Researches in Broadband Single Carrier Multiple Access Techniques Workshop on Fundamentals of Wireless Signal Processing for Wireless Systems Tohoku University, Sendai, 2016.02.27 Dr. Hyung G. Myung, Qualcomm
Outline Introduction Frequency Domain Equalization Single Carrier FDMA Summary and References 1
Prof. Adachi Prof. David Goodman NTT DOCOMO AT&T Bell Labs Tohoku U. Polytechnic U. 2
WICAT Workshop on SC-FDMA 12 oral presentations 4 IEEE Fellows including prof. Adachi (keynote speaker) 3
Outline Introduction Frequency Domain Equalization Single Carrier FDMA Summary and References 4
Number of Mobile Users 8 7 6 5 4 3 2 1 0 Global Mobile Subscribers (billions) 1980 1985 1990 1995 2000 2005 2010 2015 5
Evolution of Mobile Traffic Voice Data Multimedia Data 6
Mobile Data Increase 7
Fundamental Constraints Shannon s capacity upper bound Channel bandwidth C = min(n T, N R ) BW log 2 1 + S N Spatial multiplexing Signal quality 8
Wider Bandwidth 100 MHz Demand for higher data rate is leading to utilization of wider transmission bandwidth. 20 MHz 200 khz 1.25 MHz 5 MHz GSM IS-95 UMTS/WCDMA LTE LTE-Advanced 9
Wireless Channel Wireless channel experiences multi-path radio propagation. 10
Multi-Path Channel Multi-path channel causes: Inter-symbol interference (ISI) and fading in the time domain. Frequency-selectivity in the frequency domain. 3GPP 6-Tap Typical Urban (TU6) Channel Delay Profile 2.5 Frequency Response of 3GPP TU6 Channel in 5MHz Band 1 2 0.8 Amplitude [linear] 0.6 0.4 Channel Gain [linear] 1.5 1 0.2 0.5 0 0 1 2 3 4 5 6 Time [ sec] 0 0 1 2 3 4 5 Frequency [MHz] 11
Multi-Path Channel - cont. For broadband wireless channel, ISI and frequency-selectivity become severe. To resolve the ISI and the frequency-selectivity in the channel, various measures are used. Channel equalization in the time domain or frequency domain Multi-carrier multiplexing Orthogonal Frequency Division Multiplexing (OFDM) Frequency hopping Channel-adaptive scheduling Channel coding Automatic repeat request (ARQ) and hybrid ARQ (H-ARQ) 12
Orthogonal Frequency Division Multiplexing Orthogonal Frequency Division Multiplexing (OFDM) Divide the broadband channel into much smaller subcarriers. Flat-fading on each subcarrier. Transmit data in parallel using orthogonal subcarriers. Channel response Subcarrier Frequency 13
OFDM - cont. Design issues of OFDM Cyclic prefix (CP): To maintain orthogonality among subcarriers in the pr esence of multi-path channel, CP longer than the channel impulse response is needed. Also CP converts linear convolution of the channel i mpulse response into a circular one. High peak-to-average power ratio (PAPR): Since the transmit signal is a c omposition of multiple subcarriers, high peaks occur. Carrier frequency offset: Frequency offset breaks the orthogonality and causes inter-carrier interference. Adaptive scheme or channel coding is needed to overcome the spectral null in the channel. 14
Outline Introduction Frequency Domain Equalization Single Carrier FDMA Summary and References 15
Frequency Domain Equaliztion Channel Time domain y h x x h 1 * y x h y Fourier transform Y H X Frequency domain 1 X H Y 16
FDE - cont. For broadband multi-path channels, conventional time domain equalizers are impractical because of complexity. Very long channel impulse response in the time domain. Prohibitively large tap size for time domain filter. Using discrete Fourier transform (DFT), equalization can be done in the frequency domain. Because the DFT size does not grow linearly with the length of the channel response, the complexity of FDE is lower than that of the equivalent time domain equalizer for broadband channel. 17
FDE - cont. In DFT, frequency domain multiplication is equivalent to time domain circular convolution. Cyclic prefix (CP) longer than the channel response length is needed to convert linear convolution to circular convolution. Or, perform overlap-and-add at the receiver without using CP. K. Takeda, H. Tomeba & F. Adachi, "Iterative Overlap FDE for DS-CDMA without GI," IEEE VTC-2006 Fall, Montreal. CP Symbols 18
FDE Most of the time domain equalization techniques can be implemented in the frequency domain. MMSE equalizer, DFE, turbo equalizer, and so on. - cont. References M. V. Clark, Adaptive Frequency-Domain Equalization and Diversity Combining for Broadband Wireless Communications, IEEE J. Sel. Areas Commun., vol. 16, no. 8, Oct. 1998 M. Tüchler et al., Linear Time and Frequency Domain Turbo Equalization, Proc. IEEE 53rd Veh. Technol. Conf. (VTC), vol. 2, May 2001 F. Pancaldi et al., Block Channel Equalization in the Frequency Domain, IEEE Trans. Commun., vol. 53, no. 3, Mar. 2005 F. Adachi et al., Broadband CDMA Techniques, IEEE Wireless Comm., vol. 12, no. 2, Apr. 2005 19
Single Carrier with FDE SC/FDE x n Add CP/ PS Channel Remove CP N- point DFT Equalization N- point IDFT Detect OFDM x n N- point IDFT Add CP/ PS Channel Remove CP N- point DFT Equalization Detect * CP: Cyclic Prefix, PS: Pulse Shaping 20
SC/FDE - cont. SC/FDE delivers performance similar to OFDM with essentially the same overall complexity, even for long channel delay. SC/FDE has advantage over OFDM in terms of: Low PAPR. Robustness to spectral null. Less sensitivity to carrier frequency offset. Disadvantage to OFDM is that channel-adaptive subcarrier bit and power loading is not possible. 21
Outline Introduction Frequency Domain Equalization Single Carrier FDMA Summary and References 22
Single Carrier FDMA Single carrier FDMA (SC-FDMA) is an extension of SC/FDE to accommodate multiple-user access. It has similar structure and performance to OFDMA. SC-FDMA is currently adopted as the uplink multiple access scheme in 3GPP LTE. 23
P-to-S S-to-P S-to-P P-to-S TX & RX structure of SC-FDMA N- point DFT Subcarrier Mapping M- point IDFT Add CP / PS DAC / RF Channel Detect N- point IDFT Subcarrier Demapping/ Equalization M- point DFT Remove CP RF / ADC * N < M * S-to-P: Serial-to-Parallel * P-to-S: Parallel-to-Serial SC-FDMA: OFDMA: + 24
P-to-S Why Single Carrier FDMA? Single Carrier : Sequential transmission of the symbols over a single frequency carrier. Time domain Frequency domain Time domain N- point DFT Subcarrier Mapping M- point IDFT Add CP / PS DAC / RF FDMA : User multiplexing in the frequency domain. 25
Subcarrier Mapping Data block size (N) = 4, Number of users (Q) = 3, Number of subcarriers (M) = 12. Terminal 1 Terminal 2 Terminal 3 subcarriers subcarriers Distributed Mode IFDMA, Block-IFDMA Localized Mode LTE Uplink 26
SC-FDMA and OFDMA Similarities Block-based modulation and use of CP. Divides the transmission bandwidth into smaller subcarriers. Channel inversion/equalization is done in the frequency domain. SC-FDMA is regarded as DFT-precoded or DFT-spread OFDMA. Dissimilarities Lower transmit peak power. Equalization performance. Multi-carrier MIMO receiver algorithm. 27
SC-FDMA and DS-CDMA In terms of bandwidth expansion, SC-FDMA is very similar to DS- CDMA system using orthogonal spreading codes. Both spread narrowband data into broader band. Time symbols are compressed into chips after modulation. Spreading gain (processing gain) is achieved. 28
SC-FDMA: Comparison * Subcarrier mapping: Frequency-selective scheduling SC-FDMA * SC transmission: Low PAPR * Time-compressed chip symbols * Time-domain detection * DFT-based FDE * Block-based processing & CP OFDMA DS-CDMA /FDE 29
0 subcarrier M- 1 SC-FDMA Modulation in LTE UL Zeros Subcarrier Mapping Localized mapping with an option of adaptive scheduling or random hopping. Serial- Parallel x M- 0, x1, xn 1 to- N-DFT -to- IDFT Parallel Serial Zeros x, x, xm 0 1 1 One SC-FDMA symbol 30
LTE-Advanced: Carrier Aggregation Carrier Component (CC) 20 MHz 100 MHz 20 MHz R8 LTE 60 MHz Noncontiguous 60 MHz Contiguous Uplink multiple access scheme Clustered DFT spreading: N-times DFT-spread OFDM (per CC). 31
What s Next for Single Carrier? Ultra-wideband transmission 5G expected to increase bandwidth up to 3~400 MHz Single carrier transmission in mmwave Ultra-wideband transmission Point-to-point transmission RF contraints Single carrier multiple access for Internet-of-Things (IoT) Narrowband opportunistic transmission Low power operation Different classes of latency requirement 32
Outline Introduction Frequency Domain Equalization Single Carrier FDMA Summary and References 33
Summary Using DFT-precoding and frequency domain equalization, single carrier transmission can be as effective as multi-carrier transmission for broadband channels. Salient feature of single carrier is low peak transmit power. Well-suited for uplink or power-constraint transmission. 34
References Frequency domain equalization H. Sari et al., Transmission Techniques for Digital Terrestrial TV Broadcasting, IEEE Commun. Mag., vol. 33, no. 2, Feb. 1995. D. Falconer et al., Frequency Domain Equalization for Single-Carrier Broadband Wireless Systems, IEEE Commun. Mag., vol. 40, no. 4, Apr. 2002. F. Adachi et al., Broadband CDMA Techniques, IEEE Wireless Comm., vol. 12, no. 2, Apr. 2005. IFDMA T. Frank et al., "IFDMA: A Scheme Combining the Advantages of OFDMA and CDMA," IEEE Wireless Commun., vol.14, no.3, June 2007. SC-FDMA H. G. Myung et al., Single Carrier FDMA for Uplink Wireless Transmission, IEEE Vehicular Technology Mag., vol. 1, no. 3, Sep. 2006. H. G. Myung & D. Goodman, Single Carrier FDMA: A New Air Interface for Long Term Evolution, John Wiley & Sons, Nov. 2008. 35
Congratulations! Thank you! Workshop on Fundamentals of Wireless Signal Processing for Wireless Systems Tohoku University, Sendai, 2016.02.27 Dr. Hyung G. Myung, Qualcomm