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Upgrading your WCDMA Base Station for HSDPA? Your Radio Head Must Transmit Higher Power for Maintaining Link Quality.
Upgrading NodeB for HSDPA
Operators worldwide are in the process of upgrading the WCDMA Base stations to provide support for HSDPA. With the saturation of voice-services in developed countries, operators are looking for new revenue growth. The HSDPA-based data services open up ARPU-rich additional revenue streams for operators.
Current WCDMA technology (based on Release 99 specification) is efficient for voice services only. The HSDPA-upgrade, based on 3GPP Release 5 specifications, keeps the same voice mechanisms but adds highly efficient data capability. HSDPA-upgrade is therefore extremely important for operators.
Does the HSDPA-upgrade involve baseband Software in the NodeB's only? How about RF transmission? Can you keep the same radio?
The answer is: you need much higher transmit power.
The need for High Transmit Power and use of 16QAM
The HSDPA protocol is designed to enable operators to serve a large number of subscribers with high data rate and quality of service. HSDPA introduces a "fat pipe" of high speed data channel (HS-DSCH) that is shared by a large number of subscribers served by the NodeB. Within a cell, some subscribers will be closer to the Radio and some at the edge of the cell, so that link quality varies for individual users. The HSDPA protocol implementation requires that only users with a certain link quality, measured by Signal-to-noise ratio (Eb/N0), can experience high data rates.
To maintain service quality, an HSDPA-compliant NodeB adjusts the data rate for each user according to radio link quality as reported by the user's equipment. Since HSDPA uses "Adaptive Modulation and Coding," or AMC, the HSDPA NodeB adjusts data rate by switching the modulation from high data rate 16QAM, which is optimal for subscribers closer to the Radio, to the lower rate QPSK, which is more efficient for users at the edge of the cell. Current implementations of AMC introduce seconds of data processing delay when switching modulation formats, so switching should be minimized for good user experience.
But 16QAM is also less power efficient. In other words, the links to the high speed subscribers must have higher quality (Eb/N0) because of the use of 16QAM coding scheme. For handling many links with higher quality (Eb/N0), the operator must use a HSDPA Radio with higher transmit power to maintain high data rates and minimize AMC switching.
For example, it can be demonstrated that the required Eb/N0 is approximately 2dB higher for every step as one steps goes from Release '99 WCDMA, to HSDPA-QPSK, and then to HSDPA-16QAM. Therefore, HSDPA-16QAM requires about 4dB (2.5x) better Eb/N0 than "voice-only" WCDMA. For example, an existing WCDMA NodeB, optimized for voice-centric applications and therefore designed to transmit 20W, would take 50W to service an equivalent number of data customers when transmitting HSDPA-16QAM. If the transmit power were not increased, only a few high-speed data customers can be accommodated.
So the base station designed thermally, mechanically and electrically to handle 20W now must transmit 50 W to serve more HSDPA customers. What are the challenges for the RF designer of the base station?
Challenges of Transmitting High Power
First, the designer will need a much higher efficiency RF Transmit subsystem to transmit higher power (50W in above example) within the same enclosure originally designed to dissipate a fraction of the power (20W).
The need for high transmit power also brings up the challenge of meeting absolute RF emissions requirements that do not scale with transmit power. 3GPP TS 25.104 defines spectral emissions and absolute spurious emissions that shall not be exceeded. Since the levels of allowable emissions are the same regardless of transmitted power, the linearity requirement of the transmitter's power amplifier becomes more difficult to meet as transmit power increases.
Enter TelASIC offering High Power Efficiency and High Linearity for PA
TelASIC Communications, Inc offers Remote and Integrated Radio Head products to facilitate high power HSDPA transmission. The high level of power efficiency and high linearity is made possible by TelASIC's internally developed components for power amplifier linearization technology, based on Dynamic Crest Factor Reduction (DCFRTM), Dynamic Digital Pre-distortion (DDPDTM), and high speed, high resolution ADCs and DACs purpose-built for dynamic signals like HSDPA.
For further details, contact info@telasic.com
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