CDMA mobile phone receiver interference test and its impact

Noise effect

When testing a mobile phone, you should first understand the structure of the phone before considering the noise. The mobile phone has both a transmitter (TX) and a receiver (RX). The transmitter transmits power between +30dBm and -55dBm, and the receiver signal reception range is between -20dBm and -108dBm. The transmitter and receiver of CDMA handsets have a wide dynamic range of more than 80 dB, and the maximum power (+23 dBm for Class III handsets) and receiver sensitivity (-104 dBm) are the most important indicators to be aware of.

CDMA cell phone testing requires multiple tests to ensure that the phone meets specific standards. For example, testing the power of the mobile phone transmitter to ensure that the transmission power range is accurate over a wide dynamic range, in particular to test the maximum transmission power of the mobile phone, to ensure that the mobile phone's radiated power is close to the minimum specified by the maximum EIRP (effective isotropic radiated power), III The mobile phone's transmit power range is +23dBm to +30dBm. Noise is generally an insignificant factor in the measured total power compared to noise due to the large power measured by the instrument. In addition, the mobile phone transmitter should also be tested to see if it meets the minimum transmission requirements. The CDMA minimum power transmission requires that the mobile phone transmission must be less than -50 dBm. Even in this case, the influence of the transmission channel noise is usually negligible and will not affect the measurement of the lowest power. The problem of measuring the lowest power of CDMA is generally caused by the minimum noise limit of the power meter used.

Receiver sensitivity measurements will be affected by noise on the handset receive channel. The CDMA handset receiver sensitivity is specified at -104 dBm, and the handset must be capable of demodulating the forward link signal transmitted at -104 dBm with a frame error rate of less than 0.5%. Eb/Nt is a parameter indicating the ability of the mobile phone to accurately receive and demodulate the forward link signal, where Eb is the energy per bit of the communication channel, and Nt is the total noise on the receiving bandwidth, which is somewhat like the signal-to-noise ratio in the analog circuit ( S/N). When the Eb/Nt ratio is increased, the receiver can better demodulate the signal better; and as the Eb/Nt decreases, the handset is likely to erroneously demodulate the forward link signal. The actual bit energy of the forward link communication channel is 15.6 dB lower than the total forward link power of -104 dBm. In other words, the actual signal received by the handset during the measurement is -119.6 dBm. From this point on, we will be mixing The forward link signal is referenced in the contents of the Walsh code.

According to the tolerance of the mobile phone receiver design, its sensitivity to noise is also different. Usually, when the performance of the mobile phone is at the edge value, there will be noise problems on the forward link channel. There are many factors that affect the Nt in Eb/Nt. .

Noise source on the receiving channel

The essence of KTB noise floor Nt is thermal noise, which is always present in the environment. Thermal noise is also known as KTB noise, where:

K = Boltzmann constant (1.38 & TImes; 10-23)

T = reference temperature (Kelvin)

B = receiver effective noise bandwidth

For CDMA systems operating at 1.23 MHz bandwidth, the thermal noise is approximately -113 dBm. You might ask, how does the receiver demodulate the -119dBm communication signal with -113dBm noise floor? This is because the CDMA processing gain is nearly 21dB, and 14.4kbps/9.60kbps can be transferred to the 1.228Mcps rate.

â—†Component noise

The noise of the receiver front-end components (downconverter and amplifier) ​​also produces Nt, which affects the sensitivity of the handset, which is the specified sensitivity level of -104dBm achieved by the forward link power test stage. In addition, all other noise factors increase Nt and affect the success of the sensitivity test. Compared with mobile phones on the edge of performance indicators, mobile phones with certain tolerances have more room to accommodate increased noise.

â—†Earth noise

There are also many unknown sources of noise that also reduce the Eb/Nt of the forward communication channel. For example, any circuit with transmit power will generate spectral noise. The size depends on whether the signal itself or the intermodulation of two interfering signals has higher power falling on the measured bandwidth, and whether it appears together with the sensitivity test. External units can also generate interference noise, especially for AMPS systems when testing 800MHz CDMA handsets. There are even reports that microwave ovens can interfere with cell phone sensitivity testing. There are many production plants close to the production line for lunch. If you find a lot of sensitivity problems at noon or break time, you should know where to find the reason.

â—†Test under close contact

In a production environment, there are many test benches that test mobile phones at various stages according to the test plan. This means that a mobile phone that is being tested at one stage will interfere with another mobile phone that is tested at different stages. In general, it is the sensitivity of the interfered cell phone. Remember that the forward link is set at -104dBm. The main interference may be that the neighboring mobile phone is receiving a large forward link signal, and exactly one mobile phone is performing sensitivity test. The forward link signal is set at -104dBm. Or below. Mobile phone tests that set the forward link at a higher level include dynamic range, minimum transmit power, and open loop range. Generally, the forward link of these tests is -25 dBm.

Receiver and transmitter coupling

Another source of noise affecting the size of the Nt is the cross-coupling of the handset transmitter to the receiver, which is a problem with handset design due to the lack of proper isolation or matching between the receive and transmit channels. Because it is a design issue, the solution is costly and difficult. The forward and reverse links are separated by 45 or 80 MHz, so the interference between the unit band and the PCS band is highly isolated between the links, and the crosstalk from one link to the other is found at the front end of the mobile phone. Phenomenon, this is a problem with the design of the front end of the mobile phone.

Receiver channel sensitivity

Since the main sources of spectral noise are listed above, let's look at how these spectral noises enter the handset receiving channel.

Usually we use two methods to test the phone. The most common one is to test through a physical RF connection. This connection is often referred to as an "over-current" connection, while the less common test connection method is directly through the cell phone antenna. We assume that all of the tests discussed here are performed through a physical "over-current" connector that determines whether or not to connect the antenna based on the handset manufacturing process. In order to find the worst case, we assume that the antenna is already assembled into the phone.

In most cases, the phone may be susceptible to noise due to poorly shielded RF cables or unconnected antenna ports. In general, the antenna port is the main source of noise, especially if the antenna is connected during testing. Figure 3 shows the possible impact of a handset that is undergoing a sensitivity test on another handset with a forward link power of -25 dBm. The handset 1 is a handset under test with a high forward link power of -25 dBm. Since the transmission channel power of the mobile phone 1 is -25 dBm, assuming that the attenuation of the unconnected switch is 20 dB, the signal leaked through the unconnected port of the antenna may be as high as -45 dBm.

The magnitude of the attenuation varies by design. Since the signal leaking through the antenna is transmitted via air, its propagation attenuation can be calculated using the Friis conversion equation:

among them:

Pr=receive power

Pt = transmission power

Gr = antenna transmission gain

Gt = antenna receive gain


d=distance between Tx and Rx

Assume that in the worst case, the mobile phone 1 is 1 meter away from the mobile phone (assuming only the far-field effect, the near-field factor is difficult to consider), the two mobile phones are aligned in parallel and there is no attenuation material between the mobile phones, then the antenna of the mobile phone 2 Will receive a signal up to -83.92dBm. Thus, the receiver of the handset 2 will have an interference signal of -103.92 dBm because it is assumed that the attenuation of the antenna to the over-current switch is 20 dB. This example illustrates the general cause of noise generated by a mobile phone on the receiving channel of another mobile phone. According to the assumptions made in the example, there are still many situations in the actual execution that will cause different results, which is also to know which kind of noise will come from a The mobile phone provides the basis for accessing another mobile phone channel. Azimuth, distance, switching attenuation, shielding, antenna design and implementation, all of which play a role in the overall conversion of the introduced noise.

Points to note

• If it is tested by “over-current” connection, the antenna is not connected better than the connected one. Since the handset is designed to effectively transmit and receive signals around the required frequency band, the antenna can still receive external signals and be added to the front end of the transmitter and receiver if tested by physical over-current connections. After the antenna is connected, it is also possible to transmit a large signal to the outside at a higher power when measuring the transmitting end of the mobile phone. After eliminating antenna port matching (removing the antenna from the antenna connector) at the desired test frequency, the handset can receive and transmit signals at higher attenuation levels.

• Since external interference can enter the connection of the RF cable under test to the mobile phone, it is important to use a properly shielded cable. It is best to use an N-type connector with a triple shielded cable.

• Shielding the phone during testing can greatly attenuate external interference from other mobile phones or sources of unknown noise. The previous example of cell phone interference with the phone is useful for determining the shielding attenuation when needed for specific testing. Blocking the mobile phone from external interference is not the only way to eliminate noise. Careful handling of all measured mobile phone frequencies can also effectively avoid interference. This requires more sophisticated control software to initialize the channel number and frequency to eliminate frequency conflicts.

• Pay close attention to the distance between two mobile phones using the same frequency to achieve frequency reuse. The example of mobile phone interference with mobile phones is very useful in determining the full re-use distance.

Conclusion of this paper

When dealing with receiver interference problems, especially in production environments, it is not easy to find out why the yield is reduced due to sensitivity test failures. Factory location, working hours, test methods, and other seemingly unrelated factors. Will cause random sensitivity failure. Taking a step back to understand the main sources of interference noise and how these noise sources affect the receiver front end makes it easier to solve the problem of low sensitivity pass rate.

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