Co-existence. DECT/CAT-iq vs. other wireless technologies from a HW perspective

Transcription

1 Co-existence DECT/CAT-iq vs. other wireless technologies from a HW perspective Abstract: This White Paper addresses three different co-existence issues (blocking, sideband interference, and inter-modulation) and presents potential solutions for reducing the effect of these on the overall wireless performance of the systems that have to co-exist. The discussions in this White Paper have a DECT/CAT-iq focus, but the co-existence issues and potential countermeasures are relevant to wireless systems in general.

2 Contents 1 Preface Purpose Scope Co-existence Separation in time Blocking Phenomena Countermeasure Sideband interference Phenomena Countermeasure Inter-modulation Phenomena Countermeasures Examples DECT and Wi-Fi DECT and Bluetooth DECT and GSM/PCS Strategy for design for co-existence Co-existence design Conclusions...16 Glossary...17 Appendix A: Free space attenuation

3 1 Preface The number of different wireless systems around us is increasing rapidly, and it is likely that these systems, if not designed correctly, will have a negative impact on the total wireless performance. Coexistence, i.e. having several different wireless systems to operate in the same device or in the vicinity of each other, is therefore a key issue in robust and plug-n-play system designs especially for products targeted at the consumer space. 1.1 Purpose This white paper is intended to give a description of the various problems that can arise when multiple systems are to co-exist either in the same device or close to each other. The primary purpose of this White Paper is to ensure that co-existence problems are taken into account early in the product development phase. 1.2 Scope In the first part of the White Paper three different types of co-existence problems are described and typical countermeasures are outlined. Next some examples of co-existence with DECT/CAT-iq as the point of departure are discussed in relation to co-existence issues. The White Paper is concluded by outlining a guideline for how to structure the co-existence design to ensure a robust design that accounts for potential co-existence issues. 2

4 2 Co-existence If systems co-exist nicely there will be no degradation of performance issues when the two systems are used simultaneously. However, many wireless systems today use the same frequency band (e.g. the 2.4 GHz ISM band) or a frequency band quite close to each other. If the various systems are located far from each other there are normally no co-existence problems, but today the trend is to integrate two or more different wireless systems into the same device a perfect example is an IAD (Integrated Access Device). Consequently, the attenuation between the systems is reduced and if the antennas for the various systems are located close to each other the coupling between the antennas can be high. It is also important to emphasize here that the type approval specification for the various technologies is not enough to ensure co-existence. Therefore, even if the various systems are fulfilling their respective specifications they still can interfere and cause performance degradation in other wireless systems. In order to reduce potential co-existence issues it is paramount to address co-existence in the design of devices and products. 2.1 Separation in time The primary problem for two systems to co-exist is the situation in which one system transmits while the other system receives. Hence, a solution of having several systems to co-exist is to operate the systems in a manner in which the transmitter in one system is inactive while the other system is in the receive state. One way is to make sure that the systems are not operating simultaneously, hence leading to collisions. For TDD systems a possible approach is to synchronize the systems. This, however, requires the frame period to be equal (preferable) or a multiple of each other. If the systems are synchronized they can be scheduled to transmit and receive in different time-slots, hence avoiding collisions. This approach is particular useful in situations in which multiple devices of the same type is to be colocated. Therefore this approach is also the preferred solution for large DECT systems, hence synchronizing all the base-stations (and thus the handsets). Using this approach will also improve the average system capacity with a factor of two (and furthermore allows for seamless hand-over among the base-stations). This approach can, however, be very cumbersome or even impossible in many system designs either because of HW limitations or technology characteristics. For example, in Wi-Fi there is no frame timing since Wi-Fi uses a Carrier Sense before transmission approach. Thus it is not possible to synchronize a Wi-Fi system to e.g. a DECT or Bluetooth system. The carrier sense can, however, be used to cease Wi-Fi transmission while the other system receives, but this only solves a part of the 3

5 co-existence problem since this will degrade the throughput of the Wi-Fi system. Therefore alternative solutions are needed in these kinds of situations. 4

6 3 Blocking Blocking is defined as interference by a strong and out-of-band interferer that degrades the sensitivity of the system. 3.1 Phenomena Blocking occurs in the situation in which an interfering signal is so strong that it degrades the sensitivity of the system. The typical reason for the degradation in performance during blocking is that an amplification stage in the receiver chain is put into saturation, hence reducing the gain and increasing the noise figure of the complete receiver chain. In a typical receiver there are several filters, however, it is best to remove the unwanted signals as early as possible in the receiver chain to avoid saturation in the stages. Having all the filtering in the front-end will unfortunately also increase the noise figure since will lead to a too high insertion loss before the LNA. As it is important that a good system is immune to blocking, e.g. the DECT/CAT-iq specification has some requirements to be fulfilled for type approval. More than 100 MHz away from the DECT band ( MHz) the DECT receiver shall meet a sensitivity of at least -80 dbm with an interfere level of -23 dbm. However, as DECT receivers today have sensitivity levels of up to -95 dbm the allowed degradation is quite high, since this corresponds to a range of only 25-50% of the uninterfered range. The level of -23 dbm can also easily be found around from interferers - e.g. a GSM handset which transmits at full power (+33 dbm) will give an interferer level of -23 dbm at a distance of 16m (see Appendix A for calculation of free space loss). Thus a GSM handset can seriously degrade the range from a distance of 16m. Amplitude Interferer Wanted signal Raised noise floor due to reduced gain in receiver Interferer blocks the wanted signal due to its signal strength Noise floor Frequency 5

7 3.2 Countermeasure Since the blocking signal is a wanted signal from the interferer it cannot be filtered away in the transmitter of the interferer. However, the blocking is caused by an out-of-band interferer, and hence, it can be filtered at the receiver input instead. As always it is easier to remove interferers if they are located far away in frequency. E.g. for DECT it is straight forward to reject the 900 MHz GSM, but the 1.8 GHz GSM is quite difficult to remove as the downlink frequency band is adjacent to the DECT frequency band. As the bands are so close a very high Q (Quality factor) is needed in the filter, hence making the filter very expensive (and bulky). To remove interferers close to the wanted signal (in frequency) a more linear receiver has to be used. To lower the requirement for the Q of the filter the relative distance between the interferer and wanted signal has to be reduced. This can be done by down mixing the signal (and interferer) to a lower frequency. At this IF (intermediate frequency) a filter (e.g. SAW filter) is used to reject the interference. In modern Low-IF/Zero IF in which the IF frequency is located very close to DC to allow the use of active filters there is often more gain before the interferes are removed. However increasing the current in the mixer and filter blocks can improve blocking. The price for the increase in current is higher power consumption. If the signal is not removed before the LNA but later in the receiver chain, it is important to have adequate linearity in the front-end in order to avoid saturation before the filter in which the blocker is removed. If the LNA/mixer is saturated the gain will drop and thus increase the noise figure. In some situations it can make sense to have a high current LNA before the first filter to improve the noise figure. 6

8 4 Side-band interference Sideband interference is defined as reduction in sensitivity caused by out-of-band emission from the transmitter. This type of interference is not as intuitive as blocking, but it is often a bigger interference problem. 4.1 Phenomena All signals have energy outside the used bandwidth. This energy can be both phase noise, thermal noise and broadband switching noise for digital circuitry. A part of this energy or noise will be present in-band on the co-located system(s). This noise will raise the noise floor on the other systems and degrade their sensitivity. Even if systems pass type approval and fulfil all specifications, systems can cause interference. Most ETSI specifications (Europe) allow up to -30 dbm in 3 MHz bandwidths outside the desired band. For DECT, where the noise floor is approximately -113 dbm, this means that a -30 dbm interferer in the DECT band will degrade the sensitivity of DECT up to 64 meters away in free space. Amplitude Wanted signal SNR with side-band interference Interferer Normal Signal-to- Noise Ratio (SNR) Side-band interference Noise floor Frequency 4.2 Countermeasure Since sideband interference is in-band it cannot be filtered at the interfered system. The only way to remove the interference is either to add more attenuation between the systems (e.g. increase distance between the systems) or filter the output signal of the interfering systems. 7

9 5 Inter-modulation Inter-modulation occurs in the situation in which two signals are mixed together resulting in other signal frequencies. 5.1 Phenomena The typical problems are associated with third order non-linearity products in which the interferer signal has a frequency of 2*f1-f2 or 2*f2-f1, where f1 and f2 are the frequencies of the two interferers. In typical situations the two interferer has is close in frequency so the intermodulaiton products will look like side tones in the spectrum. If these sidetones are located at the frequency of the wanted signal we will see intermodulation interference. As the interferer signal can be close to the two interferer signals the interferer is often devices of the same type. Due to the nature of third order products the inter-modulation products will increase by a factor of 3 compared to the interfering signal. Thus the inter-modulation products will typically be generated by blocks down the receiver chain after the input signals have been amplified. Another situation in which inter-modulation can become a problem is e.g. when there is a strong and wide CDMA signal just outside the DECT band. The CDMA signal can be considered to have many signal frequencies and the inter-modulation distortion will give the CDMA signal shoulders in a nonlinear receiver. These shoulders can increase the noise floor in the DECT frequency band and degrade the sensitivity of the DECT receiver. 8

10 Undesired signal (3rd order) Frequency:(2*f 1 -f 2 ) Desired signal (f 1 ) Desired signal (f 2 ) Undesired signal (3rd order) Frequency:(2*f 2 -f 1 ) Amplitude Frequency 5.2 Countermeasure To solve inter-modulation problems the intermodulation product has to be removed. One method is to filter away one or two or the interferers before they enter the block with the most non-linearity affect. Super-heterodyne receivers do this by having a channel filter at the first IF to remove potential interferers. In modern Low-IF or Zero-IF receivers the main contributor to the intermodulation performance (the input IP3) is the mixer and/or the active filter after the mixer. Here it would require an extra IF with LO and mixers to be able to remove the interferers. In this situation it can be necessary to increase the current in some of the RX blocks to improve their IP3 performance. 9

11 6 Examples In this chapter some examples of co-existing system are described. 6.1 DECT and Wi-Fi As already suggested co-existence problems between DECT and Wi-Fi can exist especially if these two technologies are integrated into the same device (e.g. an IAD). Thus the antennas are placed quite close to each other and, consequently, there is often as little as 20 db attenuation between the antennas. Measurements on typical Wi-Fi solutions show that they have a peak sideband power of approx -60 dbm in the DECT band. This is measured in 1 MHz bandwidth, which is similar to the DECT bandwidth. With only 20 db isolation in the antenna the DECT receiver will see a noise of -80 dbm, which is well above the thermal noise floor of -113 dbm/mhz. Thus the DECT sensitivity can be degraded to -70 dbm. Since the problem is sideband interference the noise has to be filtered away between the Wi-Fi device and the Wi-Fi antenna. There is a requirement of attenuation of 35 db in the 1.9 GHz band if the DECT performance is to be unaffected. As Wi-Fi often is a module with antenna connector integrated it can be difficult to add an extra filter. Therefore, in an integrated DECT/Wi-Fi device it is wise to consider how the filtering on the Wi-Fi part can be implemented as early on in the development phase as possible. The DECT signal does not normally have a strong sideband power, so the Wi-Fi part will mostly be interfered by blocking from DECT, but here the filter used in the Wi-Fi front-end will be sufficient. Furthermore, the Wi-Fi receiver is typically more linear so blocking from DECT is normally not an issue. However, it is always desirable to have as low coupling as possible between the antennas. Placing them on opposite sides of the device will normally be a good solution as the device it self has some shielding effects. In real life this can be hard or even impossible to achieve. Furthermore, in worst case scenarios some obstructions around the device can also reduce the attenuation between the two antennas. 10

12 Band-pass filter 6.2 DECT and Bluetooth Bluetooth is widely used as technology for headsets to mobile phones. Also DECT business phones with Bluetooth headset profiles exist on the market. As both the DECT and Bluetooth have to be operating in the same small handset, some co-existence issues can be foreseen. Normally the Bluetooth module is a Class 2 device with 0 to 4 dbm output power. By placing the DECT antenna(s) and the Bluetooth antenna in separate ends of the phone, the isolation between the antennas can be in the range of 15 db. This number can in free field be approx. 25 db, but with surroundings (hands, heads, tables etc) the isolation can drop to approx. 15 db depending on the position and angle of the handset. However, since DECT and Bluetooth is located approx. 500 MHz apart in frequency it helps on the blocking performance. Many standard DECT receivers are able to have full sensitivity with a blocking level of -15 to -20 dbm interferer in the 2.4 GHz band. The DECT signal will be worse to the Bluetooth receiver. A common solution to this is to use a combined balun and filter in the receiver front-end of the Bluetooth device. Baluns with good rejection in the 1.9 GHz frequency band exists, and these can be used to suppress both DECT and cellular interference. In essence these filters are also needed for Bluetooth receivers in cellular handsets. 11

13 Band-pass filter 6.3 DECT and GSM/PCS Several types of co-existence problems between DECT and GSM can be encountered. One of the problems being, that the GSM1800 downlink band ( MHz) is located just next to the DECT band in Europe. In the USA the UPCS/DECT 6.0 band ( MHz) is located in the duplex band for the PCS band. PCS has uplink at MHz and downlink at MHz. The interference to the DECT system can be generated both by the basestations and the mobile devices. The GSM basestations are using a high output power and has a high antenna gain. A typical DCS basestation will have an EIRP of 58 dbm. So even at 250m distance there can be an interferer level of -32 dbm. And this signal can be placed just outside the DECT frequency band. In many modern DECT RF circuits interferer levels of -30 to -35 dbm in the DCS band can degrade the sensitivity. In many practical situations it helps that the DECT units are located inside a building since the DCS level is reduced, but the DCS interference must still be considered for outdoor DECT deployment. Furthermore, since the DCS band is very close to the DECT band it is difficult to remove all the DCS interference in a filter in the DECT RX path. With SAW filters on the DECT signal, half of the DCS downlink band can be removed. This solution is adequate in many situations but it cannot solve all DCS interference problems. The optimal solution is to make a very linear front-end in the DECT receiver. However, this is not possible with the current single chip radios today, since the radio must be separated from the amplifiers, mixer and filters. It is necessary to have very linear blocks until a channel filtering is done which has to take place at an IF. 12

14 There can also be co-existence problems between DCS handsets and DECT. But at least in Europe the DCS uplink frequency is placed 90 MHz further away from the DECT band so the filter does have some effect. Furthermore, a GSM handset does not always have an active call while the basestations in the GSM/DCS system always be will active on at least one of the 8 timeslots. So if a DECT and DCS system is to operate close to each other (or in the same unit) a filter would probably be required. However, since the DCS uplink traffic is more than 90 MHz away it is practical to have a protecting filter on the DECT transceiver. 13

15 7 Strategy for design for co-existence In this section some guidelines on how to make a structured effort when doing design for co-existence is given. 7.1 Co-existence design When two systems are to co-exist close to each other the following approach can be used to ensure that the systems operate with the expected performance. The two main issues are blocking and sideband noise. Intermodulation will normally not be an issue with co-locating two systems in the same device as there is a need for two signals (or a single wideband signal) close to the wanted signal. 1. Antenna placement The first task is to place the antennas in the system to maximize the isolation between them. It is often a good idea to place them as far away from each other as possible. It can also be wise to place two antennas on opposite sides of the device with the antenna pattern pointing in opposite directions. Several db s extra isolation can easily be obtained by carefully placing the antennas thus reducing the filter requirements. 2. Determine coupling between systems The coupling between the two antennas should be measured. Here it is important to measure in a realistic environment and not just a free space measurement. Objects close to the device could increase the coupling significantly. 3. Determine output power for each system For each of the systems determine the output power on the antennas in order to determine whether blocking is an issue or not. 4. Determine blocking performance The blocking performance is measured for both systems at the frequency of the other system. A way to measure the blocking performance is to perform tests by increasing the signal strength of the other system in order to determine how strong a signal the receiver can tolerate while still maintaining the desired sensitivity. Knowing these values it can be seen if the interference (output power minus isolation) is above the blocking level. If this is the case the level of required filtering can be determined. The 14

16 type approval specification is not sufficient for avoiding blocking performance, since this states test scenarios that are performed far above the sensitivity of the system. On the other hand, however, the receiver might be much more robust at the given frequency than required by the specification. 5. Measure the out of band emission for the two systems. The out-of-band emission is used to ensure that there is no side-band interference. It is the maximum power measured in the receiver band of the other system. It is preferable to do the measurement with a resolution bandwidth equal to the receiver bandwidth. It is often necessary to measure this signal as the type approval specification or technical specification gives a too high value. In order to avoid degration of the sensitivity the out-of-band emission minus the isolation should be less than the noise-floor (which is approx dbm/mhz). From this it can be seen whether or not a filter is required in the transmitter. Amplitude Margin for blocking Blocking curve Side-band emission Noise floor Wanted signal Other system Frequency Margin for side-band 15

17 8 Conclusions Having multiple systems operate in the vicinity of each other can cause problems, if these systems are not designed correctly. The transmitted signal from one of the systems can degrade the sensitivity of the receiver in another system and, furthermore, several types of interference exist in a situation with multiple systems close to each other. One interference type is blocking; here the wanted TX signal from one system causes interference in other systems. A second type is sideband noise a situation in which the transmitted signal has a high out-of-band emission level leading to an increase in the noise floor of the receiver of the other systems in the vicinity. A third interference type is intermodulation. The three types of interference problems have to be solved differently. In the case of blocking it is necessary to filter the blocked receiver, while it is necessary to filter the transmitter when out-of-band emission is the problem. A 5-step guideline for doing design for co-existence has been given in this white paper. A general rule in co-existence issues is: As long as there is a certain distance in frequency between the systems it is always possible to make them co-exist without problems. However, in all cases involving multiple wireless technologies co-existence is a serious matter that has to be address by doing some careful engineering. 16

18 Glossary CAT-iq dbm Cordless Advanced Technology Internet and Quality. CAT-iq is the next generation of DECT. db over 1 mw CDMA DECT Code Division Multiple Access. Cellular systems using wideband signals. Digital Enhanced Cordless Telecommunications. EIRP IAD ISM Band LNA Phase noise Wi-Fi Equivalent Isotropic Radiated Power. Radiated power assuming an equal radiation in all directions from a loss-less antenna. Integrated Access Device. An IAD a gateway device for residential use and it includes typically both DSL and Wi-Fi interfaces (plus additional wireless interfaces e.g. DECT). Industrial Scientific and Medical Band. Several frequency bands are defined, with the GHz being the most popular. General there a very few requirements to technologies in this band so many different systems make use of these unlicensed bands. Low Noise Amplifier. The first amplifier in a receiver chain if often referred to as the LNA. Jitter on a signal. This gives a skirt around the signal in the frequency domain. Measured in dbm/hz at a distance from the carrier, e.g. db below the carrier measure in 1 Hz bandwidth. Another name for Wireless LAN. It uses the IEEE standard (with additions). 17

19 Appendix A: Free space attenuation The free space attenuation can be calculated according to Friss equation Att = ( (λ/4π) * (1/R)) 2, Where λ is the wavelength R is the distance This can in a more handy way be explained as: 38 db on the first meter and 6 db for each doubling of distance at 1.9 GHz. 32 db on the first meter and 6 db for each doubling of distance at 900 MHz. Example: 900 MHz GSM, Distance: 16 meters (33 dbm output power) Attenuation at 16 meters: dB*log2(16m)= 32dB + 6dB*4 = 56dB Power level received on a 0 dbi antenna from a 33 dbm GSM at 16 m is:33dbm - 56dB = -23dBm 18