Designing Radio for HSDPA, HSUPA

Ashis Khan, Vice President of Marketing and Robert Narumi, Manager of Customer Support, TelASIC Communications

Market Demand for HSDPA and HSUPA

Since the first mobile voice service was introduced by NTT DoCoMo in 1979, operators worldwide have added 1 billion subscribers during the first 15 years. They will soon be adding another billion subscribers by 2007.

Voice has been the driving force so far. Soon this is about to change as we look ahead.

There is a huge pent up demand among 2 billion subscribers for high-speed data services. Operators are also scrambling to offer data services to improve ARPU (Average Revenue Per User) and hence profitability. For consumers, the availability of high-speed data services means video clips on demand, television, streaming music, satellite navigation and high quality games, as well as the ability to record and send short movies. For business users, this means high speed Internet access from a laptop without looking around for a WiFi hot spot.

The wait for such data services will soon be over. Finally, next year is now; enter HSDPA and HSUPA.

But is the implementation for HSDPA and HSUPA radios same as that of UMTS? Or is there more to it?

Challenges for HSDPA-optimized UMTS Radio Design

First, OEMs must support HSDPA and HSUPA to the legacy UMTS base stations. Second, the challenge is to build radios that can effectively handle new demands of bursty data protocols like HSDPA and HSUPA – dramatically different from well-behaved voice call patterns. The designer of a HSDPA-optimized UMTS Radio must be able to support a scenario where some slots are transmitted at high power level (e.g., HSDPA), while others are maintained at low power levels (DPCH). Meeting these two challenges puts three new requirements on HSDPA-optimized radios.

These are:

1. Higher PA Linearity than ever before

For HSDPA, one major concern is the PA headroom in a base station supporting HSDPA services.  Linear PAs have to be operated more into the back-off region to accommodate for the higher peak to average (PAR) signals like HSDPA. In other words, key to HSDPA performance will be highly linear PA.

A typical 4-carrier HSDPA/ WCDMA transmit signal has a bandwidth of 20 MHz.  When the 4 carriers containing HSDPA signals are combined and transmitted using an MCPA, the transmission would have high inter-modulation distortion (IMD).  A typical IMD level is about 30 dBc and the up to the 7th order IMD must be suppressed.  Since the required practical transmit mask for WCDMA is about 55 dBc, crest factor reduction and transmit predistortion linearizer are required, and must provide about 25 dB correction capability.

To achieve such high PA linearity to support HSDPA, the designer will have to re-design the Radio for such higher linearity and use the digital predistortion  (DPD) linearization technology. To effectively correct for memory effects, the Radio will need processing up to 9th intermod. Such processing will require very high speed ADC and DACs.

2. Instantaneous Dynamic Range

From 3GPP specification, UMTS base stations require up to 25 dB power control range- for example, an HSDPA base station can change its transmit power more than 7 dB from one time slot to the next.  Therefore, an HSDPA base station must have high instantaneous dynamic range to handle highly dynamic signaling. 

In the legacy UMTS base station, low performance ADC and DAC technologies are used.  To support the dynamic range requirements, both analog and digital gain control are used.  However, these solutions are slow and thus not suitable to dynamic transmission in HSDPA

The HSDPA base station will therefore need significantly higher instantaneous dynamic range ADC and DACs.  To meet UMTS and HSDPA's requirement, a base station will require SNR of 152 dBFS/Hz (or 86 dBFS/3.84 MHz) in an ADC and 157 dBFS/Hz (or 91 dBFS/3.84 MHz) in a DAC.

3. Indoor Coverage

Data users will be primarily located inside buildings. Improved coverage will be the key to useful HSDPA service. Operators plan to use high transmit power for HSDPA services, and hence the designer will require very high PA-chain efficiency, to keep the total power dissipation of the Radio within a practical range. PA-chain efficiency is achieved by correcting memory effects using DPD linearization and again, the designer will have to bank on high speed ADC and DACs to implement DPD.

Enter Telasic Communications

Our research shows that TelASIC Communications (www.telasic.com) has the required ADC and DAC needed for HSDPA-optimized Radio. Their ADC (TC1411) and DAC (TC2411) provide the required dynamic range while providing the required signal bandwidth allowing direct IF sampling/synthesis eliminating the need for image correction/calibration simplifying the high-speed signal processing requirements. Only a single ADC and a single DAC is required to support more than 20MHz bandwidth Tx linearization requirements.

Conclusion

Supporting HSDPA and HSUPA requires a new way of designing the Radio. The designer must address the challenge associated with bursty data. TelASIC Communications offers solution for HSDPA-optimized Radio. If any designer is interested, one can contact info4hsdpa@telasic.com.