新型的OFDM自适应调制算法研究
摘要
无线通信技术无疑是当今通信领域最活跃的一支力量，一方面公众移动通信保持增长态势，特别是在中国等国家和地区增势强劲；另一方面，除传统移动通信外，全球的宽带无线接入、无线传输等领域近年来的研究和应用十分活跃，热点不断出现，包括MIMO技术、HSDPA技术、OFDM技术、UWB技术等，呈现出百花齐放的局面。这些技术的出现和发展，给整个无线通信产业的发展带来了生机和活力，但是随着无线应用的越来越多，无线频谱资源越来越紧张。于是，各种各样的具有较高频谱利用率的通信技术不断被开发出来，OFDM是目前已知的频谱利用率最高的一种通信系统。
OFDM技术的主要思想就是在频域内将给定信道分成许多正交子信道，在每个子信道上使用一个子载波进行调制，并且各子载波并行传输。OFDM系统将实际信道划分为若干个子信道，其好处之一就是能够根据各个子信道的信道状态信息(CSI)灵活地分配信息比特，进行自适应调制。目前已经存在的OFDM自适应调制算法基本上都是由xDSL上的自适应调制技术演化而来，由于考虑到算法的复杂度，对信噪比小于调制门限的子载波采取关闭策略。实际上这些子载波依然存在着传输能力，关闭这些子载波将会损失这部分传输容量。
论文在分析无线信道的主要特征和OFDM信道模型的基础上，提出了一种新型的OFDM自适应调制算法。该算法充分利用了传输能力较弱的子载波的信道容量，而在其他自适应调制算法中，这些子载波将被关闭，所以提高了系统的传输效率。当系统中某些子载波的信噪比小于调制门限时，采用子载波分集技术，对这些子载波进行分组合并，使子载波组的信噪比达到系统所要求的调制门限，再进行自适应调制。通过理论分析和仿真实验，证明这种自适应调制算法可以提高OFDM系统的性能。
It is no doubt that Wireless communication technology is the most active force at present in the telecommunications. On the one hand, public mobile communication keep uptrend particularly in China and some countries or regions, and it is showing a strong growth trend. On the other hand, in addition to the traditional mobile communications, global broadband wireless access and wireless transmission is very active in recent years on the research and application. Hotspot emerged, which includes MIMO technology, HSDPA technology, OFDM technology, UWB technology and etc. It's a flourishing situation. The emergence and development of these technologies have brought vitality for the development of the entire wireless communications industry. However, as more and more wireless applications are required, wireless spectrum resources become more and more strained. Thus, a wide range of high spectral efficiency of communications technology has been developed. OFDM (Orthogonal Frequency Division Multiplexing) is the maximum utilization of the spectrum in the known communication system.
The main idea of OFDM technology is that a given channel in the frequency domain is divided into many orthogonal subchannels. In each subchannel, using one sub-carrier to modulate one signal, all subchannels transmit those signals in parallel. OFDM actual channel is divided into a number of subchannels. One of its advantages is that the bits can be allocated flexibily based on different CSI (Channel state Information), and then adaptive modulation is made. OFDM adaptive modulation technology basically derived from the xDSL adaptive modulation technologies which take into account the complexity of algorithm. Therefore, if the local SNR of some subchannel are lower than modulation threshold, close those subchannel which still have remained capacities. It will lose some transmission capacity due to close that subcarrier.
Based on researched into the features of wireless channel and OFDM channel model, this thesis put forward a new adaptive modulation algorithm of OFDM. The algorithm can make full use of bad subchannels' transmission capacity, but in some other adptive modulation algorithm, those bad subchannels will be closed. When some subcarriers' local SNR is lower than the modulation threshold, those subcarriers are divided into groups by OFDM subcarrier-diversity technology, so each group-SNR can reach the required threshold, and then adaptive modulation is done. Through theoretical analysis and simulation results show that this adaptive modulation algorithm can improve OFDM system performance.
OFDM技术的主要思想就是在频域内将给定信道分成许多正交子信道，在每个子信道上使用一个子载波进行调制，并且各子载波并行传输。OFDM系统将实际信道划分为若干个子信道，其好处之一就是能够根据各个子信道的信道状态信息(CSI)灵活地分配信息比特，进行自适应调制。目前已经存在的OFDM自适应调制算法基本上都是由xDSL上的自适应调制技术演化而来，由于考虑到算法的复杂度，对信噪比小于调制门限的子载波采取关闭策略。实际上这些子载波依然存在着传输能力，关闭这些子载波将会损失这部分传输容量。
论文在分析无线信道的主要特征和OFDM信道模型的基础上，提出了一种新型的OFDM自适应调制算法。该算法充分利用了传输能力较弱的子载波的信道容量，而在其他自适应调制算法中，这些子载波将被关闭，所以提高了系统的传输效率。当系统中某些子载波的信噪比小于调制门限时，采用子载波分集技术，对这些子载波进行分组合并，使子载波组的信噪比达到系统所要求的调制门限，再进行自适应调制。通过理论分析和仿真实验，证明这种自适应调制算法可以提高OFDM系统的性能。
It is no doubt that Wireless communication technology is the most active force at present in the telecommunications. On the one hand, public mobile communication keep uptrend particularly in China and some countries or regions, and it is showing a strong growth trend. On the other hand, in addition to the traditional mobile communications, global broadband wireless access and wireless transmission is very active in recent years on the research and application. Hotspot emerged, which includes MIMO technology, HSDPA technology, OFDM technology, UWB technology and etc. It's a flourishing situation. The emergence and development of these technologies have brought vitality for the development of the entire wireless communications industry. However, as more and more wireless applications are required, wireless spectrum resources become more and more strained. Thus, a wide range of high spectral efficiency of communications technology has been developed. OFDM (Orthogonal Frequency Division Multiplexing) is the maximum utilization of the spectrum in the known communication system.
The main idea of OFDM technology is that a given channel in the frequency domain is divided into many orthogonal subchannels. In each subchannel, using one sub-carrier to modulate one signal, all subchannels transmit those signals in parallel. OFDM actual channel is divided into a number of subchannels. One of its advantages is that the bits can be allocated flexibily based on different CSI (Channel state Information), and then adaptive modulation is made. OFDM adaptive modulation technology basically derived from the xDSL adaptive modulation technologies which take into account the complexity of algorithm. Therefore, if the local SNR of some subchannel are lower than modulation threshold, close those subchannel which still have remained capacities. It will lose some transmission capacity due to close that subcarrier.
Based on researched into the features of wireless channel and OFDM channel model, this thesis put forward a new adaptive modulation algorithm of OFDM. The algorithm can make full use of bad subchannels' transmission capacity, but in some other adptive modulation algorithm, those bad subchannels will be closed. When some subcarriers' local SNR is lower than the modulation threshold, those subcarriers are divided into groups by OFDM subcarrier-diversity technology, so each group-SNR can reach the required threshold, and then adaptive modulation is done. Through theoretical analysis and simulation results show that this adaptive modulation algorithm can improve OFDM system performance.
引文
[1] R.W.Chang. "Synthesis of band—limited orthogonal for multichannel data transmission", Bell System Technical Journal.Vol.46.pp.1775—1796, December 1966
[2] S.B. Weinstein and P.M.Ebert, "Data transmission by Frequency Division Multiplexing using the discrete Fourier transform", IEEE Transmission on Communications Technology, Vol.COM-19, pp.628-634, October 1971
[3] M.Faill, "Digital land mobile radio communication COST 207,"tech.Rep., European Commission, 1989
[4] RICHARD Van Nee, RAMJEE PRASAD.OFDM for wireless multimedia communications [M].Boston (MA): Artech House, 2000
[5] PAPPAPORT S Theodore. Wireless communication principles & practice [M].New York: Prentice-Hall, Inc, 1996
[6] TONG Xue-jian, JI Hong, Yue Guan-xin.A quadrature coded OFDM for reducing effects of frequency off-set in wireless transmission system [J].The Journal of China Universities of Posts and Telecommunications, 2001, 8(2):11-15
[7] PROKIS J G Digital communication [M].New York: McGraw-Hill Inc, 1995
[8] CHANG Sekchin,POWERS E J. Cancallation of inter-carrier interference in OFDM system using a nonlinear adaptive filter[A],Proceedings of 2000 IEEE International conference on Communications, ICC'2000,Vol 2[C].Piscataway (NJ): IEEE, 2000.1039-1043
[9] CHEN Bor-sen, TSAI Chang-lab. Frequency offset estimation in an OFDM system [A].Proceeding of SPAWC'01[C].Piscataway (NJ): IEEE, 2001.150-153
[10] MOOSE P H. A technique for orthogonal frequency division multiplexing frequency offset correction [JJ.IEEE Trans on Communications, 1994, 42(10):2908-2914
[11] VISSER M A, ZONG Ping-ping, BAR-NESS Y.A novel method for blind frequency offset correction in an OFDM system[A].Proceedings of Ninth IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, Vol 2 [C].New York:IEEE,1998.816-820
[12] ZHAO Y, HAGGMAN S-G Intercarrier interference self-cancallation scheme for OFDM mobile communication systems Communications [JJ.IEEE Trans on Communications, 2001, 42(7):1185-1191
[13] PAHLAVAN, KAVEH. Wireless information networks [M].New York: John wiley & Sons Inc, 1995
[14]QIN Sheng-ping, QIU Yan, YIN Chang-chuan, LI Jian-feng, Yue Guang-xin. "The Effects of Time-varying Channels on OFDM".The Journal of China Universities of Posts and Telecommunicaitons.Vol.11, No.1, Mar.2004
[15] Louis L.Scharf, "Statistical Signal Processing", Addison-Wesley, 1991
[16] J.J.van de Beek, O.Edfors, M.Sandell, S.K.Wilson, and P.O.Borjesson, "On Channel Estimation in OFDM Systems", IEEE Trans. Commun., vol.46, Jul. 1998, pp.815-819
[17] Chow P S, Cioffi J M, Bingham J AC. A practical discrete multitone transceiver loading algorithm for data transmission over spectrally shaped channels. IEEE transactions on communications, 1995; 43(2):773-775
[18] Sistanizadeh k, Chow P S, Cioffi J M.Multi-tone transmission for asymmetric digital subscriber line (ADSL). 1993 International conference on communications, Geneva Switzeland, May 1993: 756-760
[19] 吴伟陵，牛凯．《移动通信原理》．电子工业出版社，2003年．pp．338
[20] Chow, P. S.; Cioffi, J. M.; Bingham, J. A. C.; "A practical discrete multitone transceiver loading algorithm for data transmission over spectrally shaped channels" Communications, IEEE Transactions on communications, 1995, vol. 43, Issue: 2, pp. 773-775
[21] ETSI. Radio broadcasting systems: Digital Audio Broadcasting (DAB) to mobile, Portable and fixed receivers[S]. European Telecommunication Standards Institute Std ETS, 1995. 300-401
[22] ETSI. Broadband Radio Access Networks (BRAN): HIPERLAN type 2 technical specification part 1: physical layer[S]. DTS/BRAN030003-1, 1999
[23] ETSI. Digital video broadcasting: framing structure, channel coding and modulation for digital terrestrial television[S]. European Telecommunication Standards Institute Std EN, 1997. 300-744
[24] IEEE. 802. 16e. IEEE Standard for WirelessMAN Access Control (MAC) and Physical Layer (PHY) specification[S]. 2004
[25] D. Hughes-Hartogs. Ensemble Modem Structure for Imperfect Transmission Media. U. S. Patents Nos. 4, 679, 227(July 1987), 4, 731, 816(March 1998) and 4, 833, 706(May 1989)
[26] Fischer, R. F. H.; Huber, J. B.; "A new loading algorithm for discrete multitone transmission", Proc. Globecomm′96. Communications, vol. 1, pp. 724-728
[27] J. Torrance, adaptive Full Response Digital modulation for Wireless Communicaitons systems. PhD thesis, Department of Electronics and computer Sience, University of Southampton, UK, 1997
[28] Rainer Grunheid, Edgar Bolinth, Herman Rohling, Kurt Aretz. Adaptive Modulation for the HIPERLAN2 Air interface
[29] 王文博，郑侃．《宽带无线通信OFDM技术》．人民邮电出版社，2003年．pp168
[30] Goldfeld L, Lyandres V, Wulich D. Minimum BER Power Loading for OFDM in Fading Channel. IEEE Trans Commun, 2002, 50(11): 1729-1733
[2] S.B. Weinstein and P.M.Ebert, "Data transmission by Frequency Division Multiplexing using the discrete Fourier transform", IEEE Transmission on Communications Technology, Vol.COM-19, pp.628-634, October 1971
[3] M.Faill, "Digital land mobile radio communication COST 207,"tech.Rep., European Commission, 1989
[4] RICHARD Van Nee, RAMJEE PRASAD.OFDM for wireless multimedia communications [M].Boston (MA): Artech House, 2000
[5] PAPPAPORT S Theodore. Wireless communication principles & practice [M].New York: Prentice-Hall, Inc, 1996
[6] TONG Xue-jian, JI Hong, Yue Guan-xin.A quadrature coded OFDM for reducing effects of frequency off-set in wireless transmission system [J].The Journal of China Universities of Posts and Telecommunications, 2001, 8(2):11-15
[7] PROKIS J G Digital communication [M].New York: McGraw-Hill Inc, 1995
[8] CHANG Sekchin,POWERS E J. Cancallation of inter-carrier interference in OFDM system using a nonlinear adaptive filter[A],Proceedings of 2000 IEEE International conference on Communications, ICC'2000,Vol 2[C].Piscataway (NJ): IEEE, 2000.1039-1043
[9] CHEN Bor-sen, TSAI Chang-lab. Frequency offset estimation in an OFDM system [A].Proceeding of SPAWC'01[C].Piscataway (NJ): IEEE, 2001.150-153
[10] MOOSE P H. A technique for orthogonal frequency division multiplexing frequency offset correction [JJ.IEEE Trans on Communications, 1994, 42(10):2908-2914
[11] VISSER M A, ZONG Ping-ping, BAR-NESS Y.A novel method for blind frequency offset correction in an OFDM system[A].Proceedings of Ninth IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, Vol 2 [C].New York:IEEE,1998.816-820
[12] ZHAO Y, HAGGMAN S-G Intercarrier interference self-cancallation scheme for OFDM mobile communication systems Communications [JJ.IEEE Trans on Communications, 2001, 42(7):1185-1191
[13] PAHLAVAN, KAVEH. Wireless information networks [M].New York: John wiley & Sons Inc, 1995
[14]QIN Sheng-ping, QIU Yan, YIN Chang-chuan, LI Jian-feng, Yue Guang-xin. "The Effects of Time-varying Channels on OFDM".The Journal of China Universities of Posts and Telecommunicaitons.Vol.11, No.1, Mar.2004
[15] Louis L.Scharf, "Statistical Signal Processing", Addison-Wesley, 1991
[16] J.J.van de Beek, O.Edfors, M.Sandell, S.K.Wilson, and P.O.Borjesson, "On Channel Estimation in OFDM Systems", IEEE Trans. Commun., vol.46, Jul. 1998, pp.815-819
[17] Chow P S, Cioffi J M, Bingham J AC. A practical discrete multitone transceiver loading algorithm for data transmission over spectrally shaped channels. IEEE transactions on communications, 1995; 43(2):773-775
[18] Sistanizadeh k, Chow P S, Cioffi J M.Multi-tone transmission for asymmetric digital subscriber line (ADSL). 1993 International conference on communications, Geneva Switzeland, May 1993: 756-760
[19] 吴伟陵，牛凯．《移动通信原理》．电子工业出版社，2003年．pp．338
[20] Chow, P. S.; Cioffi, J. M.; Bingham, J. A. C.; "A practical discrete multitone transceiver loading algorithm for data transmission over spectrally shaped channels" Communications, IEEE Transactions on communications, 1995, vol. 43, Issue: 2, pp. 773-775
[21] ETSI. Radio broadcasting systems: Digital Audio Broadcasting (DAB) to mobile, Portable and fixed receivers[S]. European Telecommunication Standards Institute Std ETS, 1995. 300-401
[22] ETSI. Broadband Radio Access Networks (BRAN): HIPERLAN type 2 technical specification part 1: physical layer[S]. DTS/BRAN030003-1, 1999
[23] ETSI. Digital video broadcasting: framing structure, channel coding and modulation for digital terrestrial television[S]. European Telecommunication Standards Institute Std EN, 1997. 300-744
[24] IEEE. 802. 16e. IEEE Standard for WirelessMAN Access Control (MAC) and Physical Layer (PHY) specification[S]. 2004
[25] D. Hughes-Hartogs. Ensemble Modem Structure for Imperfect Transmission Media. U. S. Patents Nos. 4, 679, 227(July 1987), 4, 731, 816(March 1998) and 4, 833, 706(May 1989)
[26] Fischer, R. F. H.; Huber, J. B.; "A new loading algorithm for discrete multitone transmission", Proc. Globecomm′96. Communications, vol. 1, pp. 724-728
[27] J. Torrance, adaptive Full Response Digital modulation for Wireless Communicaitons systems. PhD thesis, Department of Electronics and computer Sience, University of Southampton, UK, 1997
[28] Rainer Grunheid, Edgar Bolinth, Herman Rohling, Kurt Aretz. Adaptive Modulation for the HIPERLAN2 Air interface
[29] 王文博，郑侃．《宽带无线通信OFDM技术》．人民邮电出版社，2003年．pp168
[30] Goldfeld L, Lyandres V, Wulich D. Minimum BER Power Loading for OFDM in Fading Channel. IEEE Trans Commun, 2002, 50(11): 1729-1733