UMTS Picocell Front End Module

UMTS Picocell Front End Module
The wireless market is expanding with significant growth likely to be in Wideband
CDMA. Universal Mobile Telecommunications System (UMTS) is the third-generation
(3G) wireless communications standard that is capable of delivering voice and highspeed data using WCDMA. The UMTS system offers a layered approach to coverage by
utilizing macrocells (towers), microcells, picocells and femtocells. Picocells are
essentially mini-basestations that deliver ~24 dBm power at the antenna. These products
fill in coverage gaps and add capacity in high-traffic, high data rate areas. Described
herein is a new front end module for the UMTS/ WCDMA picocell market. The term
“front end” module refers to the portion of an RF transceiver that extends from the
antenna to the mixers or local oscillator section. In addition, these front end modules can
also be utilized for UMTS repeaters.
The module essentially consists of a transmit (Tx) bandpass filter, a Tx power amplifier
(PA), a duplexer, a receive (Rx) low noise amplifier (LNA), and an Rx bandpass filter.
The effort of bringing these elements together for this market segment in one compact,
surface mount design was quite challenging.
UMTS uses FDD (frequency division duplexing). This means that there is a separate Tx
frequency band, 2110-2170 MHz, and a separate Rx band, 1920-1980 MHz. Therefore, a
ceramic duplexer is required at the antenna port so that the picocell can receive and
transmit simultaneously. By using a distributed filtering scheme consisting of a duplexer,
and Tx and Rx bandpass filters, we can provide UMTS attenuation requirements
normally seen in much larger solutions. The flexible design allows for a suitable PA and
LNA to be added to meet UMTS picocell power and Rx noise figure requirements. In the
transmit path, the module can deliver 24 dBm WCDMA power at the antenna port, while
achieving a typical ACLR (Adjacent Channel Leakage power Ratio) of -57 dB with a
PAR (peak to average ratio) of 8 dB. The 3GPP (Third Generation Partnership Project)
specification dictates that a minimum ACLR of -45 dB be achieved under worst case
The Model VFM1004A, which has U.S. and foreign patents pending, is designed to meet
3GPP Release 6 specifications. Its size is 31.0 mm x 25.1 mm x 6.75 mm. It can replace
all of the RF components that would be typically used in a UMTS basestation (or “Node
B”) local area front end. The front end module is shown in Fig. 1.
Fig. 1
The front end module simplified block diagram is shown in Fig. 2. The receive section is
under the metal lid. The transmit section is mainly exposed to minimize heat
Rx O/P
Tx I/P
Fig. 2
At the antenna port, the duplexer is of a ceramic monoblock construction. Its insertion
loss is typically 1.3 dB on the receive side and 1.5 dB on the transmit side (1). In the
transmit path, the GaAs power amplifier is capable of delivering the full UMTS local
area requirements. It dissipates ~6W while delivering 24 dBm at the antenna port (Test
model 1 64DPCH clipped to 8dB PAR). The Tx bandpass filter, in conjunction with the
duplexer, attenuates transmit spurious and prevents Rx de-sensitization.
In the receive path, the low noise amplifier has a noise figure of 1.3 dB and a typical gain
of 13 dB. The LNA is very linear and is designed to work within the distributed
architecture. The receive monoblock bandpass filter works in conjunction with the
duplexer to provide in excess of 65 dB “transmit to receive” isolation.
The Rx bandpass filter also provides the close-in blocking needed in order to be
compliant with the UMTS standard. With respect to receiver design, one of the most
difficult aspects of the standard is a blocking requirement that leads to a front end
attenuation of 15 dB to 20 dB at 1.9 GHz and 2 GHz (2). Table 1 is an extract of the
TS25.104-630 standards document.
Table 1: Blocking Performance Requirement for Local Area BS
Center Frequency of
Interfering Signal
1920 - 1980 MHz
1900 - 1920 MHz
1980 - 2000 MHz
1 MHz -1900 MHz
2000 MHz - 12750
Interfering Signal
mean power
-30 dBm
-30 dBm
Wanted Signal
mean power
-101 dBm
-101 dBm
Minimum Offset of
Interfering Signal
10 MHz
10 MHz
Type of
Interfering Signal
WCDMA signal
WCDMA signal
-15 dBm
-101 dBm
CW carrier
In a typical radio system design, the radio is protected from these blockers by the
duplexer. Often, the requirement of the duplexer is that it has an out-of-band attenuation
of 30 dB or better with a minimum attenuation of 20 dB at 0 Hz to 1.9 GHz, and 2 GHz
to 12.75 GHz. For a 60 MHz wide filter with a low insertion loss, this is difficult.
Duplexers that provide this performance can be as large as 28 cm x 23 cm x 7.6 cm. In
order to achieve the required close-in rejection, the front end module uses a distributed
filter approach. As a result, a similar performance can be achieved within the form factor
of this module.
The receiver sensitivity of a radio is the minimum power required at the antenna port to
achieve a specific BER (bit error rate) or SNR (signal to noise ratio) at the output. The
receiver sensitivity of the radio is related to noise figure and gain.
The worst case noise figure for the front end module receive section is 5 dB. Keep in
mind that linear multiplication is equivalent to logarithmic addition. The noise figure in
dBs is equal to 10 log(F), where F is the noise factor, i.e., SNRin / SNRout.
With an LNA gain of 12 dB, for example, the noise figure equation is as follows:
F1 = 5 dB = 3.16 (for the front end module)
F2 = 15 dB = 31.60 (for the remainder of the radio Rx section)
G1 = 12 dB = 15.84 (gain of the LNA)
The “cascaded noise figure” equation is as follows:
Fin = F1 + (F2 - 1)/G1 = 3.16 + (31.60 - 1)/15.84 = 5.09
Converting to decibels, Fin = 10 log (5.09) = 7.1 dB
Therefore, the worst case noise figure for the receive section is 7.1 dB. This translates to
“a maximum input signal level at the minimum sensitivity” of -120 dBm. This is using a
UMTS processing gain of 25 dB, and a signal of 6 dB above the noise floor. The
following equation shows how this was derived.
Receiver sensitivity equation: Sin (dBm) = NF (dB) + KTB (dBm) + Eb/No (dB) – PG
,where Sin
= input signal power, i.e., Rx sensitivity
= noise figure of receiver
KTB = thermal noise power [-174 dBm/Hz + 10 log (3.84 MHz)]
Eb/No = energy-to-noise ratio (S/N above the noise floor to insure
successful demodulation)
PG = UMTS processing gain [10 log( 3.84 MHz/12.2 kHz)]
This assumes a UMTS channel bandwidth of 3.84 MHz, and a digital voice data rate of
12.2 kbps.
Sin = 7.1 dB –108 dBm + 6 dB – 25 dB = -120 dBm. This is the maximum input signal
level at the required sensitivity for the radio.
The Tx and Rx gain at the antenna for the front end module are shown in Fig. 3. The
typical gain of the LNA in the Rx path is 13 dB. The gain of the PA in the Tx path is 15
dB. A two-stage power amplifier configuration (Model VFM1019C) could yield a gain of
28 dB.
Tx, Rx Gain
Fig. 3
The graph of Fig. 4 shows the ACLR of the transmit section of the module. The
Adjacent Channel Leakage power Ratio measures the amount of power in an adjacent
channel due to intermodulation distortion generated by a digitally modulated signal in the
main channel passing through a non-linear device, such as a power amplifier. This
parameter is a good figure of merit for the transmit path in a UMTS radio. The ACLR
has a bandwidth of 3.84 MHz with a spacing from the carrier of 5 MHz. This shows an
ACLR of -57 dB. The alternate channel power (ACP) has a spacing of 10 MHz from the
carrier and the typical value is -67 dB for this module.
ACLR @ 2140 MHz
Fig. 4
The graph of Fig. 5 shows the attenuation to 12.75 GHz in the transmit path of the
module. This shows a far out attenuation, with respect to the carrier signal, of > 60 dB.
Tx Wideband Sweep
Fig. 5
The graph of Fig. 6 shows the attenuation to 12.75 GHz in the receive path. This has an
attenuation of > 33 dB, again with respect to the carrier signal.
Rx Wideband Sweep
Fig. 6
Thermal management of the front end module is an important element in system
reliability. The front end module PCB has many metallized vias under the power
amplifier to drastically reduce its thermal resistance. The power amplifier has a junctionto-case thermal resistance (Θjc) of approximately 9.5°C/watt. At full power, the PA
dissipates 6.84 watts of heat at ambient room temperature (3). At 85°C, the PA dissipates
5.96 watts. The maximum ∆T of the power amplifier junction-to-case at room
temperature would be (6.84 watts)(9.5°C/watt) = 65°C.
The junction temperature, Tj, under worst case conditions is calculated as follows:
= ambient temperature = 85°C
P @ 85°C = power dissipated = 5.96 watts
= thermal resistance, PA junction to case = 9.5°C/watt
= thermal resistance, case to heatsink = 1.2°C/watt, assuming a screw is used
to secure the module through the motherboard to the heatsink. The heatsink is directly
contacting the motherboard.
At 85°C ambient, the calculated junction temperature is..
Tj = Ta + P (Θjc +Θc-hs)
= 85°C + 5.96W[(9.5 + 1.2)°C/watt ]
= 148.8°C
Consequently, at 85°C ambient temperature, a Tj < 150°C yields a very robust reliability
The front end module mounted onto a testboard with a heatsink is shown in Fig. 7.
Fig. 7
In this photo, the RF paths are roughly top and bottom. The DC paths are left and right
of the module.
A front end module for a UMTS picocell has been described. Along with the proper
selection of amplifiers, the concept of distributed filtering has been utilized to achieve
performance that is compliant with the stringent UMTS standards in a remarkably small
1. CTS datasheet VFM1004A, Rev. H
2. Conversation with Dr. Andrew Fox, Deltenna Ltd.
3. CTS application note “UMTS Picocell Front End Module” Rev. H