LECC 2006, Valencia Potential Upgrade of the CMS Tracker Analog Readout Optical Links Using Bandwidth Efficient Digital Modulation Stefanos Dris Imperial College London CERN PH-MIC-OE

A feasibility study for the development of a fast digital readout link for Super LHC (SLHC), based on the current CMS Tracker readout optical link components (first introduced at LECC 2005, Heidelberg). Why? A digital system based on the existing components that can deliver sufficient performance for SLHC operation could potentially be a cost- effective solution. How? The example of DSL: Advanced RF digital modulation is used to transfer data at 2-8Mbit/s over copper telephone lines (3-dB bandwidth of <30kHz). What is the data rate that we can achieve with bandwidth- efficient digital modulation over the current CMS Tracker optical link? Objectives & Motivation Introduction RF Modulation OFDM QAM Tests Conclusion

The Current CMS Tracker Optical Links Introduction RF Modulation OFDM QAM Tests Analog Pulse Amplitude Modulation (PAM) Equivalent digital resolution = 8bits Sample rate = 40MSamples/s Equivalent digital data rate = 320Mbit/s Conclusion

The Shannon Capacity Shannon determined that the capacity, C, of a noisy communication channel of bandwidth, B, is given by: C=B*log 2 (1+SNR) [bits/s] Digital Communications Introduction RFModulation RF Modulation OFDM QAM Tests 3-dB bandwidth ~150MHz (due to electronics) SNR=48dB (over 100MHz) Conclusion

Quadrature Amplitude Modulation (QAM) Passband Modulation: Data bits are modulated on top of a higher frequency sinusoidal carrier. We transmit symbols which represent several bits each. Each symbol corresponds to a combination of amplitude and phase. Compared to just varying the amplitude, more bits/symbol can be transmitted. Quadrature Amplitude Modulation Introduction RF Modulation OFDM QAM Tests Conclusion

QAM is simply the simultaneous amplitude modulation of two sinusoidal carriers which are 90° to each other. Quadrature Amplitude Modulation Introduction RF Modulation OFDM QAM Tests Q=+3V I=-1V Cos(2πf c t) Sin(2πf c t) Q*Sin(2πf c t) I*Cos(2πf c t) I*Cos(2πf c t) + Q*Sin(2πf c t) In-Phase Component Quadrature Component Looks like Euler’s formula! e jω =cos(ω)+j sin(ω) Conclusion

Quadrature Amplitude Modulation Introduction RF Modulation OFDM QAM Tests We can represent QAM symbols in the complex plane since they each have a unique amplitude and a phase. It is used like an eye diagram in binary systems. Conclusion

Quadrature Amplitude Modulation Introduction RF Modulation OFDM QAM Tests Example: 16-QAM received constellation diagram. log 2 (16)=4 bits/symbol 1000 symbols transmitted through a noisy channel Conclusion

Quadrature Amplitude Modulation Introduction RF Modulation OFDM QAM Tests The number of bits/symbol (4 bits/symbol) and the symbol rate (1MS/s) determine the total data rate (4Mbits/s), and channel bandwidth required (~4MHz). Conclusion

Quadrature Amplitude Modulation Introduction RF Modulation OFDM QAM Tests We can use more bits/symbol to increase the data rate in the same bandwidth, but at the cost of higher bit error rate (BER). 64-QAM: 6 bits/symbol, symbol rate=1MS/s, data rate=6Mbits/s Conclusion

Multi-Carrier Systems Introduction RF Modulation OFDM QAM Tests A page out of the ADSL book: Orthogonal Frequency Division Multiplexing (OFDM) Given the available bandwidth of our channel, we can: a)Use a single QAM carrier with a high symbol rate occupying the entire frequency range. b)We can split the channel into smaller slices. Each slice can have a low symbol rate QAM carrier. In systems such as ADSL and Wi-Fi, multiple carriers are used. Conclusion

Multi-Carrier Systems Introduction RF Modulation OFDM QAM Tests A page out of the ADSL book: Orthogonal Frequency Division Multiplexing (OFDM) Given the available bandwidth of our channel, we can: a)Use a single QAM carrier with a high symbol rate occupying the entire frequency range. b)We can split the channel into smaller slices. Each slice can have a low symbol rate QAM carrier. In systems such as ADSL and Wi-Fi, multiple carriers are used. Conclusion

Multi-Carrier Systems Introduction RF Modulation OFDM QAM Tests A page out of the ADSL book: Orthogonal Frequency Division Multiplexing (OFDM) Given the available bandwidth of our channel, we can: a)Use a single QAM carrier with a high symbol rate occupying the entire frequency range. b)We can split the channel into smaller slices. Each slice has a low symbol rate QAM carrier. In systems such as ADSL and Wi-Fi, multiple carriers are used. Conclusion

QAM Laboratory Tests Introduction RF Modulation OFDM QAM Tests Objective To test the feasibility of using QAM over a CMS Tracker readout optical link, and determine the data rate. We cannot test a high-speed multi-carrier system at this point (if we could…). We can, however, pass one QAM carrier at a time through the link: In each test, change the carrier frequency and analyze each carrier separately. Hence we are emulating an OFDM system by independently testing each of the carriers. Conclusion

Test Setup Introduction RF Modulation OFDM QAM Tests Modulator: Agilent E4438C Vector Signal Generator. Capable of QAM signal generation at symbol rates up to 50MSymbols/s, and 256-QAM (8bits/symbol). Maximum data rate = 50M*8 = 400 Mbits/s. Generates random data patterns internally. Demodulator: Agilent E4440A Spectrum Analyzer. Capable of QAM demodulation (up to 256-QAM), with 10MHz analysis bandwidth. Both instruments are made for telecom applications, and can work with carrier frequencies up to 27GHz. Conclusion

Test Setup Introduction RF Modulation OFDM QAM Tests Conclusion

Example Data Introduction RF Modulation OFDM QAM Tests 32-QAM (5bits/symbol), Carrier Frequency=320MHz, Symbol Rate=1MS/s, Power=-30dBm From the received constellations, we can estimate the SNR. Conclusion

Results Introduction RF Modulation OFDM QAM Tests Signal to Noise Ratio as a function of carrier frequency and transmission power We have essentially characterized the entire frequency range of the channel. This is all the information needed to calculate the achievable data rate using QAM over the Tracker optical links. (See S. Dris et al., LECC 2005 for details of the calculation involved) Conclusion

Results Introduction RF Modulation OFDM QAM Tests Conclusion Achievable data rate for various target BERs The total data rate achievable using QAM-OFDM also depends on the transmission power used which is implementation specific. The (most likely) achievable data rate is estimated at 3-4 Gbits/s.

We have shown that data rates in the Gbit/s range are possible over the CMS Tracker optical links, using a QAM-based digital modulation scheme. Forward Error Correction (FEC) has not been considered in this study. We could expect a lower BER for the same data rate. This upgrade path would require creating a proprietary and novel system (i.e. forget about COTS components and ‘following’ industrial trends). More R&D and collaboration with industry and engineering departments will be necessary if we are to use this concept in future readout systems. What will be? We have proved that the principle works, but the complexity and cost (as well as the projected power consumption) of hardware implementation needs to be investigated next. We will then decide on whether this is worth pursuing further… Conclusion Introduction RF Modulation OFDM QAM Tests Conclusion