Transmitter Implications

Co-location Deployment Considerations for Direct RF Sampling Transceivers (WP541)

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The following two figures illustrate the typical RF signal lineup for transceivers with direct RF conversion and ZIF conversion architectures, respectively. The RF components for both architectures are very similar, with overall transceiver performance largely determined by the dynamic range and impairments of the data converters. Besides the usual non-linear harmonic distortions of the data converters, the impairments might include converter alias products, DC offsets, and images created from IQ imbalance, where applicable.

Figure 1. Typical Transceiver RF Lineup with RF Sampling Data Converters
Figure 2. Typical Transceiver RF Lineup with Zero IF Architecture

The co-location requirement for the transmitter is straightforward. On the transmit direction, the design must ensure the output power density in the band from 1850 MHz to 1910 MHz be less than –98 dBm/100 kHz according to Table 1. Because this is a wide area type 1-H BTS with 32 possible active transmitters, it must also conform to the sub-clause for BTS type 1-H in in [REF 1]. Specifically, it must account for emissions from more than one transmitting antenna. Using the second criteria in the conformance requirement, the emission limit becomes:

–98 dBm/100 kHz – 10log10(8) = –107 dBm/100 kHz

The value 8 in the logarithm term represents the number of counted transceiver array boundary (TAB) connectors in the TAB connector TX minimum cell group [REF 1].

This out-of-band emission requirement is largely the responsibility of the antenna filter in the radio architectures shown in the previous figures. The bandpass filter (BPF) following the DAC and balun output helps, but only to the degree that it filters the PCS band noise down to below the thermal noise level of –174 dBm/Hz. Because the noise spectral density (NSD) at the output of the DAC is roughly –160 dBm/Hz, any BPF with a reasonable rejection performance works. Inexpensive filters such as the Johanson 3750BP14D0900 [REF 2] in a 0603 footprint or similar are good candidates. This particular filter provides more than 50 dB of rejection in the PCS band.

Typical values for the TX cascade gain stages that are commercially available are shown in the following table.
Table 1. Typical Cascade Gain Stage Parameters
Gain Value
Total gain at 3.8 GHz 63 dB
Total gain at 1.9 GHz 63 dB
RMS output power 41 dBm
# of gain stages Three or four stages
Wide-band cascade noise figure 6 dB

Without loss of generality, the total gain of the amplifier stages is assumed to be the same at both 1.9 GHz and 3.8 GHz because most low-power gain blocks have higher gain at 1.9 GHz than 3.8 GHz, while it is the opposite for the final power amplifier. The output noise level at 1.9 GHz after the circulator (with IL = –0.3 dB) is:

–174 dBm/Hz + 6 dB NF + 62.7 dB Gain = –105.3 dBm/Hz or –55.3 dBm/100 kHz

To meet the –107 dBm/100 kHz requirement with, for example, 10 dB of design margin, the antenna filter needs to provide 62 dB (–55.3 – (–107) + 10) of rejection in the PCS band. This level of rejection is not very difficult to achieve given the PCS band is 1.8 GHz away from the lower edge of the C-band. The typical rejection value for commercially available antenna filters is in the 75 dB range. The cost and complexity of the antenna filter is often constrained by the close-in operating band unwanted emissions (OBUE) rather than the co-location requirement.