation of sensitivity means that the desired signal level must be 6 db stronger for the communication distance to remain the same at 1% PER. 6 db requires a quadrupling of the power level. According to Friis equation a quadrupling of the received power requires a reduction of the original distance with a factor 2. This is a rule of thumb that gives reasonable estimates when based on empirical range measurements. An example: if you experience a meter range indoors with your ZigBee device, a 6 db degradation of the sensitivity level will reduce the range to about meter. The following sections also contain graphs showing expected free space range vs. distance between the interferer and the desired signal receiver. The graphs are calculated based on 123 meter as the maximum interference free range. The minimum distance between interferer and receiver for no degradation in transmission is calculated. For each halving of the interferer to receiver distance, the range of the desired signal is halved. No interference, dbm transmit power and -82 dbm minimum receiver input level give a maximum range of 123 meter. The -82 dbm is the level of the desired signal as defined in [1] when performing rejection measurements. Note that the sensitivity of CC24 is typically -95 Application Note AN41 (Rev. 1.) SWRA94 Page 15 of 22

16 dbm and outdoor line of sight range is measured to be more than meter with 1.6 db antenna gain on receiver and transmitter. The graphs can be an aid in practical installations. In instances where a ZigBee transceiver must be installed close to an interferer another ZigBee device can be installed further from the interferer to extend the range of the close-by device, i.e. act as a repeater. The following example illustrates this. According to Figure 23: Range vs. distance between WLAN interferer and receiver of desired signal, 35 db rejection., a ZigBee device 1 meter from a WLAN device will achieve a range of approximately 1 meter. Installing a second ZigBee device as a repeater 1 meter from the first device will extend the range. Interferers can be overcome by a higher density of nodes. There are several mechanisms that reduce the effect of an interferer. This application note considers only difference in frequency. CC24 offers excellent rejection of interferers just a few MHz from carrier and higher rejection at larger offsets. For interferers that operate at or close to the desired signal frequency, other mechanisms aid in improving coexistence. DSSS and CSMA-CA are implemented both in ZigBee and WLAN. Frequency hopping is used in Bluetooth. 6.1 ZigBee vs. ZigBee The rejection of a ZigBee interferer is better than 35 db of an interferer at the adjacent channel (+/-5 MHz) and better than 5 db at the alternate channel (+/-1 MHz). For interferers located at or further away than +/-15 MHz from the desired channel, the rejection is more than db. Assuming that a typical ZigBee interferer transmits dbm of output power, the minimum distance between an interfering transmitter and the receiver that does not result in increased PER can be calculated. The results are listed in Table 1. Difference between desired signal and interferer. Rejection at the frequency offset. Minimum distance interferer and receiver. +/- 5 MHz 35 db 2.2 m +/- 1 MHz 5 db.4 m +/- 15 MHz db 12 cm Table 1: Minimum ZigBee interferer to receiver distance for no impact on packet losses. ZigBee interferer offset +/- 5 MHz, 35 db rejection Range [m] Distance between RX and ZigBee interferer [m] Figure 18: Range vs. distance between ZigBee interferer and receiver of desired signal, 35 db rejection. Application Note AN41 (Rev. 1.) SWRA94 Page 16 of 22

17 ZigBee interferer offset +/- 1 MHz, 5 db rejection Range [m] Distance between RX and ZigBee interferer [m] Figure 19: Range vs. distance between ZigBee interferer and receiver of desired signal, 5 db rejection. ZigBee interferer offset +/- 15 MHz, db rejection Range [m] Distance between RX and ZigBee interferer [m] Figure : Range vs. distance between ZigBee interferer and receiver of desired signal, db rejection. 6.2 Bluetooth vs. ZigBee For Bluetooth interferers at +/-5 MHz or more from the desired signal the rejection is better than 35 db. The rejection is typically better than db of interferers at +/- 1 MHz or larger offsets. A typical transmitter level for a Bluetooth class 2 device is dbm. The minimum distance between interfering transmitter and desired receiver in this case is listed in Table 2. Application Note AN41 (Rev. 1.) SWRA94 Page 17 of 22

18 Difference between desired signal and interferer. Rejection at the frequency offset. Minimum distance interferer and receiver. +/- 5 MHz to +/- 1 MHz 35 db 2.2 m +/- 1 MHz db 12 cm Table 2: Minimum Bluetooth interferer to receiver distance for no impact on packet losses. Bluetooth interferer offset +/- 5 MHz, 35 db rejection Range [m] Distance between RX and Bluetooth transmitter [m] Figure 21: Range vs. distance between Bluetooth interferer and receiver of desired signal, 35 db rejection. Bluetooth interferer offset +/- 1 MHz, db rejection Range [m] Distance between RX and Bluetooth transmitter [m] Figure 22: Range vs. distance between Bluetooth interferer and receiver of desired signal, db rejection. Application Note AN41 (Rev. 1.) SWRA94 Page 18 of 22

19 6.3 WLAN vs. ZigBee Application Note AN41 For WLAN interferers at +/- 12 MHz or further from the desired channel, the rejection is better than 35 db. It is better than 55 db for channels at +/- 22 MHz or larger offsets and for +/- 33 MHz and further, the rejection is better than db. A typical transmitter level for a WLAN node is 14 dbm [1]. The minimum distance between interfering transmitter and desired receiver in this case are listed in Table 2. Difference between desired signal and interferer. Rejection at the frequency offset. Minimum distance interferer and receiver. +/- 12 MHz to +/- 22 MHz 35 db 11. m 1 +/- 22 MHz to +/- 33 MHz 55 db 1.1 m +/- 33 MHz db 62 cm Table 3: Minimum WLAN interferer to receiver distance for no impact on packet losses. WLAN interferer offset +/- 12 MHz offset, 35 db rejection Range [m] Distance between RX and WLAN transmitter [m] Figure 23: Range vs. distance between WLAN interferer and receiver of desired signal, 35 db rejection. 1 This distance is significantly smaller in a real environment as Friis equation does not include fading losses. Application Note AN41 (Rev. 1.) SWRA94 Page 19 of 22

20 Closest WLAN offset +/- 22 MHz, 55 db rejection Range [m] Distance betw een RX and WLAN transmitter [m] Figure 24: Range vs. distance between WLAN interferer and receiver of desired signal, 55 db rejection. WLAN interferer offset +/- 33 MHz, db rejection Range [m] Distance between RX and WLAN transmitter [m] Figure 25: Range vs. distance between WLAN interferer and receiver of desired signal, db rejection. 6.4 ZigBee in a WLAN environment The IEEE 82.11b standard defines 14 channels with overlap. The power spectral density (dbm/hz) drops rapidly at +/-11 MHz from the carrier frequency. There is still power at higher offsets, but channels with more than 22 MHz difference between carriers are considered nonoverlapping and will give a situation with minimum interference. From MHz to MHz there is only room for 3 channels without overlap. Figure 26 shows the optimum channel selection for North America and Figure 27 the selection for Europe. Figure 28 shows the possible ZigBee / IEEE channels. The following IEEE channels falls clear of the North American channel selection: 15,, 25 and 26. The corresponding channels in Europe are 15, 16, 21 and 22. All these channels are at least 12 MHz offset from the WLAN carrier, hence the receiver rejection is at least 35 db of the WLAN signal. If there is one WLAN installed, db rejection can be achieved or 55 db with two WLANs interfering. Application Note AN41 (Rev. 1.) SWRA94 Page of 22

21 Channel Frequency [MHz] Table 4: IEEE 82.11b channels [2] Figure 26: IEEE 82.11b North American channel selection (non-overlapping) [1]. Figure 27: IEEE 82.11b European channel selection (non-overlapping) [1]. Figure 28: IEEE channel selection [1]. Application Note AN41 (Rev. 1.) SWRA94 Page 21 of 22

22 7 CONCLUSION CC24 demonstrates excellent blocking performance, which is important in order to operate reliably in the presence of interfering 2.4 GHz systems such as Bluetooth, and WLAN. This application note shows the test results obtained and interprets the results for use in practical installations. 8 REFERENCES 1. IEEE Std IEEE Std 82.11b-1999/Cor GENERAL INFORMATION 9.1 Document History Revision Date Description/Changes Initial release. Application Note AN41 (Rev. 1.) SWRA94 Page 22 of 22

23 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are sold subject to TI s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI s standard warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where mandated by government requirements, testing of all parameters of each product is not necessarily performed. TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and applications using TI components. To minimize the risks associated with customer products and applications, customers should provide adequate design and operating safeguards. TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right, or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI. Reproduction of information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive business practice. TI is not responsible or liable for such altered documentation. Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not responsible or liable for any such statements. Following are URLs where you can obtain information on other Texas Instruments products and application solutions: Products Applications Amplifiers amplifier.ti.com Audio Data Converters dataconverter.ti.com Automotive DSP dsp.ti.com Broadband Interface interface.ti.com Digital Control Logic logic.ti.com Military Power Mgmt power.ti.com Optical Networking Microcontrollers microcontroller.ti.com Security Low Power Wireless Telephony Video & Imaging Wireless Mailing Address: Texas Instruments Post Office Box Dallas, Texas Copyright 6, Texas Instruments Incorporated

THE GC5016 AGC CIRCUIT FUNCTIONAL DESCRIPTION AND APPLICATION NOTE

THE GC5016 AGC CIRCUIT FUNCTIONAL DESCRIPTION AND APPLICATION NOTE Joe Gray April 2, 2004 1of 15 FUNCTIONAL BLOCK DIAGRAM Nbits X(t) G(t)*X(t) M = G(t)*X(t) Round And Saturate Y(t) M > T? G(t) = G 0 +A(t)