1、 Rep. ITU-R M.2034 1 REPORT ITU-R M.2034Impact of radar detection requirements of dynamic frequency selection on 5 GHz wireless access system receivers (2003) 1 Introduction Recommendation ITU-R M.1652 recommends that, in order to facilitate sharing with radars, mitigation techniques be implemented
2、by wireless access systems including radio local area networks (RLANs) (WAS) in the bands used by radars at 5 GHz. It describes a mitigation technique, named dynamic frequency selection (DFS), which requires a radar detection mechanism to be implemented. In addition, the Recommendation also gives th
3、e detection and response requirements with which the DFS should comply with. This Report considers how radars operating in the 5 GHz band can be detected by WAS without extreme constraints on the RF front-end design or on the system capacity of the WAS. Radar detection may only be possible during th
4、e time there is no active transmission in the WAS cell. During channel availability check time, there is no transmission for a significant time (e.g. 60 s). During in-service monitoring the detection can take place during moments of no uplink or downlink traffic. The detection of a radar pulse of po
5、wer above the so-called DFS threshold triggers a DFS procedure, and the access point must leave the channel within a short delay. In this Report the interplay of DFS threshold TDFS, WAS false alarm rate and radar detection failure rate by WAS are presented and discussed from the point of view of imp
6、lementation of radar detection in a WAS receiver. 2 Abbreviations and acronyms ACT: (channel) availability check time DF: detection failure probability DFS: dynamic frequency selection FA: false alarm probability OFDM: orthogonal frequency division multiplexing PDF: probability density function PRF:
7、 pulse repetition frequency RLAN: radio local area network RSS: received signal strength TDFS: DFS threshold WAS: wireless access system This Report was jointly prepared by Radiocommunication Study Groups 8 and 9, and any futher revision will also be undertaken jointly. 2 Rep. ITU-R M.2034 3 Assumpt
8、ions and methodology Radar detection by a WAS is characterized by two performance metrics: 3.1 Detection failure probability (DF) This is the probability that a radar is present in the channel but no radar pulse is detected by the WAS. DF must be as low as possible not to interfere with radars. A va
9、lue of DF = 1% is set as a target. The probability of detection varies with the power of the radar signal relative to the detector threshold setting in the receiver a higher power level increases the detection probability. 3.2 False alarm probability (FA) This is the probability that when a radar si
10、gnal is not present in the channel, interference pulses are detected by the WAS, triggering an unnecessary DFS procedure. This can happen when a noise or interference burst is mistaken for a radar pulse. Such a burst may be caused by WAS devices in the vicinity operating on the same channel or on th
11、e adjacent RF channels of the WAS system. A high interference/noise floor ratio in the WAS receiver will obviously increase the FA. However, this probability should be low as possible in order not to trigger unnecessary DFS procedures, which would severely limit the WAS capacity especially in dense
12、deployment scenarios. During channel availability check time, it is sufficient to be sure that the channel is not measured as occupied for some small percentage of the time. It will be seen that this percentage is a very sharp function of the WAS DFS threshold setting, with a few dB altering the FA
13、probability by many orders of magnitude. The WAS performs power measurements averaged over a duration rss_meas_duration, the measurements signal_rss being spaced by the averaging duration. Therefore, during channel availability check time (ACT), for example, the number of measurements performed is:
14、nb_trials = round (ACT / rss_meas_duration) The radar signal is received at a level of radar_ pulse_ power (dBm), (equivalent to TDFS in the notation of Recommendation ITU-R M.1652) and pulses have a duration of radar_ pulse_duration (ns). The radar signal bandwidth (MHz) is denoted by B. Many radar
15、s send pulses periodically, at a pulse repetition frequency (PRF) rate. For a rotating radar, the WAS will periodically receive a burst of pulses, such that the number of pulses occurring during the channel ACT, denoted by nb_RADAR_ pulses_ per_ACT will be greater than one. The 5 GHz orthogonal freq
16、uency division multiplexing (OFDM) RLANs sampling period used is T = 50 ns. 4 Detection criteria The detection criterion used here is based on absolute signal power measurements. If the measured power exceeds a certain threshold, then a radar is assumed to be detected. It is assumed that the signal
17、samples are averaged white Gaussian noise. Therefore, a measurement performed during rss_meas_duration consists of (rss_meas_duration / T) independent complex Rep. ITU-R M.2034 3 Gaussian variables. Thus, the probability density function (PDF) of the noise is 2centred on noise_floor with: noise_free
18、_deg = 2rss_meas_duration / T degrees of freedom. There is no data available on received signal characteristics or statistics of a radar pulse, propagated over an urban, rural or other environment. In some cases, the radar pulse duration can be shorter than rss_meas_duration. The radar signal bandwi
19、dth is normally smaller than an OFDM signal bandwidth of 18 MHz, so that radar pulse samples collected during the power measurement will be correlated. If the radar pulse was sampled at a rate of T_RADAR = 1 / B, the samples would be uncorrelated. It is assumed here that the PDF of the power when me
20、asuring a radar pulse is 2centred on: meas_avg_radar_ power = radar_ pulse_ power + 10 log10(min(radar_ pulse_duration, rss_meas_duration) / rss_meas_duration) with degrees of freedom equal to: radar_free_deg = 2 round(min(radar_ pulse_duration / T_RADAR, rss_meas_duration / T_RADAR) The PDF of the
21、power measurements noise_rss_ pdf and radar_rss_ pdf are plotted in dBm in Fig. 1 for a radar of type K received at a level of 67 dBm, and an interference/noise floor of 84 dBm (see 5). Once the two distributions noise_rss_ pdf and radar_rss_ pdf are known, the DF and FA probabilities can be compute
22、d. DF = (prob(radar_rss_ pdf rss_threshold)nb_trialsNote that rss_threshold is the WAS receiver RSS threshold setting, whose value should be set between noise_floor and meas_avg_radar_ power such as to meet the required DF and FA probability criteria. 5 Results The following values are used in the c
23、alculations: rss_meas_duration = 1 s radar_ pulse_ power = 67 dBm, 64 dBm, 62 dBm radar_ pulse_duration = 1 s interference/noise_floor = 84 dBm B = 4 MHz ACT = 60 s nb_RADAR_ pulses_ per_ACT = 1, 10 Figures 1 to 6 plot FA and DF using various values chosen from the above list. It can be seen that th
24、e form of DF steepens as the number of pulses increases. Increasing TDFS by a number of dB moves the DF curve the same number of dB along the x-axis. It should be noted that an interference/noise floor increase will move the FA curve horizontally to the right also. 4 Rep. ITU-R M.2034 Rap 2034-01100
25、 90 80 70 60 50 40FIGURE 1PDF of the power levels of radar pulses and interference/noise floor0.450.350.250.150.050.40.30.20.10NoiseRadarMeasured level (dBm)PDFRap 2034-02101011021031041051065055606570758085FIGURE 2DF and FA, single radar pulse, TDFS= 67 dBmFalse alarmDetection failurePower comparis
26、on threshold (dBm)ProbabilityRep. ITU-R M.2034 5 Rap 2034-03101011021031041051065055606570758085ProbabilityFIGURE 3DF and FA, 10 radar pulses, TDFS= 67 dBmFalse alarmDetection failurePower comparison threshold (dBm)Rap 2034-04101011021031041051065055606570758085FIGURE 4DF and FA, single radar pulse,
27、 TDFS= 64 dBmFalse alarmDetection failurePower comparison threshold (dBm)Probability6 Rep. ITU-R M.2034 Rap 2034-05101011021031041051065055606570758085FIGURE 5DF and FA, single radar pulse, TDFS= 62 dBmFalse alarmDetection failurePower comparison threshold (dBm)ProbabilityRap 2034-061010110210310410
28、51065055606570758085ProbabilityFIGURE 6DF and FA, 10 radar pulses, TDFS= 62 dBmFalse alarmDetection failurePower comparison threshold (dBm)Rep. ITU-R M.2034 7 6 Summary of simulation results False alarm rate The FA rate in the WAS receiver with interference/noise floor 84 dBm is 1% for a receiver de
29、tection level (rss_threshold) of 79 dBm (Fig. 2). The curve falls very sharply, but in order to ensure acceptable service quality level during WAS transmission when in normal operation, the FA should be several orders of magnitude lower. Detection circuit implementation margins are not included, and
30、 the interference floor is assumed to be equal to the highest receiver sensitivity, corresponding also for example to interference from an adjacent channel access point 30 m distant. Radar detection performance If only one pulse is detected, with the radar signal level 67 dBm, the detection failure
31、rate DF is 1% with rss_threshold 74 dBm. (Fig. 2). Thus there is less than 2 dB only of margin for the FA and DF for rss_threshold set to a value between the two criteria. In practice, variations in operating conditions will easily cause this narrow margin to be exceeded. In addition, the typical to
32、lerance specified for RSS measurements being 5 dB in the 5 GHz RLAN standards and the cost of imple-menting tighter tolerance in devices manufactured in large scale production would be prohibitive. If several radar pulses are available, detection probability improves. The effect of increasing the radar signal level that must be detected is shown in Figs. 4, 5 and 6 for TDFSof 64 dBm and 62 dBm. Figure 6, where 10 pulses are detected with TDFS= 62 dBm yields an improved margin for implementation.
