UMTS base station receiver occupies only half a square inch

How high can the integration be achieved under the premise of meeting the performance requirements of the macrocell base station? Process technology still defines certain important features that must be fabricated using a special process: GaAs and SiGe processes in the radio frequency (RF) field, high-speed ADCs in thin-line CMOS processes, and high-quality factor (High-Q) filters. It is well implemented using semiconductor materials. In addition, the market's demand for increased integration has not stopped.

In view of the above issues, Linear Technology decided to use system-in-package (SiP) technology to develop receivers with a footprint of approximately 1/2 square inch (just over 3 cm2). The receiver has a 50Ω RF input, a 50Ω LO input, an ADC clock input, and a digital ADC output at the boundary. This boundary is left to add low noise amplifier (LNA) and RF filtering for input, LO and clock generation, and digital processing of digital outputs. Within the 15mm x 22mm package is a signal chain using SiGe high frequency components, discrete passive filtering and thin line CMOS ADCs.

This article will analyze the design of the LTM®9004 micromodule (μModule®) receiver (a direct conversion receiver).

Design goals

The design goal is the Universal Mobile Telecommunications System (UMTS) Uplink Frequency Division Duplex (FDD) system, especially for medium coverage base stations in the working band I (see 3GPP TS25.104 V7.4.0 specification for details). Sensitivity is a major consideration for receivers. When the input signal-to-noise ratio (SNR) is -19.8dB/5MHz, the required sensitivity is ≤-111dBm. This means that the effective noise floor at the receiver input must be ≤-158.2dBm/Hz.

Design Analysis: Zero IF or Direct Conversion Receiver

The LTM9004 is a direct conversion receiver with an I/Q demodulator, baseband amplifier and dual 14-bit 125Msps ADC (shown in Figure 1). The LTM9004-AC low-pass filter has a 0.2dB corner at 9.42MHz, allowing for 4 WCDMA carriers. The LTM9004 can be used with an RF front end to form a complete UMTS band uplink receiver. The RF front end consists of a duplexer and one or more low noise amplifiers (LNAs) and ceramic bandpass filters. To minimize gain and phase imbalance, the baseband link uses a fixed gain topology. Therefore, an RF variable gain amplifier (VGA) needs to be placed before the LTM9004. Typical performance examples for such front ends are given here:

Receive (Rx) frequency range: 1920MHz to 1980MHz

RF gain: 15dB (max)

Automatic Gain Control (AGC) Range: 20dB

Noise figure: 1.6dB

IIP2: +50dBm

IIP3: 0dBm

P1dB: -9.5dBm

Suppression at 20MHz: 2dB

Suppression on the transmit (Tx) band: 96dB

UMTS base station receivers occupy only half a square inch of floor space

Figure 1: Direct conversion architecture implemented in the LTM9004 micromodule receiver

Considering the effective noise impact of the RF front end, the maximum allowable noise caused by the LTM9004 must be -142.2dBm/Hz. The typical input noise of the LTM9004 is -148.3dBm/Hz, resulting in a system sensitivity of -116.7dBm.

Typically, such receivers can benefit from DSP filtering of certain digitized signals after the ADC. In this case, assume that the DSP filter is a 64-tap RRC low-pass filter with α = 0.22. In order to operate in the presence of co-channel interference signals, the receiver must have sufficient dynamic range at maximum sensitivity. The UMTS specification requires a maximum co-channel interference of -73 dBm. Note that for a modulated signal with a 10dB crest factor, the input level of -1dBFS is -15.1dBm within the IF passband of the LTM9004. At the LTM9004 input, this is equivalent to -53dBm, or a digitized signal level of -2.6dBFS.

When the RF Auto Gain Control (AGC) is set to the minimum gain, the receiver must be able to demodulate the expected maximum signal from the handset. This requirement ultimately sets the maximum signal size that the LTM9004 must provide at -1dBFS or below. The minimum path loss required in the specification is 53 dB, and the average power of the handset is assumed to be +28 dBm. Then at the receiver input, the maximum signal level is -25dBm. This is equivalent to a peak of -14.6dBFS.

Several blocking signals are detailed in the UMTS system specification. In the presence of such signals, only a specified size of desensitization is allowed, with a sensitivity index of -115 dBm. The first of these is an adjacent channel 5 MHz apart with a level of -42 dBm. The peak value of the digitized signal level is -11.6 dBFS. DSP post processing adds 51dB rejection, so this signal is equivalent to a -93dBm interference signal at the receiver input. The final sensitivity is -112.8dBm.

Moreover, the receiver must also compete with a -35dBm interference channel that is >10MHz apart. The IF rejection of the μModule receiver will attenuate this interfering channel to a digital signal level equivalent to a peak of -6.6dBFS. After DSP post processing, it is equivalent to -89.5dBm at the receiver input, and the final sensitivity is -109.2dBm.

In addition, out-of-band blocking signals must also be considered, but the levels of these out-of-band blocking signals are the same as the in-band blocking signals already discussed.

In all of these cases, the LTM9004's -1dBFS typical input power average is much higher than the maximum expected signal level. Note that the crest factor of the modulated channel will be approximately 10dB ~ 12dB, so at the output of the LTM9004, the largest of them will reach a peak power of approximately 6.5dBFS.

The largest blocking signal is a -15dBm continuous wave (CW) tone (beyond the receive band edge ≥ 20MHz). The RF front end will provide 37dB of rejection for this tone, so it will be -32dBm when it appears at the input of the LTM9004. At this point, the signal of this level value still does not allow to reduce the sensitivity of the baseband μModule receiver. The equivalent digitization level peak is only -41.6dBFS, so there is no effect on sensitivity.

Another unwanted source of interfering signal power comes from the transmitter's leakage. Since this is an FDD application, the receiver described here will be coupled to a transmitter that operates simultaneously. The transmitter's output level is assumed to be ≤ +38dBm, while the "send to receive" isolation is 95dB. Then, the leakage at the LTM9004 input is -31.5dBm and the offset from the received signal is at least 130MHz. The equivalent digitization level peak is only -76.6dBFS, so the sensitivity is not reduced.

One challenge with direct conversion architectures is second-order linearity. Undesired second-order linearity will allow any desired or undesired signal to enter, which will cause DC offset or pseudo-random noise on the baseband. If such pseudorandom noise is close to the receiver's noise level, then those blocking signals discussed in detail above will reduce sensitivity. In all cases where these blocking signals are present, the system specification allows for a reduction in sensitivity. The -35dBm blocking channel reduces sensitivity to -105dBm as specified by the system specification. As we saw above, this blocking signal forms a -15dBm interference signal at the receiver input. The second-order distortion produced by the LTM9004 input is approximately 16 dB lower than the thermal noise. As a result, the predicted sensitivity is -116.6 dBm.

A -15dBm CW blocking signal will also result in a second order component; in this case the component is a DC offset. DC offset is undesirable because it reduces the maximum signal that the A/D converter can handle. A reliable way to mitigate the effects of DC offset is to ensure that the second-order linearity of the baseband μModule receiver is high enough. At the input of the ADC, the predicted DC offset due to this signal is <1mV.

Please note that transmitter leaks are not included in the system specifications. Therefore, the sensitivity drop due to this signal must be kept to a minimum. The output level of the transmitter is assumed to be ≤ +38dBm, while the "send to receive" isolation is 95dB. The loss of sensitivity due to second-order distortion generated in the LTM9004 will be <0.1dB.

There is only one requirement for third-order linearity in the specification. This is in the case where there are two interfering signals, the sensitivity must not fall below -115dBm. The two interfering signals are a CW tone and a WCDMA channel, both of which are -48 dBm in size. These interfering signals appear at the input of the LTM9004 at -28dBm. Their frequencies are 10 MHz and 20 MHz apart from the desired channel, so the third-order intermodulation components will be on the baseband. At this time, this component still appears in the form of pseudorandom noise, which causes the signal to noise ratio to decrease. The third-order distortion produced in the LTM9004 is approximately 20 dB lower than the thermal noise floor and the expected sensitivity reduction is <0.1 dB.

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