Multicarrier WCDMA Feasibility, Part 3

See Part 1 for an analysis of receiver performance specifications.
See Part 2 for more on the variable gain amplifier (VGA) implementation and spurious free dynamic range (SFDR).
See Part 4 to learn about PA linearization.
See Part 5 for an an in-depth look at transmit modulation and direct conversion.

There are several options for the architecture of the transmit signal path. Figure 7 shows a direct-conversion architecture for an initial point of reference only. Section 6 of 3GPP TS 25.104 describes the transmit signal requirements. Throughout all of the architectures to be discussed, there is an assumption that there is a channel filter at the output of the power amplifier (PA) that is sharp enough so as not to desensitize the receive path and to ensure spurious emissions, when colocated, are filtered sufficiently.

Frequency Error
The specification mandates that the same source is used for RF frequency and data clock generation. This implies that with a 3.84 Mb/s data rate, all IF and RF sources should have 3.84 MHz as an integer divisor. As a consequence, converter sample rates of 30.72 MSPS, 61.44 MSPS, 76.8 MSPS, 122.88 MSPS, and 245.76 MSPS, which represent multiplication factors of 8, 16, 20, 32, and 64 are common in WCDMA applications.

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Power Control
In terms of power control, the maximum output power is defined as the mean power level per carrier measured at the antenna. For a wide area base station this should be greater than +38 dBm with an integration bandwidth of 3.84 MHz. The specification allows for power control to be applied to each carrier at the antenna output, and on a code channel basis for user quality of service control.

The per carrier power control needs to have a minimum of 18 dB dynamic range. For a system using a single carrier per digital/analog converter (DAC), the dynamic power control is best placed in a variable gain amplifier (VGA), in order to optimize the dynamic range requirements of the DAC. For a multicarrier system in which there is a common power control setting for all carriers, this should be adjusted in the VGA.

It is possible that all but one carrier of a multicarrier system can be 18 dB below the single carrier (see Figure 8). If the spectral performance for a single carrier and for multiple carriers each at maximum dynamic power can be achieved, this scenario would not stress the DAC's dynamic range requirements any further. The dynamic range can be incorporated into the DAC's requirements; however, this would increase the dynamic range requirements of the DAC. (This is a possibility with high dynamic range DACs such as the AD9786 and AD9726, but for the following analysis it is assumed that an analog VGA will be present.)

When closed-loop power control is implemented, the base station keeps lowering a code channel's power until the user equipment (UE) detects an increase in the error rate. The UE closes the loop with the base station and in such a way maintains a specified quality of service.

Inner loop power control is the base station's part of the closed-loop code-channel power control and the specification mandates a 1 dB step size with a range of ±12 dB in extreme conditions and ±9 dB for normal conditions. This power control is performed at the code channel level, before the composite carrier is formed. If the code channels required for UE synchronization (P-CPICH, P-SCH, S-SCH, PCCPCH) are used, the power level of the code channel has only a small effect on the composite carrier's peak-to-average ratio (PAR), and hence, marginally negligible effect on the dynamic range requirements of the analog downlink blocks.

Peak-to-Average Ratio (crest factor)
The power amplifier that drives the antenna has opposing performance metrics when considering efficiency and linearity. The amplifier is most efficient when driven into saturation, but also has its worst linearity in saturation. Conversely, an amplifier driven for linearity is highly inefficient. Typically, a compromise is found between linearity and efficiency. This results in amplifiers that are operated in a mode where the average operating point is set such that the signal crests are just less than the maximum saturated output power that the amplifier can deliver.