Measuring Blocking of CDMA Receivers

Overview

The CDMA wireless system was designed to operate in the same radio frequency band as the older Advanced Mobile Phone System (AMPS), which operated in the U.S. cellular band before the introduction of CDMA technology. The AMPS radio scheme divides the frequency into many adjacent relatively narrowband FM channels, while the CDMA radio scheme uses a small number of wideband radio channels. As a result, the CDMA channel plan has to include the existing AMPS channels, and AMPS will act as an interferer to degrade the CDMA link.

Here we discuss two main mechanisms that affect the design and performance of cellular-band CDMA handsets:

• Reciprocal mixing, where the LO phase noise can block the received RF signal in the presence of frequency interference.
• Cross-modulation, where leakage from the handset transmitter overloads the receiver's low-noise amplifier.

This paper also demonstrates the excellent performance of the system by measuring a real system.

Background of Cellular Band Frequency Planning

AMPS service is in the US 850MHz cellular band:

• 824MHz to 849MHz uplink (mobile phone transmitter reverse channel)
• 869MHz to 894MHz downlink (mobile phone receiver forward channel)

AMPS channels are spaced 30kHz apart, and each channel occupies approximately 24kHz at peak frequency deviation.

CDMA services occupy the same U.S. cellular bands, with CDMA channels aligned to the 30kHz spacing of AMPS (i.e., each channel spans multiples of 30kHz), but each CDMA channel occupies 1.23MHz of bandwidth. To manage this distribution, mobile operators allocate 12.5MHz of frequency bands, with the nearest AMPS channel located 285kHz away from the nearest CDMA channel, the band boundary (i.e., the CDMA boundary is nine 30kHz AMPS channels plus 15kHz away from the center of the AMPS channel), as shown in Figure 1.

Figure 1. Relationship between a CDMA channel and the nearest AMPS carrier, which is an interferer to the CDMA channel.
 

When the nearest AMPS channel is much stronger than the CDMA signal level, it is a single-tone interference to the CDMA channel, and the interference frequency offset is shown in Equation 1:



285kHz + 615kHz = 900kHz, which is the frequency difference between the nearest AMPS interference channel and the center of the specified CDMA channel. The power intensity of this interference source relative to the sensitivity of the interfered CDMA channel (-101dBm) is defined in the 3GPP2 air interface standard as: -30dBm in the worst case.

Blockage indicators for CDMA phones

Blocking is a measure of the ability of a mobile phone operating on a channel to receive a CDMA signal when a narrowband jammer is located at a given frequency offset from the center frequency of the channel. Receiver blocking is measured in terms of the frame error rate (FER).¹

The advantage of CDMA system is that more than 25 mobile phones can work simultaneously on the same frequency (i.e. on the same channel center frequency). Code division multiplexing (channel division) means that the uplink and downlink carrier frequencies of each mobile phone use different orthogonal spreading codes.

To achieve this goal, the CDMA base station must accurately control the power emitted by each mobile phone transmitter to ensure that all users receive the same power level. Accordingly, the mobile phone receiver must have a wide gain control range. When the mobile phone receiver is farthest from the base station, the typical signal strength of the forward link is only -110dBm.

The problem stems from the fact that the adjacent AMPS system does not manage the uplink power of the mobile phone in the same way as CDMA. When the CDMA mobile phone is receiving at its extreme sensitivity, the nearby AMPS base station may emit a strong interference. This situation is particularly prone to occur at the cell boundary.

Fortunately, the characteristics of the downlink spreading code make the mobile phone receiver relatively immune to adjacent channel interference. Narrowband AMPS interference is "spread" by the mobile phone's correlator, so its impact is reduced by the processing gain (about 25dB). Because the interference is significant, 3GPP2 specifies a test to ensure that the CDMA receiver can fully handle adjacent channel interference. The CDMA2000 standard in 3GPP2 specifies the following blocking test conditions:

For the US CDMA system, the cellular band test requirements specify a minimum effective co-directional radiated power of +23dBm. The PCS band test requirements specify a minimum effective co-directional radiated power of +15dBm (tests 1 and 2), or +20dBm (tests 3 and 4). The power of the jammer is specified to be -30dBm  (tests 1 and 2), or -40dBm (tests 3 and 4)².

When testing a CDMA front-end IC or a zero-IF receiver for blocking, it is important to be aware of the interference components caused by a single-tone jammer and to recreate those effects in the test setup. The two main factors that affect blocking are reciprocal mixing and cross-modulation.

Reciprocal Mixing

Reciprocal mixing occurs when a single-tone jammer mixes with the receiver's local oscillator (Rx LO). The Rx LO has finite phase noise and mixes with the single-tone jammer to produce an interference component at the intermediate frequency (IF), which for a zero-IF system is at baseband (Figure 2).

Figure 2. Reciprocal mixing in the presence of blocking.

The receiver's blocking specification is a key performance parameter that sets the LO phase noise requirement. For accurate blocking measurements, the single-tone jammer's own phase noise will also contribute to the overall interference level. Therefore, in the lab, you should choose a low-phase-noise RF signal source to ensure that the main source of blocking is the phase noise in the Rx LO, rather than the RF signal generator.

For example, referring to Maxim's superheterodyne CDMA reference design (Rev. 3.5), using the MAX2538 front-end IC and the MAX2308 IF demodulator, its cascaded noise figure is less than 3dB in the cellular band. If we assume that the duplexer loss in the phone is about 3dB, we get:



If the RF generator phase noise is 10dB below the receiver noise floor, then:



Here -30dBm is the specified single tone strength for Tests 1 and 2 (Table 1). Therefore, the noise floor of the new receiver is:



It can be seen that the RF generator's -148dBc/Hz phase noise has a relatively small impact on the receiver sensitivity (only 0.4dB degradation).

The CDMA phone standard requires a minimum phase noise of -144dBc/Hz at 900kHz offset. Assuming a flat response to far-end phase noise (-144dBc/Hz over the entire band), the calculated result gives a receiver noise floor of -167dBm/Hz, 1dB worse than the interference-free noise floor of -168dB/Hz. Therefore, the CDMA standard allows the receiver sensitivity to degrade by 1dB due to the generation of RF interference.

Table 1. Minimum requirements for blocking in CDMA phones3

Parameter		Units	Tests 1 and 3	Tests 2 and 4
Tone offset from carrier	SR1	kHz	+900 (BC 0, 2, 3, 5, 7 and 9) +1250 (BC 1, 4 and 8)	-900 (BC 0, 2, 3, 5, 7 and 9) -1250 (BC 1, 4 and 8)
	SR3	kHz	+2500	-2500
Tone power		dBm	-30 (Tests 1 and 2) -40 (Tests 3 and 4)
		dBm/1.23MHz	-101
		dB	-7
		dB	-15.6 (SR1) -20.6 (SR3)

Cross-modulation interference

Cross-modulation occurs when a strong transmitter leakage signal is present at the receiver LNA input. This modulated interferer and the 900kHz AMPS signal cross-modulate in the LNA to produce third-order nonlinear products, resulting in an increase in noise power in a given RF channel in the receiver. Even though the receiver IP3 is primarily the mixer IP3, most of the cross-modulation occurs in the LNA because the transmitter leakage reaching the mixer input is very small due to the presence of a bandpass filter between the LNA and the mixer4. In order to include this effect in the receiver test setup, a CDMA reverse channel modulated signal must be injected into the receiver. For the cellular band, the transmit power injected into the LNA input is given by Equation 5:

Assuming a 52dB isolation between the duplexer transmit and receive ports and a 2dB loss from the antenna to the duplexer transmit port.

Test example using the CNR method

Figure 3 shows the complete setup for testing blocking of a cellular band CDMA receiver. The same setup can be used for PCS band testing, but the frequency offset and power level of the jammer and the power level of the transmitted signal must be set accordingly as shown in Table 1. In this test setup, we use the CNR (Carrier to Noise Ratio) method to measure blocking.

Figure 3. Cellular band single-tone blocking test setup.

Sensitivity is defined as the minimum received power at which the frame error rate (FER) is ≤ 0.5% for 95% of the time. In the CNR measurement, we note that in the RF configuration 1 of the 3GPP2 standard, the Ec/Ior of the traffic channel is -15.6dB, and the Eb/Nt of the traffic channel is 4.5dB for a data rate of 9600bps. The processing gain is 10log (1.2288Mcps/9600bps) = 21.072dB, which gives Equation 6:

Therefore, the CNR required to demodulate the CDMA signal over a 1.23MHz channel width is -1dB. In our test setup, we use a 3kHz RBW and test by comparing the spot frequency test signal power (at 250kHz) and the total noise power over the entire 615kHz I channel bandwidth. Because the given received signal power is -101dBm, and the total allowed noise power is -100dBm , we can see that in order to meet the system sensitivity requirement, the CNR is -1dB.

To illustrate this approach, consider the measurements of Maxim's N-CDMA V4.1 reference design, which uses a zero-IF monolithic receiver IC (MAX2585) with an internal VCO (Figure 4). The green line shows the given signal when there is no blocker or transmit signal (for the given signal, we used a -101dBm tone offset from the channel frequency by 250kHz as the modulating signal for the CDMA forward channel). The blue line represents the noise rise when both the blocker and CDMA transmit signals are turned on simultaneously. The following steps summarize the test setup:

Figure 4. Noise rise caused by single-tone blocking and CDMA transmit signals.

• The system gain was adjusted to receive a -101dBm signal relative to the input of a 3dB pad, which was used to simulate the duplexer losses. For the MAX2585 receiver IC, the gain was set to give a nominal output signal level of 8.5mVRMS (-28.5dBm into a 50Ω load).
• Turn on the -24dBm CDMA transmit signal (45MHz lower than the receive channel frequency at the 3dB attenuator).
• Turning on a -30dBm blocking signal relative to the 3dB attenuator input, the noise floor is observed to rise.
• The level of the spot frequency jammer is adjusted to raise the noise floor until the total noise power from 0 to 615kHz is 1dB higher than the given signal level. In this example, we integrate the noise from 25kHz to 615kHz to avoid DC leakage in the spectrum analyzer.
• Record the level of the interfering transmitter at -1dB CNR and calculate the blocking margin.

In this example, the total noise power from 25kHz to 615kHz is -27.5dBm, and the received tone at the output is -28.5dBm, which meets the -1dB CNR requirement. The single-tone jammer level is -27dBm at -1dB CNR, indicating that the MAX2585 meets the blocking requirement at the measured frequency with a 3dB margin.

in conclusion

This article discusses blocking in terms of the 3GPP2 standard, discusses the main sources of blocking, and presents a practical method for measuring blocking in a CDMA receiver. For more information on Maxim's superheterodyne and direct-conversion ICs, visit the Maxim website: Wireless, RF, and Cable.