Amateur radio users (HAM) can be harmful because they can be located just a few meters from unshielded telephone wire, and the transmitting power in European is usually below 400W PEP (Peek Envelope Power) while it can be as high as 1500W in the US. Amateur radio users are allowed to use several different frequency bands, calls HAM bands, in the range 1.81MHz~30MHz.
The international HAM bands are listed in Table 2.2 [3]. The VDSL spectrum varies with number of sub-carriers N’. There are 2 operating modes: N’=2048 and N’=4096, the sub-carriers spacing ∆ =f 4.3125KHz. The VDSL uses a large frequency band ranging from 138KHz to 17.66MHz, and depending on N’, there are 3 (when N’=2048) or 5 (when N’=4096) HAM-bands that overlap VDSL spectrum.
HAM-band Band start (MHz) Band stop(MHz) Comments
HAM-band 1 1.810 2.000
HAM-band 2 3.500 3.800
HAM-band 3 7.000 7.100
HAM-band 4 10.100 10.150
HAM-band 5 14.000 14.350
VDSL in HAM band
HAM-band 6 18.068 18.168
HAM-band 7 21.000 21.450
HAM-band 8 24.890 24.990
HAM-band 9 28.000 29.100
VDSL out of HAM band
Table 2.2 Amateur Radio Bands defined by ANSI [3]
The simulation block of amateur radio (RFI) generator uses a simulated speech signal transmitted with Single Side Band (SSB) modulation. The simulated speech signal m(t) is constructed from white Gaussian noise which is band limited to 4KHz by passing through a special speech simulation filter [4]. The filtered signal is then switched on and off to simulate speech activity. The speech is on for 5 seconds and off for 10 seconds. During the speech activity period of 5 seconds the speech should be switched on for 50ms and off for 150ms repeatedly to simulate syllables [5]. The block diagram of amateur radio simulation shows as Figure 2.2-1. The Power Spectrum Density of RFI simulation result shows as Figure 2.2-2.
( ) ( ) cos(2 ) ( ) sin(2 ). (2.2-1)
rfi c c
S t =m t πf t −m t πf t
( )
m t Srfi( )t
Figure 2.2-1. Simulation block diagram of amateur radio
Figure 2.2-2 The Power Spectrum Density of RFI simulation (RFI power =-10dBm)
Figure 2.2-3 Amateur radio vs AM model 3 PSD
In Figure 2.2-3, the PSD of amateur radio is lager than AM broadcast radio.
Therefore, the main source of RFI is considered to be the amateur radio transmitters. In this thesis, we will focus on dealing with amateur radio.
Figure 2.2-4 The PSD level of RFI vs Received signal
(RFI power = -10dBm VDSL mask =-60dBm Channel: VDSL_TP1_1000 )
In the Figure 2.2-4 we can observe that RFI significantly degrades VDSL system performance, since RFI which is not periodic in the sub-carriers spacing gets spread to many sub-carriers. This is because the segmentation of the received data and removal of cyclic prefix is equivalent to multiplying the received signal (including RFI noise) by a rectangular window. Multiplication in time is equivalent to convolution in the frequency-domain with a sinc-like function. Since the sidelobes of the sinc are relatively high and decay rather slowly, RFI can effect a large number of sub-channels [11].
In order to satisfy the RFI egress requirements, DMT tones within HAM-band 1~5 are not modulated as shown in Figure 2.2-4. As a result, these unused tones can be used to measure the RFI, and RFI models can be estimated with the help of the measurements.
Chapter 3
Cancellation Of RFI
Since the power of RFI signal may be high. In order to prevent RFI signal clipping in ADC, a front-end analog cancellation which can suppress the RFI power level is needed. DMT-tones in the HAM-bands must be unused in order to avoid RFI egress. So we can use unused tones in the HAM-bands to estimate the RFI signal model. The block diagram of RFI canceller shows as Figure 3-1.
Figure 3-1. Block diagram of RFI canceller
As shown in Figure 3-1, the RFI cancellation process has to be conducted symbol by symbol. The derivation of the RFI signal model from one DMT symbol period is the beginning of our RFI cancellation algorithm. The simplest model is zero-order with a constant RFI envelope in one VDSL symbol period [9]. But in ETSI VDSL systems, the symbol periods are several times longer than that of Zipper VDSL. The RFI envelop in one DMT symbol may not be constant. By the parameters listed below, the relationship between RFI bandwidth and VDSL sub-carrier spacing is explained.
1. RFI bandwidth < VDSL sub-carrier spacing
Zipper-based VDSL’s sampling rate is fixed at 22.08MHz.
2 2 .0 8
If the RFI bandwidth is smaller than the inter-tone spacing of the DMT system, than the base-band function m(t) will not change much with the duration of one DMT frame [8]. Therefore, the RFI envelope is constant.
2. RFI bandwidth ≅ VDSL sub-carrier spacing
ETSI VDSL’s sub-carriers spacing is fixed at 4.3125KHz.
3 5 .3 2 8 systems the RFI envelop in one DMT symbol may not be constant. The simulation of RFI signal in one DMT symbol is shown in Figure 3-2. Therefore, the use of zero-order model in digital RFI cancellation is not fully justified. A
more accurate first-order model is instead adopted by names [11]. In Section 3.2, 3.3, 3.4 we will introduce the previous works in digital RFI cancellation. And our RFI cancellation algorithm is discussed in Section 3.5 and 3.6. In Section 3.7, the reduction of computation cost of our RFI cancellation algorithm will be presented.
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 -0.04
Sampling point (Sampling rate=1/35.328MHz)
Voltage
RFI in one DMT symbol
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 -0.025
Sampling point (Sampling rate =1/35.328MHz) RFI in one DMT symbol
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 -0.03
Sampling point (Sampling rate = 1/35.328MHz)
Voltage
RFI in one DMT symbol
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 -0.025
Sampling point (Sampling rate = 1/35.328MHz)
Voltage
RFI in one DMT symbol
3860 3880 3900 3920 3940 3960 3980 4000 4020 4040 4060 -0.02
Sampling point (Sampling rate = 1/35.328MHz)
Voltage
partial magnification
3850 3900 3950 4000 4050 4100
-6
Sampling point (Sampling rate = 1/35.328MHz)
Voltage
partial magnification
Figure 3-2. RFI in one DMT symbol period