US009136937B1 (12) United States Patent (10) Patent No.: US 9,136,937 B1 Cheng et al. (45) Date of Patent: Sep. 15, 2015 (54) SYSTEMAND METHOD FOR PROVIDING (56) References Cited ANTENNA DIVERSITY IN MULTIPLE SUB-CARRIER COMMUNICATION SYSTEMS U.S. PATENT DOCUMENTS 7.305.056 B2 * 12/2007 Kroeger ........................ 375,348 (75) Inventors: Hao-Ren Cheng, Miaoli County (TW); 7,636,593 B2 * 12/2009 Kurioka . 455,575.7 William J. McFarland, Los Altos, CA 8,755,476 B2 * 6/2014 Li et al. ....... 375/347 2004,022.9650 A1* 11, 2004 Fitton et al. ... 455,561 (US) 2005/0107045 A1* 5/2005 Kroeger .......................... 455.83 2005.0245224 A1* 11/2005 Kurioka ...... 455,272 (73) Assignee: Qualcomm Incorporated, San Diego, 2005/0254608 A1* 11/2005 Lee et al. .... 375/347 CA (US) 2007/0230604 A1* 10, 2007 Nakamura .. 375,260 2008/005104.6 A1* 2, 2008 Ruckriem ... ... 455,132 2008. O139153 A1* 6, 2008 Tuo et al. .... 455/2772 (*) Notice: Subject to any disclaimer, the term of this 2010/009 1904 A1* 4/2010 Wang et al. ... 375,296 patent is extended or adjusted under 35 2010, 0183099 A1* 7, 2010 Toda et al. .. ... 375/340 U.S.C. 154(b) by 680 days. 2010/0195754 A1* 8, 2010 Li et al. ......................... 375,267 (21) Appl. No.: 12/706,141 FOREIGN PATENT DOCUMENTS (22) Filed: Feb. 16, 2010 WO WO 2008133298 A1 * 11, 2008 * cited by examiner (51) Int. Cl. H3K9/00 (2006.01) Primary Examiner — Linda Wong HO3D 3/00 (2006.01) (74) Attorney, Agent, or Firm — Bay Area Technology Law H04B I/44 (2006.01) Group PC H04B 7/85 (2006.01) H04B 7/06 (2006.01) (57) ABSTRACT H04B 7/08 (2006.01) A system and method to provide antenna diversity by Switch (52) U.S. Cl. ing between antenna feeds to optimize signal quality. Antenna CPC .......... H04B 7/18545 (2013.01); H04B 770602 conditions are determined during an OFDM guard interval to minimize disruption of symbol transmission. When the signal (2013.01); H04B 7/0802 (2013.01); H04B quality assessment determines that an improved signal is 7/0805 (2013.01); H04B 7/1855 (2013.01) available on an alternative antenna, an antenna change is (58) Field of Classification Search performed to Switch to the alternative antenna, also during a USPC ......... 375/219, 295,316, 328,346, 351, 220, GI, again minimizing disruption in information transmission. 375/222, 240.26-240.28, 254, 259,285, Preferably, the process is continually applied, so that antenna 375/284, 278, 299, 307, 324,339, 340, 348, conditions are monitored for changing conditions and the 375/354; 455/3.01, 3.04, 428,403, 13.3, 7, antenna experiencing the best signal is selected. 455/39, 63.3, 73,78, 83,560 See application file for complete search history. 22 Claims, 6 Drawing Sheets 41 thresholds?doppler and delay \. 402. Determine current antenna power 403. 'ss Current antenna power below feshold 4. Test atternate antenna Alternate antana power above threshold? Switch to new antenna s Hold at new antenna U.S. Patent Sep. 15, 2015 Sheet 2 of 6 US 9,136,937 B1 307 N. 306 305 w M 309 FIG. 3 U.S. Patent Sep. 15, 2015 Sheet 3 of 6 US 9,136,937 B1 401 - Doppler and delay thresholds? 402 --- Determine current antenna power 403 Current antenna power below threshold? Test afternate antenna Alternate antenna power above threshold? 408 Switch to new antenna Hold at new antenna FG. 4 U.S. Patent Sep. 15, 2015 Sheet 4 of 6 US 9,136,937 B1 -3 -SS - - - - - - - - - - - - - - - - - - - - - - - - 1 - - - - - Doppler Frequency (Hz) F.G. 5 ls -8) 8S - - - - - - F - -S 10' Oppler Frequency (Hz) FIG. 6 U.S. Patent Sep. 15, 2015 Sheet 5 of 6 US 9,136,937 B1 i L r Doppler Frequency HZ ) FIG 7 .3 - - - - - - - - - - - - - - - - - L- - - - - - - - - - - -9. Doppler Frequency (HZ) FIG. 8 U.S. Patent Sep. 15, 2015 Sheet 6 of 6 US 9,136,937 B1 RSS (13-seg) O YO 8 sec US 9,136,937 B1 1. 2 SYSTEMAND METHOD FOR PROVIDING characteristics In pilot-based systems, a known symbol, or ANTENNA DIVERSITY IN MULTIPLE "pilot, is transmitted at given Sub-carrier frequencies and at SUB-CARRIER COMMUNICATION SYSTEMS given times. Since the receiver knows the transmitted symbol, any errors to the transmitted pilot due to Sub-carrier condi FIELD OF THE PRESENT INVENTION tions can be estimated and an appropriate correction calcu lated. Channel conditions for all Sub-carriers and times can The present invention relates to improving signal quality in likewise be interpolated from the pilot information, allowing mobile digital communication systems in general and more equalization of the signal and Subsequent coherent demodu specifically to methods and systems for providing antenna lation diversity in mobile, wireless OFDM systems. 10 Further details regarding the design of OFDM systems can be found in co-pending, commonly-assigned U.S. patent BACKGROUND OF THE INVENTION application Ser. No. 12/272.629, filed Nov. 17, 2008, Ser. No. 12/277,247, filed Nov. 24, 2008, Ser. No. 12/277,258, filed Advanced multimedia services continue to drive require Nov. 24, 2008, Ser. No. 12/365,726, filed Feb. 4, 2009, and ments for increasing data rates and higher performance in 15 Ser. No. 12/398,952, filed Mar. 5, 2009, all of which are wireless systems. Current technologies for high performance hereby incorporated by reference in their entirety communication systems, such as those specified by the Japa Antenna diversity is a well recognized technique that gen nese integrated services digital broadcasting terrestrial stan erally employs two or more antennas to provide improved dard (ISDB-T), employ communication methods based on quality and reliability by reducing multipath interference. Orthogonal Frequency Division Multiplexing (OFDM). The multiple antennas give the receiver different observations As known to those of skill in the art, multipath interference of the same signal, each of which are subject to different presents a significant impediment to effective wireless com conditions. Accordingly, even if one antennais Suffering from munication. Since a clearline-of-sight rarely exists between a poor reception, an alternate antenna may still have good sig transmitter and receiver, a signal typically gets reflected by nal quality. buildings and other various obstacles. As a result, multiple 25 One class of existing diversity reception technologies versions of the signal travel along different paths before arriv includes systems that combine the signals received from each ing at the receiver. Each path is subject to different conditions antenna. An example of this technique involves daisy-chain which can introduce phase shifts, time delays, attenuations, ing receivers so that a multiple receivers associated with each and other distortions that can destructively interfere with one antenna combines the signal with the others in series A more another at the receiver. For example, the variable transmission 30 Sophisticated system uses maximum ratio combining (MRC) delays can result in inter-symbol interference (ISI) when the to selectively weight and add the signals coherently to pro different data signals arrive at the receiver simultaneously vide better results than simple addition Such methods are In OFDM multiple sub-carrier systems, a higher rate data widely used in mobile and portable digital TV receivers How signal is divided among multiple narrowband Sub-carriers ever, any system that relies on combining signals must have that are orthogonal to one another in the frequency domain. 35 duplicate a significant portion of the signal processing archi Thus, the higher rate data signal is transmitted as a set of tecture for each antenna, depending upon the stage at which parallel lower rate data signals carried on separate Sub-carri the signals are combined. Accordingly, such techniques offer CS good performance but at a Substantial cost, typically almost a A received OFDM symbol in an OFDM system generally multiple equivalent to the number of antennas employed consists of both data and pilot synchronization information 40 compared to a single antenna system. transmitted on the multiple sub-carriers multiplexed together Another strategy for diversity reception relies on Switching and spanning multiple sample periods. Modulation and between antennas based upon the quality of the signal being demodulation in an OFDM system uses an inverse fast Fou received. This technique offers appreciable cost savings but rier transform (IFFT) at the transmitter and a fast Fourier represents implementation and performance challenges, pri transform (FFT) at the receiver. At the transmitter, a cyclic 45 marily due to loss of data and channel information during the prefix of a section of the IFFT output for each OFDM symbol Switching process. is typically appended to the beginning of the OFDM symbol Thus, it would be desirable to provide a system and method as a guard interval (GI). The length of the OFDM symbol of antenna diversity using antenna selection that offers before adding the guard interval is known as the useful sym increased performance. In particular, it would be desirable to bol period duration. At the receiver, the cyclic prefix is 50 provide antenna Switching while minimizing disruption of removed prior to the FFT demodulation by the appropriate data reception or channel estimation. positioning of an FFT window, with size equal to the useful symbol period duration, along a received sample sequence. SUMMARY OF THE INVENTION The FFT demodulation transforms the window of received time domain samples, in the received sample sequence, to a 55 In accordance with the above needs and those that will be frequency domain (OFDM) symbol. mentioned and will become apparent below, this disclosure is A principle advantage of this type of communication sys directed to a method for providing antenna diversity in a tem is that the lower data rate occupies alonger symbol period multiple Sub-carrier digital communication receiver compris than in a higher rate single carrier system.