Single-chip WiMAX transceivers offer distinct cost and area savings over discrete designs, but strict transmit performance requirements have traditionally precluded their use in base station designs. Some newer devices offer vastly improved Tx SNR performance, however, making them useful in applications ranging from femtocells to microcells.
Basestation Cell Types and Typical Requirements
The wireless industry divides basestations into several broad and overlapping categories based on maximum output power and coverage area; this is shown in Table 1, which provides ballpark estimates of these parameters. The design examples that follow use the power output estimates. Price and equipment size are critical in all types of cells. Femtocell basestations require low cost and small size just to be viable in the market; while picocell and macrocell basestations have a distinct market advantage when their price and size beat the competition. Single-chip transceivers help to reduce both price and size, but these advantages only apply if the basestation meets its performance requirements.
Transmit Signal Chain Architecture1 Transmit Signal Chain Block Diagram
Figure 1 shows a simplified block diagram of a typical direct up-conversion transmit signal path used for wireless communications, with a dashed rectangle around the blocks recently integrated into WiMAX CPE transceivers. Base station transmitters, especially those designed for larger cell coverage, typically use discrete components to achieve high linearity and low noise.
As Figure 1 shows, however, single-chip transceivers can offer significant cost and area advantages. These CPE transceivers combine several function blocks, including a digital interface, data converters, analog filters, gain stages, mixers, and pre-drivers, into a single mixed-signal integrated circuit. Incorporating the data converters and digital interface allows the baseband processor (BBP) to be purely digital, enabling the use of state-of-the-art fine-line CMOS processes to decrease cost, power, and size. Some integrated transceivers also incorporate two direct down conversion receivers, further reducing overall area and cost.
Tx Power and Noise
Transmitter noise is a critical parameter when comparing transceivers to discrete designs for a specific application, but noise is only one consideration. The noise must be specified at a particular output power, because the transceiver output power determines the gain required from the PA. For example, if transceiver "B" has a few dB lower noise than transceiver "A" but requires several dB more gain to achieve the same output power, the extra gain added to "B" will result in a higher system noise. The relationship between noise and gain is especially important when considering absolute emissions limits, such as those described in the design examples.
Table 2 shows this relationship using a quantitative example.Table 2
When working through the design process, a plot of Tx noise vs. frequency offset can be helpful. Regulatory spurious emission limits scale to dBm/MHz, allowing a quick determination of whether a transceiver will work in a given application. Figure 2 shows the plot for a multiple-input, single-output (MISO) WiMAX/WiBRO RF transceiver at a 2500MHz carrier frequency and 10MHz signal BW. Note that the frequency offset is the center of the 1MHz integration BW. Thus, if the center is 5.5MHz, the edge of the integration BW is 5MHz. 5MHz is the channel edge for a 10MHz BW signal of interest.
Although the 10MHz in-channel power output is -3 dBm, the in-channel emissions is -13 dBm, as shown in Figure 2, because the measurement integrates over 1MHz, not the entire 10MHz channel.
The very steep roll-off at the band edge in Figure 2 is due to the on-chip interpolating digital filters. Incorporating these on the transceiver chip relieves the BBP from this responsibility, and halves the data rate between BBP and transceiver due to the 2x interpolation.
2. Tx Noise vs. Integration BW Offset
Regulatory agencies place limits on the maximum output power in a particular band, as well as the maximum out-of-band (OOB) and out-of-channel emissions. These limits depend on the country of operation, as well as the frequency bands used. This article focuses on the FCC limits in the 2.4 to 2.7GHz range. In the United States, licensed WiMAX deployments fall between 2496MHz and 2690MHz. Unlicensed spectrum is available in the 2.4GHz ISM band (2400 MHz to 2483.5 MHz).
The FCC states maximum power and OOB limits using a variety of units and methods. The sections below list the limits for several frequency ranges used by WiMAX base stations. When the limit is in terms of attenuation below in-band output power, the integration BW must be the same for both the in-band power and the spurious emission measurements.
From 2496 MHz to 2690 MHz, the maximum Equivalent Isotropic Radiated Power (EIRP) band power output is 63 dBm. Spurious emissions of a base station must be attenuated at least 43 + 10log(P) dB at the channel edges, where P is the band power output in watts.
2.4 GHz Unlicensed Band
In this ISM band, the restrictions are more severe, but the maximum power output is also much lower. From 2400 MHz to 2483.5 MHz, the maximum conducted power output is +30dBm and the maximum EIRP is +36dBm for point-to-multipoint (PTMP) base stations . Point-to-point (PTP) base stations must abide by the same conducted power limit, but the maximum theoretical EIRP is limitless. For a PTP base station, the EIRP must be reduced by 1 dB for every 3 dB increase in directional gain of the antenna over 6 dB. Spurious emissions, out-of-channel but still within the ISM band, must be attenuated 20 dB below the signal of interest . Emissions outside of the ISM band must be attenuated 50 dB below the band power, or to a level of -41.25 dBm/MHz, whichever results in the lesser attenuation . Further, operators using the ISM band must abide by the restricted band limits mentioned below.
The FCC has labeled certain bands as "restricted." Emissions in these bands can only be spurious, and must always be equal to or less than the absolute limit of -41.25 dBm/MHz for operators in the ISM bands . Two restricted bands affect the 2.4GHz ISM band: from 2310 MHz to 2390 MHz, and from 2483.5 MHz to 2500 MHz. Licensed bands adjacent to restricted bands do not have to abide by these limits.
For quantitative evaluation of single-chip CPE transceivers in various scenarios, the power amplifier (PA) is assumed to be an ideal gain stage. In practice, the PA will add noise and nonlinear effects, but a PA can be designed for each of the examples below such that the performance of the complete transmit path meets the requirements. Each example uses a CPE transceiver set for a maximum output of -3.0 dBm in a BW of 10 MHz.
The first example is a 1W point-to-multipoint microcell in the ISM band transmitting in a 10MHz BW centered at 2417 MHz. The first regulation to meet is the out-of-channel attenuation of 20 dB. This attenuation from the in-channel power of -13 dBm/MHz results in the limit of -33 dBm/MHz, shown as the "20dB attenuation limit" in Figure 3.
Figure 3 includes Analog Devices' AD9354 performance curve from Figure 2, allowing a quick comparison of the regulatory limits and the transceiver emissions. Since the AD9354 out-of-channel energy is much lower than the -33 dBm/MHz limit line, the transceiver satisfies the 20dB attenuation requirement.
The second limit is the OOB attenuation of 50 dB or -41.25 dBm/MHz, whichever is the lesser attenuation. With a Pout = 30 dBm, 50 dB is by far the lesser attenuation. The frequency span between the signal-of-interest BW center and the ISM edge is 17.5 MHz (2417 MHz " 2400 MHz + 500 kHz). 50 dB attenuation from the -13 dBm/MHz in-channel power results in a -63 dBm/MHz limit, shown as the "50dB attenuation limit" in Figure 3. By inspection, the emissions from the AD9354 are much lower than this limit.
3 Tx Emissions vs. Frequency Offset with Regulatory Limits
Lastly, the FCC requires an attenuation of spurious emissions to a -41.25 dBm/MHz level at the 2390 MHz restricted band edge. The frequency span is 27.5 MHz (2417 MHz to 2390 MHz + 500 kHz). Because this is an absolute limit, the PA gain is taken into account. A gain of 33 dB is required to raise the in-band output power from "3 dBm to 30 dBm. Referencing this back to the transceiver output effectively moves the regulatory limit to -74.25 dBm/MHz (-41.25 dBm/MHz -33 dB). This is shown as the "-74.25dBm/MHz" line in Figure 3.
Femtocell in the Licensed Band
Femtocells are the most cost sensitive of all base station types, so it is imperative that a single-chip transceiver meet the requirements of this application. From table 1, a typical femtocell base station may provide up to 20 dBm output power in band. At the channel edges, the spurious emissions need to be reduced by 33 dB (43 + 10*log(0.1)). 33 dB attenuation from the in-channel power of -13 dBm/MHz results in a regulatory limit of -46 dBm/MHz, shown as a horizontal dotted line on Figure 3.
Picocell in the licensed band
Picocells encompass a very wide range of maximum output power levels, ranging from 20 dBm to 30 dBm. For this example, with the power output set to 27 dBm, the required attenuation is 40 dB (43 + 10*log(0.5)) at the channel edge. -13 dBm/MHz in-channel power - 40 dB attenuation results in a regulatory limit of -53 dBm/MHz. This limit is the horizontal line at -53dBm/MHz on Figure 3.
Microcell in the licensed band
Traditionally the domain of expensive and physically large discrete transmitter designs, a microcell requires significantly more gain and power output than the smaller cell sizes. The highest power output from table 2.2 W (33 dBm), requires an attenuation of 46 dB (43 + 10*log(2)). Apply the same logic as for the previous two examples, 46 dB attenuation below the in-channel power of -13 dBm/MHz results in a regulatory limit of -59 dBm/MHz. In Figure 3, comparing the AD9354 emissions curve to the horizontal dashed line at -59dBm/MHz shows that there enough margin to satisfy this requirement.
The transmitter performance of WiMAX CPE transceivers has improved such that these devices are now suitable for applications ranging from picocells to microcells. In addition to the cost and area savings afforded by integrating the transmit path, single-chip transceivers bring along two receivers which further reduce cost and area compared to discrete designs. This article discusses transmitter performance. A similar analysis on receivers is available by sending an email to email@example.com requesting the "Dynamic Range Whitepaper."
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About the Author
Patrick Wiers is a senior applications engineer focusing on integrated WiMAX transceivers at Analog Devices. He holds a B.S. in Electronic Engineering from California State Polytechnic State University in San Luis Obispo, CA. firstname.lastname@example.org