Digital Automatic Gain Control Integrated on WLAN Platform

In this work we present a solution for DAGC (Digital Automatic Gain Control) in WLAN receivers compatible to IEEE 802.11a/g standard. Those standards define communication in 5/2.4 GHz band using Orthogonal Frequency Division Multiplexing OFDM modulation scheme. WLAN Transceiver that we have used enables gain control over Low Noise Amplifier (LNA) and a Variable Gain Amplifier (VGA). The control over those signals is performed in our digital baseband processor using dedicated hardware block DAGC. DAGC in this process is used to automatically control the VGA and LNA in order to achieve better signal-to-noise ratio, decrease FER (Frame Error Rate) and hold the average power of the baseband signal close to the desired set point. DAGC function in baseband processor is done in few steps: measuring power levels of baseband samples of an RF signal,accumulating the differences between the measured power level and actual gain setting, adjusting a gain factor of the accumulation, and applying the adjusted gain factor the baseband values. Based on the measurement results of RSSI signal dependence to input power we have concluded that this digital AGC can be implemented applying the simple linearization of the RSSI. This solution is very simple but also effective and reduces complexity and power consumption of the DAGC. This DAGC is implemented and tested both in FPGA and in ASIC as a part of our WLAN baseband processor. Finally, we have integrated this circuit in a compact WLAN PCMCIA board based on MAC and baseband ASIC chips designed from us.

Authors:

• Emilija Miletic
• Milos Krstic
• Maxim Piz
• Michael Methfessel

Keywords:

• WLAN
• AGC
• RSSI
• baseband processor

References:

[1] A. Troya, K. Maharatna, M. Krsti¶Çâ╝, E. Grass, U. Jagdhold, R. Kraemer,
Low-Power VLSI Implementation of the Inner Receiver for OFDMbased
WLAN Systems, IEEE Transactions on Circuits and Systems I,
March 2008, Vol. 55, No. 2, pp. 672-686.
[2] P. Singerl, C. Vogel, A Fast and accurate Automatic Gain Control for a
Wireless Local Area Network Receiver, Proceedings of 2005 Global
Mobile Congress (GMC 2005), Chongqing (China), 10-12 Oct. 2005,
pp. 34-38, Special Session of Fourth Generation Mobile Forum
[3] D. Filipovic, "Digital Automatic Gain Control", patent application
WO/2003/105433.
[4] A. Troya, K. Maharatna, M. Krsti¶Çâ╝, E. Grass, U. Jagdhold, R. Kraemer,
Efficient Inner Receiver Design for OFDM-based WLAN Systems:
Algorithm and Architecture, IEEE Transactions on Wireless
Communications, Vol. 6, No. 4, Apr 2007, pp. 1374-1385
[5] S. S. Das, R. V.Rajakumar, M. I.Rahman, A. Pal, F. H.P.Fitzek, O.
Olsen, R. Prasad, "Low Complexity Residual Phase Tracking Algorithm
for OFDM-based WLAN Systems", Proceedings WPMC 2005, Aalborg,
Denmark, September, 2005.
[6] MX268x30A / MX860x30A Wireless LAN Measurement Software,
technical note, Anritsu Corporation.
[7] H. Rohling, D. Galda, R. Gr├╝nheid , OFDM: A Flexible and Adaptive
Air Interface for a 4G Communication System, Proc. International
Conference on Telecommunications (ICT) 2002, Beijing, June 2002
[8] A. Pandey, S. Ratan Agrawalla, S. Manivannan, VLSI implementation
of OFDM modem, white paper Wipro Technologies.
[9] M. Wouters, G. Vanwijnsberghe, P. Van Wesemael, T. Huybrechts, S.
Thoen, Real Time Implementation on FPGA of an OFDM based
Wireless LAN modem extended with Adaptive Loading, In Proc. Solid-
State Circuits Conference, 2002. ESSCIRC 2002. pp. 531- 534
[10] MAX2828/MAX2829 - Single-/Dual-Band 802.11a/b/g World-Band
Transceiver Ics, datasheet, www.maxim-ic.com.