RF Receiver System Design for Wireless Local Area Network Bridge
at 5725 to 5825 MHz
N.A. Shairi1, T. Abd. Rahman1 and M.Z.A Abd. Aziz2
1
Wireless Communication Center,
Faculty of Electrical Engineering,
Universiti Teknologi Malaysia,
81300 Skudai, Johor, Malaysia
2
Faculty of Electronic & Computer Engineering,
Universiti Teknikal Malaysia Melaka,
Locked Bag 1200, 75450 Ayer keroh, Melaka, Malaysia
[email protected], [email protected], [email protected]
Abstract - In data communication system, the
outdoor of Wireless Local Area Network (WLAN)
technology can be implemented in bridge connection.
It allows LANs in separated buildings to be connected
over the distance ranging in several hundred meters. In
RF system design level, the performance of the WLAN
bridge also relies on the RF receiver system where it
must be well designed to minimize distortions in the
system. This paper presents a RF receiver system
design for WLAN bridge at 5725 – 5825 MHz based
on IEEE 802.11a standard. At 54 Mbps data rate, the
receiver can receive minimum input power of –69.1
dBm with 19.84 dB SNR and 47.3 dB dynamic range
which is comply with the standard. Beside that, the
prototype of the RF receiver has been measured to
verify with the simulation results in terms of gain
compression and spurious responses.
Keywords: Receiver system, RF receiver, WLAN system.
WLAN outdoor.
must extract the desired RF signal in the presence of
potential interference. Consequently, the first
component of the receiver, a band select filter (BS
filter) attenuates out-of-band signals received by the
antenna. Therefore, the BS filter only selects the RF
band of 5.725 – 5.825 GHz. Two low-noise amplifiers
(LNA) boost the desired signal level while minimally
adding the noise of the RF signal. To overcome image
frequency problem, image reject filter (IR filter) is
used to eliminate the image before down conversion.
The mixer down-converts the RF signal to a lower
intermediate frequency (IF) by mixing the RF signal
with a LO signal. Then, the channel select filter (CS
filter) attenuates all unwanted frequency components
generated by the mixer and any signal in the adjacent
channels. To boost the IF signal to the desired level the
cascaded power amplifiers is used so that the I and Q
demodulator (in Indoor Unit) can detect and downconvert the IF signal to the I and Q signal. Finally, the
signals are further processed by OFDM base-band in
order to get actual base-band data.
1. Introduction
In WLAN outdoor, the transmitter of WLAN
bridge system consists of antenna, indoor unit (IDU)
and outdoor unit (ODU). This paper presented the RF
receiver system design of ODU for WLAN bridge at
5725 – 5825 MHz which is based on the
commercialized products (off-the-shelf) of the
subcomponents. The RF distortions such as image
frequency, inter-modulation distortion (IMD), spurious
responses and Noise Figure (NF) should be minimized
with the proper design and selection of receiver
architecture, subcomponents, frequency plan and RF
budget so that the RF receiver system complies with
IEEE 802.11a standard.
2. Receiver Architecture
Figure 1: Receiver of WLAN bridge system
3. Frequency Plan
Frequency plan is an aspect of the RF receiver
design especially for architectures of superheterodyne
receiver. The selections of LO and IF frequency in
designing of superheterodyne receiver are an important
decision to eliminate image frequency.
Figure 1 shows the superheterodyne receiver
architecture for WLAN bridge system. The receiver
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In order to ensure that the image frequency is on
the outside of the RF bandwidth of the receiver, it is
necessary to have
fIF > BRF ,
(1)
2
where fIF is the IF and BRF is RF bandwidth of the
receiver [1]. RF bandwidth is referred to the 100 MHz
bandwidth of 5 GHz upper U-NII band. Therefore, IF
frequency of 815 MHz is selected with the trade-off
among three parameters; the amount of image noise,
the spacing between the desired signal and image
signal; and the loss of the image reject filter. The
higher Q in the CS filter is required if IF is too high
and leads to high insertion loss in the CS filter [2].
Low-side injection of LO is selected in order to
get lowest signal power of image frequency. For lowside injection (LOlow),
4. RF Receiver Budget
In RF receiver of ODU, a large system gain is
required to restore the weak power level of the RF
signal to a level that can be detected by the
demodulator. The P1dB inputs of demodulators in the
market typically range from -5 to 10 dBm. Therefore,
the output power of the RF receiver should be
approximately less than –5 dBm. The required total
gain of the RF receiver must be distributed throughout
the RF and IF stage. According to David M. Pozar [1],
higher gain occur in IF stage because of two reasons.
First, the low frequency amplifier is cheaper for high
gain compared to the high frequency amplifier.
Second, the requirements of high IP3 and P1dB of the
individual components at early stage of RF receiver
are less.
Overall, total gain of the RF receiver is obtained
by adding the gain of every RF component,
(2)
RF = LOlow + IF,
i −1
GT = ∑ Gj
where the location of image (Imagelow) is
(3)
Imagelow = LOlow – IF,
(4)
= 2IF –RF,
Therefore, the frequency plan according to the
Table 1 is constructed to cover four operating RF
channels of the standard. By using equation (2), the
local oscillator has four distinct frequencies to translate
four operating RF channels to an IF frequency of 815
MHz. Table 1 lists all the image frequencies by using
equation (3). As shown in Figure 2 the image
frequencies fall in the 3700 - 4200 MHz band where
the application of this band that follows ITU-T
regulation is for fixed-satellite of space-to-earth link
[3]. It is significantly yield very low signal level of
downlink received at earth station if compared to
uplink signal.
Table 1: Frequency plan for RF receiver design
Channel
Number
1
2
3
4
IF
(MHz)
815
815
815
815
LO
(MHz)
4930
4950
4970
4990
Image
(MHz)
4115
4135
4155
4175
RF
(MHz)
5745
5765
5785
5805
(5)
j
where GT is the system cascaded gain in dB and Gj is
the component gain for j from the system input.
The budget and analysis of gain and IP3 for the
cascaded RF receiver chain is presented in Table 2.
The system gain of the RF receiver is 54.5 dB is found
by using equation (5). Therefore, according to IEEE
802.11a standard, if the minimum input (sensitivity) of
the RF receiver is –65 dBm at 54 Mbps of data rate,
the output power of the RF receiver is –10.5 dBm,
which is lower than P1dB input of demodulator (-5 to
10 dBm).
Table 2: Gain and IP3 of RF receiver chain
Input Power (Pin) = -65 dBm
Stage
1
2
3
4
5
6
7
8
9
System
Gain (GT)
54.5 dBm
Component
BS Filter
LNA 1
LNA 2
IR Filter
Mixer
CS Filter
Power Amp. 1
Power Amp. 2
Power Amp. 3
System IIP3
-21.8 dBm
G (dB)
-1.4
12
12
-4.05
-6.5
-9.55
16
16
20
System
OIP3
32.69 dBm
IIP3 (dBm)
∞
8
8
∞
18
∞
10
10
13
System
Pout
-10.5 dBm
In order to analyze the linearity of the RF
receiver, the procedure of equivalent system input IP3
calculation in [5] is used. System input IP3 (IIP3)
calculation is obtained using equation (6),
Figure 2: The exact location of image frequencies
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


,
1

I I P 3 = 10 log
1
1
1


+
+ ... +


IPn 
 IP1 IP 2
Table 3: In-band spurs of RF receiver system
(6)
where IIP3 is equivalent system input intercept point
(in dBm), IP1 is input IP3 of first stage (in mW) and
IPn is input IP3 of last stage (in mW). The system
output IP3 (OIP3) of cascaded RF receiver is
calculated by using equation (7),
OIP3 = IIP3 + GT ,
(7)
As a result, the system IIP3 of the receiver is
–21.8 dBm while the system OIP3 is 32.69 dBm.
Approximately, to avoid IMD, the maximum power
input for the RF receiver cannot be higher than –21.8
dBm.
5. RF Spurious Responses
Spurious responses (so called spurs [1]) are the
responds of two input frequencies in mixer that
produce unwanted frequencies [4] including harmonic
mixing of the frequencies [5]. Any RF receiver
frequency that satisfies the following relationship is a
potential receiver spurious response [5],
± mf RF ± nfLO = ± fIF ,
(8)
fRF is any incoming frequencies into the mixer
RF port, fLO is local oscillator frequency, fIF is
where
desired IF frequency, m is integer multiplier of RF
frequency and n is integer multiplier of LO frequency.
Therefore, RF receiver will respond to undesired
signal at RF frequencies within its tuning range and
produce spurs within the IF passband.
In order to determine the potential receiver
spurious responses that may fall at IF of 815 MHz,
equation (8) is rewritten as a result of two possible RF
spurious frequencies (all numbers are positive) [5].
fRF 1 =
nfLO − fIF
m
(9)
fRF 2 =
nfLO + fIF
m
(10)
By using equation (9) and (10), the RF filters (BS
filter and IR filter) generally can attenuate most of the
spurious responses in the RF stage because the
spurious responses are located outside the 5.725 –
5.825 GHz band. However, there are some of the
spurious responses that fall within the RF band so
called in band spurs. Table 3 lists two in band spurs for
each operating RF channel that may have a potential of
distorting the desired IF signal.
RF
LO
IF
(MHz)
(MHz)
(MHz)
Channel
1
5745
4930
815
Channel
2
5765
4950
815
Channel
3
5785
4970
815
Channel
4
5805
4990
815
In band
spurs
Mixing
order
(MHz)
5753
5750.7
5777
5773.5
5801
5796.4
5825
5819.2
-5RF + 6LO
7RF + (-8LO)
-5RF + 6LO
7RF + (-8LO)
-5RF + 6LO
7RF + (-8LO)
-5RF+6LO
7RF + (-8LO)
The power level of the mixing frequencies that
falls in IF channel is further analyzed by using ADS
2002C software. As a result, the power level of the
mixing frequencies are -350 dBm (average) and would
not interfere the IF channel because it is dominated by
the noise floor (-101dBm).
6. Noise Figure and Sensitivity Analysis
Sensitivity (also known as minimum detectable
signal (MDS) [1]) is defined as the minimum signal
level that the system can detect with acceptable signalto-noise ratio [2]. In wireless digital communication
system, sensitivity is the minimum received signal
level that produces a specified BER when the signal is
modulated with a bit sequence of data [6]. It is limited
by IF bandwidth, noise figure and minimum SNR. The
IF bandwidth is usually fixed on the channel
bandwidth used in the RF system (ie IEEE 802.11a
uses 20MHz channel bandwidth). The level of
minimum SNR for receiver sensitivity depends on the
type of modulation (i.e. QPSK, BPSK and16-QAM)
[7] because SNR is related with bit energy to noise
power spectral density, Eb/no of the modulation [1].
Therefore, in RF system design, noise figure plays an
important role to ensure that the value of noise figure
does not reduce the sensitivity of RF system. The total
noise figure of the RF system is known as Friss
equation [5] by the given equation,
NFtotal = NF 1 +
(NF 2 − 1) + (NF 3 − 1) + ...,
G1
(11)
G1G 2
where NFtotal is equivalent input noise factor (linear),
Fi is stage noise factor (linear) and Gi is stage gain
(linear).
In order to meet the requirement of the sensitivity
specification (measured in Packet Error Rate (PER)) in
IEEE 802.11a standard, the RF receiver system is
designed to have a noise figure of less than 10 dB.
By using equation (11), the noise figure of the RF
receiver system is 7.057 dB, which is 2.943 dB lower
than that specified in the standard (see Table 4). The
noise figure of passive components such as filters and
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mixer equals their loss in decibels [5]. The total noise
figure (NFT) is a logarithmic (decibel) to the linear
sum of noise terms at each cascaded stage according to
the Friss equation (11).
The cascaded LNAs play a paramount role of
reducing the system noise figure. This is because if the
RF receiver employs only one LNA with the same
component parameter (NF = 2.5 dB), the system noise
figure will be 16.33 dB, which is 6.33 dB higher than
10 dB NF. The cascaded of two LNAs provides 9.273
dB improvements from 16.33 dB NF, which is a
significant change for the system noise figure.
The cascaded LNAs play a paramount role of
reducing the system noise figure. This is because if the
RF receiver employs only one LNA with the same
component parameter (NF = 2.5 dB), the system noise
figure will be 16.33 dB, which is 6.33 dB higher than
10 dB NF. From this analysis, the cascaded of two
LNAs provides 9.273 dB improvements from 16.33
dB NF, which is a significant change for the system
noise figure.
Table 4: Noise figure of RF receiver chain
G (dB)
Component
BS Filter
-1.4
LNA 1
12
LNA 2
12
IR Filter
-4.05
Mixer
-6.5
CS Filter
-9.55
Power Amplifier 1
16
Power Amplifier 2
16
Power Amplifier 3
20
Total NF (NFT)
NF (dB)
1.4
2.5
2.5
4.05
6.5
9.55
6.5
6.5
3.5
7.057 dB
As further analyzed in ADS 2002C software,
Figure 3 is a result of sensitivity performance on the
RF receiver where PER versus input power level
(received power) over eight different data rate is
plotted. The system noise figure in the simulation is
7.056 plus 5 dB implementation margin. The assumed
5 dB implementation margin has to take into account
as required in the standard. Besides, there is assumed
no phase noise. Sensitivity of the RF receiver is
marked at 10 % PER. Table 5 lists the approximated
sensitivity for all data rate according to the graph. The
simulated sensitivity is lower than the specified input
levels (minimum sensitivity) at 10% PER, hence
fulfilled the performance requirement according to
IEEE 802.11a standard
Figure 3: RF receiver’s sensitivity performance
Based on simulated result of sensitivity, minimum
SNR output for the RF receiver can be obtained by
using equation (12) as written below
SNRo min = Si min+ 174dBm / Hz −10 log B − NFsys (12)
where B is 20 MHz channel bandwidth and NFsys is
7.056 dB plus 5 dB implementation margin. As listed
in Table 5, the approximated minimum SNR output
(SNRomin) is calculated for eight different data rates.
Since the RF receiver can receive any signal level
above sensitivity level, dynamic range for the RF
receiver has to be calculated in determining the ability
of RF receiver to receive minimum and maximum
input power. According to David M. Pozar [1],
receiver dynamic range (DRr) of a RF system is the
range of signal power that is limited at the low end by
minimum detectable signal power (sensitivity) and at
the high end by the input third order intercept point
(IP3),
DRr (dB) = IP3input (dBm) – Si(dBm)
(13)
The input IP3 is chosen as maximum allowable
signal power because this would be the maximum
input power before intermodulation distortion becomes
unacceptable [1]. By using equation (13), Table 5 lists
the approximated dynamic range for the RF receiver
according to simulation result of sensitivity and input
system IP3. The input system IP3 is taken at –21.8
dBm for all data rate.
Table 5: Sensitivity, minimum SNR and dynamic range
of RF receiver
Data
rate
(Mbps)
6
9
12
18
24
36
48
54
Simulated
result of
sensitivity
(dBm)
-85.5
-84.3
-82.4
-80.4
-77.3
-74.7
-70.0
-69.1
Minimum
SNR output
(dB)
Dynamic
Range
(dB)
3.44
4.64
6.544
8.544
11.644
14.244
18.944
19.844
63.7
62.5
60.6
58.6
55.5
52.9
48.2
47.3
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7. Simulation and Measurement Results
The comparison between simulation and
measurement result are reported in this section. In the
simulation stage, the RF receiver has been modeled in
ADS 2002C software using behavioral models. The
system measurement and simulation are performed on
the RF receiver without several filters.
7.1 Gain Compression
The system gain compression is measured at 5765
MHz where Dielectric Resonator Oscillator (DRO) is
used as a LO source. The measured output power
includes cables and connector losses. As shown in
Figure 4 the measured output power in linear region is
slightly higher than the simulation result. With the
measured of 57.5 dB system gain, the P1dB in the
measurement result is -38 dBm input power or at
18.447 dBm output power which is lower than the
P1dB in the simulation.
Table 6: Spurious responses of RF receiver
Channel 1
5745 MHz
In band
spurs
(MHz)
Output
Freq.
(MHz)
Output
Power
simulation
(dBm)
Output
Power
measured
(dBm)
5753
823*
-8.658
-8.317
5750.71
820.7*
-8.108
-7.998
Channel 2
5773.57
823.6*
-9.531
5765 MHz
* Output frequency that falls in the IF channel
(805 – 825 MHz or 20 MHz bandwidth)
-8.570
8. Conclusions
A RF receiver for WLAN bridge system operating
at 5725 – 5825 MHz band has been designed with the
selected off-the-shelf components. Superheterodyne
architecture has been chosen in this RF receiver
design. RF budgets such as gain, noise figure and IP3
have been calculated and further analyzed in ADS
2002C software for output power of spurious
responses, gain compression and sensitivity. It is
shown that the RF receiver has 54.5 dB system gain
with NF of 7.057 dB. Thus, at 54 Mbps data rate, the
receiver can receive minimum input power of –69.1
dBm with 19.84 dB SNR and 47.3 dB dynamic range,
which has met the IEEE 802.11a standard. Beside that,
the prototype of the RF receiver has been measured to
verify with the simulation results in terms of gain
compression and spurious responses.
9. Future Works
Figure 4: Comparison of gain compression
7.2 Spurious Responses
The comparison is made between simulation and
measurement result where the input power is set to -65
dBm assuming it operating at 54Mbps data rate. The
mixing order of 1RF-1LO has a high potential of
interfering the IF channel. As listed in as Table 6, the
frequencies of the mixing order fall not exactly on the
815 MHz but as the IF bandwidth is 20 MHz wide,
three of the output frequencies (823 MHz, 820.7 MHz
and 823.6 MHz) fall in the channel band (805 to 825
MHz). Therefore, a proper site survey is needed to
minimize the potential interference of the spurious
responses.
The RF receiver prototype has been measured
without BS and IR filters (see Figure 5). This is due to
the time it takes to purchase all off-the-shelf
components. Therefore, once the filters are received
they have to be integrated for complete RF receiver
prototype. The RF receiver is then should be measured
for performance verification according to the IEEE
802.11a standard.
Figure 5: RF receiver prototype
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