WCDMA - HSDPA Physical Layer

UbiNetics Academy organized the session on “WCDMA/HSDPA Physical Layer Design” at The Atria Hotel in December 2004. UbiNetics - CTO , Mr. Jon Burrell was the speaker.

He started with industry overview. Different mobile operators in different regions are implementing different technologies. He explained, the basics of WCDMA(Wideband Code Division Multiple Access) , UE (User Equipment = UMTS mobile handset) design, HSDPA extension in UE design.  

There will be around 100 million UMTS handset (UE) in year 2006. They will be multimode mobiles. HSDPA. WCDMA, EDGE (Enhanced Datarate for GSM/Global Evolution), GPRS (General Packet Radio Services) . WCDMA has more data rate because of QPSK (Quadrature Phase Shift Keying) modulation, Turbo coding and Multiple Transport Channel Combination. For Fwd. Link OVSF(Orthogonal Variable Spreading Factor)  is used to separate mobile users. Scrambling

Code is used to separate different cells, and identify the cell. For Rev. Link Scrambling is used to distinguish mobile users. Power Control is very important at Rev. Link. So 70 mico second fast feedback mechanism is used. 

He explains the basic hardware and software architecture for UMTS mobile set. There will be two clock pulse generators. 32 KHz and 13 to 17 MHz. So if HSDPA is not used, then one clock is stopped to conserve the battery.

Renesas is chip designing company, which supplies hardware for UbiNetics UEs. For HSDPA, at UE side , only Rake receiver is not sufficient. For short SF, in case of OVSF, the Rake receiver cannot perform well. So Equalizers

and G-Rake are also used. They are LMS based and MMSE based. MMSE equalizer needs to inverse a long matrix. One dedicated module at UE side for MAC (Medium Access Control) and periodic acknowledgements is implemented. It

also re-shuffles the out-of order data. Many mobile operators and technology solution providers including Lucnet, has used UEs from UbiNetics to test HSDPA implementation trial.

HSDPA, there will be 12 categories of terminals. AMC (Adaptive Multirate Coder) and HARQ (Hybrid Automatic Repeat Request) are key feature. In AMC, the transmitting power is not changed, but data rate is changed as per RF

conditions. More data and less protection bits for good RF condition and visa versa. UE can support upto 15 OVSF code in HSDPA. That means UE can receive Downlink Data from 15 different coded channels. UE sent CQI (Channel

Quality Indication) as feedback. It is 20 bits long, transmitted with SF = 256. HSDPA Physical layer has down link (FL) Shared Control Channel: HS-SCCH. It transmits that, next frame is dedicated to so and so UE, with so and so format, modulation, code etc. At Node B (UMTS BTS) side, scheduling algorithm decides, the candidate UE for downlink data. Round Robin(RR) and Proportional Fair (PF) are two major algorithms. In RR, all UEs, get equal data in Fwd. Link, regardless of their RF condition. So the UE, with poor RF condition, may request for re-transmission and overall cell capacity reduced. In PF, the UE, with good RF condition gets the priority. He presented bar graphs of cell capacity with both scheduling algorithms. He explained with time diagram for UMTS frame and time slots, that there will

be 5 milliseconds round trip time from Node B to UE, to get acknowledge of transmitted data. HARQ improves the cells capacity significantly. At UE side chase combining and Incremental Redundancy (IR) used. In the former, the UE

combines, the same data from different frame as per SNR. While in later, same data is re-transmitted with less data bits and more protection bits. So I asked him, the IR may be reducing the cell capacity. He replies, it can

increase also, as IR insured less re-transmission.

After HSDPA fundamentals, he again showed the same diagram of different hardware and software modules at UE. This time, the new modules for HSDPA implementation were highlighted. They are CQI reporting, Equalizers, HS-SCCH

detector, HS-PDSCH de-spreader, HARQ rate match, Layer one MAC-HS. QualComm comes up with HSDPA ASIC chip. The UE architecture is scalable from WCDMA to HSDPA implementation. In future, it will be extended for HSUPA (High Speed Up-link Packet Access). Most of the algorithm is implemented at hardware

side only. So there will be less load on DSP (Digital Signal Processor)

At the end, there was an interesting and interactive question answer session. I asked some questions, publically as well as personally. Ec/Io must increase for higher rates because the spreading gain reduces and this is usually done by increasing power. Soft Handoff is not possible for HSDPA data channel. At present, in the market UEs are available which supports 5 multiple codes. There will be practically 9 fingers at UE’s RAKE receiver. Some of them are

dedicated to keep track of pilot signals only.

I also asked him, about future of wireless. What next, after HSDPA? Will UbiNetics implement MIMO technology at UE side? Lucent Technologies/Bell Labs is pioneer for MIMO with BLAST ASIC chip implementation. He forecast,

HSUPA, OFDM (Orthogonal Frequency Division Multiple Access) will key technology for 4G networks at Radio side.

At last we had dinner. However, I started dinner so late because of my curiosity and questions on the topics, that when I finished, the waiters from Atria Hotel were waiting to close the hall. :)

WCDMA HSDPA physical layer design Power Point Presentation

Mastering HSDPA

I attended one training “Mastering HSDPA” by Awards Solutions, at our Lucent Technologies on 27-28 April 2005. The overall content was technical and mainly covered physical layer aspects. Yet some interesting matter I come to know, that I want to share with you.

·        HSDPA, EvDO all these technology promises very high data rate, it is just theoretical sector throughput. It is achievable only with high-end handset, no other traffic in that sector, and very good RF condition (may be line of sight just near to cell site). However, in reality, the user experience of throughput is quite different.

·        He also compared UMTS and cdma2000. He told, UMTS has more signaling overhead. UMTS is occupying 3 times more bandwidth. Yet it cannot provide 3 times data rate compare to cdma2000!!

·        HSDPA comes after UMTS. He discussed many major changes on Radio side compare to UMTS and HSDPA. There are few minor changes in core network side also. HSDPA standard moves scheduler from RNC to cell site, (Node B) for faster response to changing RF condition. We listed out inputs and outputs for scheduler. We could appreciate the high design complexity involves for this scheduler.

·        As the name suggests HSDPA is just downlink high-speed data solution. It does not address VoIP’s requirement of low latency. So VoIP over HSDPA should be avoided.

·        If new standard like HSDPA increase RF link capacity then backhaul capacity must be increased. Cell site is connected to core network by leased line. In North America, the price for leased line is $600 to $700. If network has 2000 cell sites then just rent for lease lines will be $ 6000 * 200

·        Generally all physical channel can be discriminated from logical or transport channel by its name. Physical channel contains P in their name. HS-SCCH is exception. It is a physical channel. Yet P is absent!!!

·        Handoff between two antennas belongs to same cell site is softer handoff. For Softer Handoff, the physical channels are different, but transport channel (lease line from network to Cell site) remain one only.

·        He talked about IP and politician comparison!! IP promises ‘Best effort delivery’ It means nothing. Yet do most of the things. Exactly opposite to our politicians. They Promises many things but …… :)

·        He shared his view about future technology. It will be any flavor of OFDMA. OFDM is more suitable for broadcast. At present OFDM is at the same stage where cdma was 12 years ago. Wi-Max also uses OFDM.

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