Search mrcy.com

Innovation That Matters
TM
MICROWAVE PRODUCT DIGEST

Future PA Requirements: More performance, smaller size

Competition among the major wireless carriers over the past few years has brought about dramatic decreases in the cost of purchasing mobile phone plans. The recent price trends towards greater affordability have put at least one wireless phone in approximately 70% of U.S. households. In this massive group of users lies a key demographic of 15 to 19 year old teenagers, 40% of which are wireless subscribers. The mobile phones that they use have evolved into sophisticated devices that not only provide high quality digital voice calls, but support a range of data driven applications, such as text messaging, photo and video capture, games, ring-tones, even MP3 storage and playback capability. Their processing power has spawned many new development environments geared specifically towards handsets. More complex applications are being created for wireless devices every day and younger generations that are quick to utilize such technology have created a billion-dollar market. As mobile phones delve deeper into the realm of full-fledged multimedia devices, they are becoming more limited by the air interface over which they operate. Innovations in hardware and software for mobile devices have grown by leaps and bounds; the latest offerings of wireless companies include streaming video and music services.

With present third generation (3G) technologies offering marginal performance for these higher data rate requirements, carriers have a multitude of standardized and soon-to-be standardized choices for a better physical layer. Macro-cell technologies utilizing various transmission schemes, such as UMTS-TDD and WiMAX, show great potential with respect to both data rates and spectral efficiency and aim to eventually replace current networks by providing everything from mobile telephony to last-mile IP delivery.

For systems that utilize such standards, some of the more difficult challenges have been encountered in the design, development and implementation of the RF subsystems. Broadband is a relative term when used to describe data propagation and each time a faster system is made available, a benchmark is set. Consumers will not consider a technology state of the art unless its performance exceeds what is currently on the market, which in this case is the ~400 to 800 kbps offered by their cable operator or DSL provider.

The RF subsystems of 3G and future 4G networks not only need to meet these stringent performance requirements, but must operate under harsher conditions than today’s infrastructure. Instead of the equipment being located inside an air-conditioned shed at the base of the tower, it will now be on roof-tops, telephone poles, and other outdoor locations. Worse yet, the equipment must work properly in extreme environments, from the Arizona desert to the most northern locations on the globe. Since the base stations, also known as Node Bs, are located in obscure areas, cooling fans are typically not designed in. This creates an added obstacle for the Node B designer, since the heat of internal components can only be removed by normal convection.

Another challenge is that the base station needs to be small in size in order to be located in the required areas of operation. 19 inch rack mount components no longer can be implemented since the entire base station is typically the size of a small suit case. The end result is a size constraint on all of the internal components.

Cost is yet another challenge since more wideband Node Bs are required in a given area in order to broadcast the complex modulations used in 3G networks. In order for service providers to have reasonable ROIs after deployment, they have pushed base station costs down to nearly half of what they were a few years ago. Even with lower prices many large service providers are hesitant to deploy new technologies in their current networks since their ROI is not guaranteed. In the interim, they are touting new “high speed” services using their current networks but at data rates significantly lower than true 3G/4G technologies. In order for new technologies to gain acceptance and find providers willing to spend the necessary dollars, their equipment costs must be minimal.

The sub-systems/components used in these base stations must perform at the new levels described above in order for the OEM to be successful. They must operate over a wide temperature range, be smaller in size than present technologies, be affordable, and most importantly, provide advanced performance in order to meet the high linearity requirements of the transmission method. As many manufacturers already know, the performance requirements of the power amplifier (PA) are among the most difficult specifications to meet. The PA is typically the most non-linear component, and has a significant influence upon system temperature. Amplifier costs are beginning to come down, due to availability from far-eastern manufacturers, but quality and mean time to failure have not caught up to western designs. Other issues facing the PA in these networks are bandwidth capabilities, efficiencies and output options, none of which are easily met in one module.

In order to satisfy all of the OEMs requirements, Stealth Microwave has developed the LS Series of power amplifiers (see Figure 1). These new modules presently cover select bands from 1.9 to 2.7 GHz with output powers at 1 dB compression measuring 6 to 16 Watts.
Figure 1
The LS Series uses the latest GaAs FET technology along with a built-in predistortion linearizer which reduces the unwanted intermodulation distortion products (IMDs). The predistorter uses circuitry that reduces the IMDs created by the PA, which increases the theoretical intercept point. Using a two-tone comparison, the third order output intercept point (OIP3) of the LS series verses a similar non-linearized unit is dramatically improved by 7 dB. Due to this improvement in linearity, the LS models can deliver more than four (4) Watts of COFDM power (see Figure 2) and better than two (2) Watts of WCDMA power. Amplifiers that use feed forward as a method of correction also provide excellent linearity performance, but are typically too large and temperature sensitive to be used in outdoor applications.
Figure 2

COFDM performance plot for the SM2025-42LS
Other specifications of the LS Series include 52 dB of linear gain, ± .5 dB gain flatness and gain variation from 0? to +55?C of only ± .5 dB. The LS Series have several built in features, including a single DC supply of +12 Volts, over voltage protection and thermal protection which will turn the unit off if the case temperature exceeds +70 o C and will automatically turn on once the temperature has fallen back to normal levels. Logic On/Off control is also standard, allowing the end user to turn the unit on or off via a TTL signal. One optional feature is RMS Forward Power Detection which allows the end user to monitor output power via an analog voltage.

Frequency bands and output powers depend on the model chosen. Typical frequency bands are 2.0 to 2.5 GHz and 2.3 to 2.7 GHz. A wideband version covering 2.0 to 2.7 GHz is also available. Output powers at P1dB range from 6 Watts to 16 Watts. See Tables 1 and 2 for two examples of models already operating in the field.

SM2025-42LS
ULTRA LINEAR AMPLIFIER SPECIFICATION

Parameter Specification
Frequency Range2.0 – 2.5 GHz
Pout (P1dB) +42 dBm (min.)
Output Third Order Intercept Point (OIP3)+61 dBm
Linear Gain52 dB
Gain Flatness (over full band)± .5 dB
Gain Change (over temperature)± .5 dB
VSWR (Input/Output)1.8:1 / 1.5:1
DC Input Voltage+12 Volts
DC Input Current (operational)
5.5 Amperes
Mechanical Dimensions6.0 x 2.5 x .56 inches
RF ConnectorsSMA Female
Operating Temperature0º to +55ºC
Operating Humidity95% Non-condensing
Operating AltitudeUp to 10,000 feet above Sea Level


Table 1
SM2327-38LS
ULTRA LINEAR AMPLIFIER SPECIFICATION

Parameter Specification
Frequency Range2.3 – 2.7 GHz
Pout (P1dB)+39 dBm (typ.)
Output Third Order Intercept Point (OIP3)+58 dBm
Linear Gain 52 dB
Gain Flatness (over full band)± .5 dB
Gain Change (over temperature)± .5 dB
VSWR (Input/Output)1.4:1 / 1.5:1
DC Input Voltage +12 Volts
DC Input Current (operational)3.0 Amperes
Mechanical Dimensions6.0 x 2.5 x .56 inches
RF ConnectorsSMA Female
Operating Temperature0º to +55ºC
Operating Humidity95% Non-condensing
Operating AltitudeUp to 10,000 feet above Sea Level
Table 2


Besides the exceptional performance, the compact size of these new units sets them apart from the competition. Measuring only 6.0 L x 2.5 W x .56 H inches, the modules are approximately 1/5 the size of standard amplifiers with similar OIP3 capabilities. See Figure 3 for a size comparison of one of Stealth’s standard 2.5 GHz amplifiers with an OIP3 of 61 dBm, and the SM2327-42LS with the same OIP3. The 100W unit draws ~30A, while the LS only draws 5.5A.
Figure 3

Size comparison of an LS model (shaded) vs. a standard amplifier with comparable OIP3
The compact size allows OEMs to incorporate the modules into small, high density Node Bs which in-turn, allows them the freedom of placing the base station in discreet locations, such as telephone poles, or church steeples. Since the units draw less current than non-linearized units, they do not need additional cooling from fans or air conditioners. Their extended bandwidths also allow the system designers to use one module to cover multiple frequency bands versus several narrow band units. This saves on space, DC consumption and heat, all which translate into purchase savings for the OEM. The end result is lower fees from the service provider

Conclusion

In order for 3G/4G services to be successful, highly linear base stations, with large operating bandwidths are essential. In order to accomplish this, several small base stations are required to provide coverage. This in turn causes Node Bs to be deployed in remote areas, such as roof tops and telephone poles. The components inside need to provide exceptional performance over a wider temperature range than in the past. The power amplifier is typically the most non-linear element in the radio and usually creates the most heat. The Stealth LS series of power amplifiers solves these problems by providing amplifiers that are very small in size, draw a fraction of the current needed for standard power amplifiers, and are inexpensive in comparison to large amplifiers, especially units that utilize feed forward technology.