Bridging the Gap from the CMOS DSP to the Antenna in OFDM Systems
By Henrik Morkner, Gary Carr, Alan Rixon, Avago Technologies

Wireless data devices are fast encroaching on territory historically covered by wired systems.  New systems using WiFi or WiMAX OFDM modulation schemes can be tasked with handling large amounts of data for entertainment applications, like HDTV and gaming, making data access ubiquitous for everyday needs. OFDM system specifications present unique challenges for system designers. Avago Technologies is helping to drive this communications revolution by supplying captive-fabricated GaAs PHEMT products that effectively address performance, design and cost requirements for OFDM applications ranging from 2.4GHz to 50 GHz.

OFDM Landscape
Below 6 GHz, the trend for high volume applications has been to integrate the CMOS DSP to the antenna interface with a single FEM (Front End Module). This FEM must be very small to enable multiple-input, multiple-output (MIMO) placements in a laptop or other portable device. Additionally, the FEM should contain multiple functions including, Tx and Rx signal amplification, filtering, and switching. For the 6 to 33 GHz range, the trend has been to integrate single functions such as LNA, power amplifiers, drivers, and mixers into surface mount packages. Above 33 GHz, the market is still dominated by chip-and-wire, but innovative package solutions are on the horizon. The challenge for designers is to identify and leverage full-featured compact solutions that overcome OFDM’s technical issues while meeting system cost objectives.

CMOS Limitations for OFDM
DSP (Digital Signal Processing) within any OFDM (Orthogonal Frequency Division Multiplexing) scheme, such as WiFi (802.11), WiMAX (802.16) or licensed point-to-point radios, is dominated by CMOS processors. The strength of these processors is in their clever base band and RF band signal processing in standard CMOS. While CMOS makes an excellent signal processing medium, it falls short in the last stages of transmit power amplification, incoming low noise receive amplification, and signal up/down block conversion. For this last gap between the CMOS DSP and the antenna, Avago Technologies offers a variety of low cost solutions that complement CMOS in the quest to effortlessly move large amounts of wireless data.

Challenges with OFDM Power Transmission
The OFDM modulation scheme works by splitting the radio signal into multiple smaller sub-signals that are transmitted simultaneously at different frequencies to the receiver. OFDM reduces the amount of crosstalk in the system by the arrangement of the magnitude and phases into an optimum bit pattern. An example of a measured OFDM signal for WiFi is shown in Figure 1.

The fundamental challenge with an OFDM signal is the PAR (Peak to Average Ratio). In an ideal data frame, the PAR is about 10.8 dB. When the average transmit power is 0.1 W (typical for WiFi), the resulting peak power is 1 W, which poses an incredible challenge to the final transmit power amplifier. A simple CMOS class A amplifier would consume over 8 W of DC power (and dissipate resultant heat) to serve as the final transmit stage. This would be very impractical.

In most applications, GaAs-based power amplifiers working in class AB mode with distortion compensation are used. Here the PAR can often be reduced to a theoretical 6.6 dB if a base level of 2.6 dB EVM is acceptable through peak amplitude clipping. An example is shown in Figure 2. Generally, GaAs amplifiers can provide enough gain in 2 or 3 stages while CMOS takes 6 to 8 stages. The distortion compensation uses the natural AM-AM and AM-PM distortion of one stage and forces the following stage to distort in reverse. This technique allows the same average transmit power of 0.1 W in WiFi to be transmitted at 0.4 W peak power. Combined with class AB operation and distortion compensation, a modern WiFi power amplifier can accomplish this with only 0.66 W of DC power.

An example of an Avago Technologies’ class AB WiFi power amplifier with distortion compensation is shown in Figure 3. In this case, the amplifier is implemented in two stages of enhancement-mode PHEMT, but the same techniques could be used in other technologies. The main feature is that the quiescent current is set as low as possible, allowing the current to increase naturally as power level increases. As the first stage goes into AM-AM and AM-PM distortion, the second stage goes into reverse distortion. The result is a distortion compensation scheme that works over a limited frequency and power transmit range. This technique is used in all Avago Technologies’ WiFi and WiMAX power amplifiers and FEMs and the advantages are shown in Figure 4.

For frequencies above 6GHz, distortion compensation can be very hard to design and manufacture. Consequentially, the typical offering is class A power amplifiers with their associated high current. Since nearly all OFDM applications above 6 GHz (LMDS, Point-to-Point Radio, etc.) use a fixed-point base station, thermal and DC power management is easier. Avago Technologies has an entire family of class A power amplifiers that cover 6 GHz to 42 GHz in separate bands. Figure 5 shows an example of the AMMP-6408 MMIC. For the same 0.1 W average power transmission, this amplifier is backed off 10 dB from its 1 dB compression point. The resultant DC power consumption is 3 W, of which 90% must be dissipated away as heat from the 5x5 mm package.

Challenges with OFDM Signal Reception
Since receive signals are at such a low power level, the PAR is normally not an issue unless the transmitter is very close to the receiver. The most common problem faced in the reception of OFDM signals is the dynamic range of the receiver. The system noise figure determines the lowest signal that the OFDM system can detect and process. The filtering and input of 1 dB system compression determine the largest signal that can be processed. Unfortunately, the two factors can often be dynamically opposed.

In the upper level of signal reception, the desired signal may include low power, interferers, self-mixing products, and out-of-band transmitters (from cell phones to air traffic control radars) that can saturate the receiver. To reduce the effect of these unwanted signals, the best filters and switches combined with the highest 1 dB compression/lowest gain LNA (Low Noise Amplifier) must be used. A typical input 1 dB compression for an OFDM system is -2dBm in band.

The lowest level of signal reception is determined by the system noise figure. Unfortunately, the addition of filtering and switching in front of the LNA directly increases the system noise figure. Elements after the LNA contribute less to the system, since their effects are reduced by the LNA gain. If the LNA gain is more than 10 dB, then post LNA effects can be ignored. A typical goal for the system noise figure is under 4 dB. A quality LNA runs 1 to 2 dB of noise figure, which translates to a maximum loss of 2 to 3 dB in the filter and switch before the LNA. To use a lower quality LNA with a higher noise figure means that higher quality switches and filters must be used to meet the same system specs and vice-versa.

Avago Technologies also offers a variety of LNAs and discrete FETs that can be used to build LNAs for OFDM systems. The ATF-54xxx and ATF-55xxx ePHEMT discrete FET series are popular with many custom 0.1-18 GHz systems designed for high performance and low cost. These can be easily used in designs for non-standard frequencies or power levels. A typical application is shown in Figure 6. Avago’s MGA (0.1-6GHz) and AMMP-62xx (6-42 GHz) series are very popular with developers who want a higher level of integration for more standard applications. These MMICs often incorporate biasing, bypass switching, power down with full or partial RF matching to simplify the system design and create more consistent performance. An example of a popular MGA is shown in Figure 7 and a typical AMMP-62xx series LNA is shown in Figure 8.

Putting Transmit and Receive Together in ASIC Chips
When volumes are high enough, a custom ASIC (Application-Specific Integrated Circuit) may be justified. While cell phones have long used ASICs, especially for transmit and filtering functions, the trend is now taking hold in OFDM applications, such as WiFi 802.11.a/b/g/n FEMs. These might integrate all the active functions, including SPDT switches, low noise amplifiers, linear power amplifiers, power-down logic, and power-detection. This MMIC can be combined with a package and filter set to form a complete FEM. Figure 9 shows a single-chip solution based on ePHEMT that conforms to standard WiFi transmit and receive requirements. An ASIC bridges the gap in the final data transmission that effectively improves performance, saves footprint and speeds design time as shown in Figure 10.

Conclusion
Whether it be WiFi, WiMAX, or point-to-point radio, OFDM data formatting is the dominant way to wirelessly transmit and receive data. While CMOS provides excellent DSP functionality, it has difficulty in bridging the gap to the antenna. GaAs amplifiers or modules have a proven record to fill this gap. For transmit, GaAs circuits can provide power efficient solutions for high PAR. On the receive side, GaAs provides the dynamic range and resistance to saturation. Additionally, if integration of transmit and receive with multiple functions is desired, careful design of ASICs can provide a very cost-effective, high-performance solution for OFDM system requirements. Addressing the gap in OFDM systems will help to ensure high quality when transmitting wireless data.

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