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Clinton Chan - TelASIC Communications
Emerging wireless data services such as HSDPA, LTE, and WiMAX promise to provide mobile high speed access to services such as music and video download and full-featured Web browsing. The data usage demand by its very nature is unpredictable and bursty when compared to stable and predictable voice traffic. This places new demands on the wireless infrastructure network. One such demand is being especially careful with Spectral Emission Mask (SEM) violations. The purpose of this article is to identify transient SEM violations while transmitting bursty broadband networks. We will focus specifically on HSDPA networks although the same considerations apply to LTE and WiMAX networks.
WCDMA vs. HSDPA Waveforms
The following diagrams show the differences in signal characteristics of voice (WCDMA) and data (HSDPA) transmission.
The Voice (WCDMA) waveform on the left is relatively static while the data (HSDPA) waveform on the right is bursty (up to 6.6 dB amplitude change in one time slot of 666us). Bursty operation presents special challenges for base station downlink transmitters deploying power amplifiers suitable only for voice. For data applications, a transmitter's power amplifier must maintain linearity during signal power fluctuations caused by bursty data. Failure to do so will result in violation of SEM (spectral emissions mask) requirements during power level transition periods.
Analysing and Measuring SEM Violations
To analyze SEM violations caused by bursty data transmission, it is necessary to capture the transmitter output signal with high speed, wide bandwidth, and high dynamic range spectrum analyzers such as the Tektronix RSA6100A.
The above setup is used to test for SEM violation: the transmitter under test is fed by a signal source generating bursty HSDPA (modified TM5) signals. The PA output is captured using the Tektronix RSA6100A. A power meter is used to monitor the output RF power. SEM compliance can be determined by examining the PA output signal captured by the RSA6100A.
We obtained the following measured results after running three cases:
Case 1: Transmitting a non-bursty TM5 (HSDPA) test signal with conventional PA results in no SEM violations.
Case 2: Transmitting a bursty TM5 test signal with conventional PA results in SEM violations during power transition periods.
Case 3: Transmitting a bursty TM5 test signal using TelASIC's remote radio head featuring dynamic DPD PA linearization technology optimized for bursty operation results in no SEM violations at any time.
Recommendations
We recommend you use the set-up described above to see whether your radio network is going to violate SEM when using HSDPA transmission.
Avoiding SEM Violations
Rapid power fluctuations induce thermal effects in the transmit power amplifiers and makes linearization difficult. Specifically, power fluctuations cause self-heating in the PA output transistor, thus changing device characteristics. Speclinearization techniques are needed to compensate for these transistor changes in order to maintain proper operation and deliver high power efficiency while not violating SEM.
TelASIC's remote radio head (RRH) products incorporate unique power amplifier linearization techniques targeted for HSDPA today. TelASIC's digital predistortion (DPD) engine adaptively tracks signal level variation to kepower amplifier at optimal linearity at all times. As a result, TelASIC's RRH products achieve leading-edge power efficiency with high-quality RF performance not only with voice signals, but maintain the same high-qperformance in the face of bursty HSDPA data.
Conclusion
The bursty nature of broadband data usage can cause basestation transmitter power amplifiers to violate SEM specifications during power fluctuations. Hperformance test equipment is needed to analyze transmitters for correct behaviorwhen sending bursty data. In addition, special linearization techniques are needed to avoid SEM violations while achieving high power efficiencies. TelASIC has developed remote radio heads featuring PA technology optimized for emerging mobile broadband data applications such as HSDPA, LTE and WiMAX.
Author Biography
Clinton Chan is manager of customer solutions at TelASIC Communications. He received his MSEE from the University of Southern California in 1990. He also has a BSEE from UCLA (1985), and attended the Anderson School of Management at UCLA. He joined TelASIC Communications, Inc. in February of 2002. Clinton Chan holds a US patent in the field of phased array antennas and has over 20 years experience in the communications electronics industry. He formerly held positions at Multilink Technology and Boeing Space and Communications, working in design engineering and in project management.
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