Understand receiver dynamic range and AGC tradeoffs in WiMAX femtocells and picocells (Part 1 of 2)

These smaller base stations, sometimes called microcells or picocells, are typically operator-installed equipment. Femtocells and picocells are small and ideally the size of a WLAN access point (20 cm × 15 cm × 5 cm = 1500 cm³), so they can be mounted easily on walls of buildings, on top of light posts, or in tunnels. They have limited power budgets (often powered over Ethernet) and must achieve excellent performance, as this will enhance the customer's experience while minimizing capital expense (CAPEX) requirements.

In addition to deploying macrocells, microcells, and picocells, some operators are investigating consumer purchased and installed femtocells to improve coverage in a home or small office. When a WiMAX subscriber enters the home, the consumer terminal will connect to the femtocell, which then connects to the network through a backhaul, such as an existing DSL or cable modem. The femtocell will provide several benefits for both the subscriber and the operator. The subscriber will benefit by getting excellent in-home coverage and maximum data rates, while the operator benefits from extended network coverage, more efficient spectrum utilization, and reduced customer churn.

As a consumer product, the femtocell base station market is predicted to reach two million units in 2012 (from ABIResearch: "WiMAX Market Analysis and Forecast"), but in order to achieve the projected volume, the femtocell must be affordable. Reaching these consumer price targets will require an understanding of the standard and of the specific usage scenarios expected in femtocell deployments. This article proposes cost- and power-efficient techniques that will allow system manufacturers to develop high-performance femtocells and picocells using WiMAX transceivers, such as the AD935x from Analog Devices.

The WiMAX standard In order to start the femtocell dynamic range analysis, the key performance requirements for the WiMAX standard are identified in Table 1. These requirements come from the WiMAX Forum's Mobile RCT (Radio Conformance Testing) document.

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Some observations about these requirements:

• The sum of the noise figure (NF) and implementation loss is 13 dB. It is unlikely that a system that barely meets the minimum requirements will be competitive in the marketplace. Based on this, a 5.5-dB noise figure (NF) (3.5-dB RFIC NF + 2-dB front-end loss) and 1 dB implementation loss will be assumed. (Note: NF is 3.5 dB at maximum gain.)
  While femtocells are intended to cover a relatively small area, state-of-the-art sensitivity is still important in order to reduce the mobile subscriber (MS) transmit power requirement, reducing network interference while maximizing mobile subscriber battery life.
• With a 10-MHz desired channel and using the assumptions above, the QPSK sensitivity is –94.6 dBm and the 16QAM sensitivity is –84.8 dBm.
• The base station must be able to receive a –45-dBm signal in addition to achieving state-of-the-art sensitivity.
• The minimum MS transmit power is –22 dBm (23-dBm maximum power minus 45 dB of power-control range). The minimum path loss assumed in this paper is 23 dB. Since a femto base station could be on a desk, it is possible for the mobile subscriber antenna to be very close to the base station antenna.

With 3.5-MHz bandwidth, the input-referred noise power with a 5.5-dB NF is about –103.1 dBm, so the ADC noise, referred to the antenna, must be less than –113.1 dBm. The maximum signal power at the antenna is –45 dBm and the peak-to-average ratio (PAR) used in this calculation is 8 dB, although it could be as large as 12 dB.

Allowing 3-dB margin for gain variation means the ADC's antenna-referred full scale must be –34 dBm. Thus, as shown in Figure 1, the ADC's dynamic range must be about 79.1 dB (effective number of bits, ENOB = 12.8 bits) for the 3.5-MHz bandwidth mode without utilizing any AGC. In 10-MHz bandwidth mode the noise is relaxed by 4.5 dB, but the sample rate increases, holding the required ADC figure-of-merit constant.

Figure 1: ADC dynamic range requirements for a 3.5-MHz signal bandwidth
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In practice, the calculated dynamic range could be inadequate. The –45 dBm maximum input signal may not be adequate for a femtocell application because the base station may be on a desk, and it is conceivable that the mobile subscriber could be as close as 10 to 20 cm from the base station. As illustrated in Figure 5 (further below), the path loss could be as small as 20 dB, resulting in a –42 dBm input power. The problem could be further exacerbated by higher antenna gain.

These practical considerations could drive the ADC ENOB requirement to more than 14 bits. Compounding this further is the requirement for most femtocells to support multiple-input, multiple-output (MIMO) transceivers. A MIMO implementation would require up to four relatively expensive 14-bit ADCs, an impractical cost for a consumer targeted device.

Given the impractical nature of the implementation described above, other implementations need to be explored to achieve not only the performance required, but also the targeted price points. To start, the above dynamic range calculation assumes constant receiver gain. A close reading of the RCT reveals that it is not really necessary to receive the –45 dBm signal in the same gain configuration as the signal at the specified sensitivity level.

Automatic gain control (AGC) can be used to dramatically reduce the required dynamic range. Most system designers know that adding variable gain is much less expensive than increasing ADC dynamic range. The disadvantage of using variable gain is that all signal levels cannot be received at the same time. If, for example, the gain is reduced to receive large signals, the increased noise figure will make it difficult to receive small signals. The impact of variable gain on WiMAX base station performance is discussed in the following sections.

Low-cost WiMAX base station using an integrated transceiver
One possible solution to address the cost versus dynamic range performance trade-off is to consider an integrated transceiver that offers excellent performance and built in AGC. High-performance, highly integrated, CMOS direct-conversion transceivers, such as Analog Devices' AD935x, cover all WiMAX profiles in the 2.x GHz and 3.x GHz frequency bands.

Unlike most other RFICs, these devices include on-chip ADCs and DACs with on-chip digital filtering. The JESD207 interface is used for easy connection to digital baseband modems. This class of RFIC device contains optional system-level features including autonomous AGC, dc-offset calibration, closed-loop transmit power control, and radio calibration hooks. The high level of integration enables the reduction of the radio bill of materials. The chip partitioning enables an all-digital modem IC manufactured in the latest 65 nm or 45 nm CMOS technology, which can further reduce system cost and power dissipation. A block diagram of such a transceiver is shown in Figure 2.

Figure 2: Integrated WiMAX Transceiver block diagram.
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To demonstrate how the AGC can be used to extend dynamic range, Figure 3 shows the receiver error vector magnitude (EVM) vs. input power for various gain settings. The best sensitivity is achieved at the maximum gain setting of 75 dB, but the EVM falls below –15 dB with a –67 dBm signal. Reducing the gain to 54 dB, for example, increases the maximum signal power to –45 dBm, at the expense of a 4 dB sensitivity degradation.

Figure 3: Receive EVM versus input power with 75 dB and 54 dB gain settings
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Two methods can be used to increase the dynamic range. The simplest is to use slow AGC, which updates the gain on a frame-by-frame basis. The gain is constant during a frame, so a gain reduction that results from a very large input can impair the receiver's ability to service distant mobiles. This near-far problem is discussed in the next section. A more robust technique is to use a combination of MAC-assisted grouping of users and the fast AGC mode. This technique is described in a subsequent section.

Femtocell near/far problem
Figure 4 shows the performance implication of extending dynamic range by using AGC.

Figure 4: Near/far problem.
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The following questions need to be answered:

• If mobile subscriber #1 is transmitting at minimum power, how close can the base station be such that the received power is –45 dBm?
• If the base station is receiving a –45 dBm signal, how much gain reduction is required and how much is the NF impacted?
• How does the base station NF degradation affect how far mobile subscriber #2 can be from the base station?

Figure 5: Line-of-sight (LOS) and non-line-of-sight (NLOS) path loss in a 2-floor office environment
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If the mobile subscriber is transmitting –22 dBm and the maximum BS input power is –45 dBm, the minimum path loss is 23 dB, which occurs at 15 cm. Although this is quite close, it is possible, since the BS could be on a desk in a residential femtocell application.

Table 2 summarizes the calculated maximum range and the range with a –45 dBm input with various modulations and a 10-MHz bandwidth. For this case, the 10-MHz bandwidth mode is chosen as the wider signal bandwidth results in shorter range.

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The impact of using gain control rather than a very-high-resolution ADC to maximize dynamic range is this: First, by implementing AGC, very high gain can be used to completely suppress the ADC noise under small signal conditions. In transceivers such as the AD935x, the ADCs contribute just 0.15 dB to the NF at maximum gain. If, instead, a high-resolution ADC were used, at least 0.5 dB of NF would have to be budgeted to the ADC because ADC dynamic range is so expensive. Since path loss with distance is proportional to 30 log(d), the 0.5 dB higher NF of the fixed gain system would reduce the maximum range by about 4%.

These calculations assume a 0 dBi antenna, but real systems could use higher-gain antennas, particularly in picocell deployments. With higher-gain antennas, the base station will see a larger signal at the receiver when the mobile subscriber is transmitting at minimum power from a given distance from the base station. With the higher signal present, the base station will have to further reduce the receiver gain, resulting in increased base station receiver NF. In this scenario, the fast AGC helps to prevent clipping for high input signal levels.

Conclusion
Using a fully integrated transceiver solution with a built-in, flexible AGC will allow system developers to meet strict performance requirements while maintaining acceptable price points for picocell or femtocell equipment. However many system developers want to add margin into their system design beyond what the RCT requires.

(Part 2 of this article will present media access controller (MAC) algorithms to show how additional system margin can be achieved; click here to read Part 2)

About the authors
Tony Montalvo is the Director of Analog Devices' Raleigh Design Center in Raleigh, North Carolina. Before joining Analog Devices, Tony. led the RF IC group at Ericsson Inc and was involved with the design of Flash memories at Advanced Micro Devices.. He received a B.S. in Physics from Loyola University in 1985, an M.S.E.E. from Columbia University in 1987 and a Ph.D. from North Carolina State University in 1995 where he is also an Adjunct Professor.

Manish Manglani is a systems design engineer focusing on wireless infrastructure products at Analog Devices Inc. He graduated with a Master's degree in Electrical Engineering from Virginia Tech in 2001.

Chris Cloninger is the marketing manager for the WiMAX transceiver products at Analog Devices Inc. Previously, Chris worked as the Marketing and Application Manager for Analog Devices' High Speed Analog-Digital Converters group. He holds a Bachelor's degree in Computer Engineering from Clemson University.

For questions or comments, please send an email to wimaxtransceivers@analog.com.