Abstract:
A method for antenna diversity includes: determining a first strength of the first antenna according to a received signal received by the first antenna; receiving the guard interval of a first symbol or a second symbol or both using the second antenna and determining a second strength of the second antenna; and determining which one of the first and the second antennas to receive the useful duration of the second symbol according to the first and the second strengths.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a communication system, and more particularly, to method and apparatus for antenna diversity in a communication system.  
         [0003]     2. Description of the Prior Art  
         [0004]     Currently, conventional analog TV signal broadcasting is gradually transforming into digital video broadcasting (DVB). In the DVB-T standard, signals are processed using a COFDM (Coded Orthogonal Frequency Division Multiplexing) technique. The DVB-T standard is a continuous OFDM system.  
         [0005]     The DVB-T system is able to support mobile receipt, and typically uses twin antennas to receive signal.  
         [0006]     According to the IEEE 802.11a or 11g standard, a symbol is composed of three parts: a preamble, a guard interval (GI) and data. A conventional antenna diversity method applied in the IEEE 802.11a or 11g standard measures strength of the antennas during the receipt of the preamble. However, in the DVB-T standard, a symbol is composed of a guard interval and a useful duration but no preamble. Additionally, in TV broadcasting systems, the facts that video transmission cannot be interrupted and signals cannot be re-transmitted should also be considered.  
         [0007]      FIG. 1  depicts a schematic diagram of a conventional DVB receiver  100 . The conventional DVB receiver  100  has two antennas  110  and  140 , two tuners  120  and  150 , and two demodulators  130  and  160 . The receiver  100  employs a comparing circuit  170  to compare the strength of the output signal of the first demodulator  130  and the second demodulator  160  so as to select either the antenna  110  or the antenna  140  to receive signals.  
         [0008]     In other words, the conventional receiver  100  uses two receiving modules that have the same properties to process the signals received by the first antenna  110  and the second antenna  140 , respectively. As a result, the cost and circuitry size of the conventional receiver  100  are accordingly increased.  
       SUMMARY OF INVENTION  
       [0009]     It is therefore an objective of the claimed invention to provide a method and apparatus of antenna diversity capable of reducing circuitry complexity and cost.  
         [0010]     According to a preferred embodiment of the present invention, a receiver comprises: a first antenna and a second antenna; a switch for selecting one of the first and second antennas to receive an incoming signal according to a control signal; a tuner for converting the incoming signal; a demodulator for demodulating the signal output from the tuner; and a detecting unit for respectively measuring strengths of the incoming signal received by the first and second antennas, and generating the control signal according to the strengths of the incoming signal received by the first and the second antennas; wherein the switch switches the antennas during a guard interval of the incoming signal.  
         [0011]     According to the embodiment of the present invention, a method of antenna diversity in a receiver is disclosed. The method comprises: receiving a useful duration of a first symbol of an incoming signal using a first antenna; detecting a signal strength of the first symbol before a useful duration of a second symbol to produce a first strength; detecting a signal strength for a second antenna before the useful duration of the second symbol to produce a second strength; and comparing the first and the second strengths to determine which one of the first and second antennas receives the useful duration of the second symbol.  
         [0012]     According to the embodiment of the present invention, another method of antenna diversity in a receiver is disclosed. The method comprises: determining a first strength of the first antenna according to a received signal received by the first antenna; determining a second strength of the second antenna when at least one of the guard interval of the first symbol and the second symbol is received by the second antenna; and determining which one of the first and the second antenna to receive the useful duration of the second symbol according to the first and the second strengths.  
         [0013]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]      FIG. 1  is a schematic diagram of a conventional DVB receiver.  
         [0015]      FIG. 2  shows a schematic diagram of an incoming signal of the DVB-T standard.  
         [0016]      FIG. 3  is a block diagram of a DVB receiver according to a preferred embodiment of the present invention.  
         [0017]      FIG. 4  is a flowchart of antenna diversity according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]     In the DVB-T standard, the transmitted signal is organized in frames. Each frame consists of 68 OFDM symbols. Each symbol is composed of two parts: a guard interval used for preventing an ISI (Inter-Symbol Interference) problem, and a useful duration used for transmitting data.  
         [0019]     Please refer to  FIG. 2 , which shows a schematic diagram of an incoming signal  200  defined in the DVB-T standard. As shown, a first symbol  210  comprises a guard interval  212  and a useful duration  214 , and a second symbol  220  comprises a guard interval  222  and a useful duration  224 . Each of the guard intervals  212  and  222  has duration of Tg, and each of the useful durations  214  and  224  has duration of Tu. In other words, each of the first symbol  210  and the second symbol  220  has duration of TS (TS=Tg+Tu).  
         [0020]     In the DVB-T standard, the guard interval of each symbol is the cyclic prefix of the useful duration of the symbol. Depending on the transmission mode, the duration Tg of the guard interval can vary, and may have values such as ¼Tu, ⅛Tu, 1/16Tu, 1/32Tu and so on.  
         [0021]     Please refer to  FIG. 3 , which depicts a block diagram of a DVB receiver  300  according to a preferred embodiment of the present invention. The receiver  300  comprises a first antenna  310 , a second antenna  320 , a switch  330  used for switching between the first and second antennas  310  and  320 , a tuner  340  used for down-converting a received incoming signal, a demodulator  350  for decoding and demodulating the signal output from the tuner  340 , and a detecting unit  360  used for detecting and comparing the signal strength of the first antenna  310  and the second antenna  320  to control the switch  330 . In practical implementations, the tuner  340  typically uses an AGC (Automatic Gain Control—not shown) circuit to adjust the gain of the received incoming signal.  
         [0022]     Please refer to  FIG. 4 , as well as  FIG. 2 .  FIG. 4  is a flowchart describing the antenna selection made by the receiver  300  according to one embodiment of the present invention. The steps of the flowchart are described as follows:  
         [0023]     First, the receiver  300  of the present invention selects one antenna (such as the first antenna  310 ) to perform signal synchronization and boundary acquisition.  
         [0024]     After the boundary acquisition is finished, the receiver  300  performs step  404  to use the first antenna  310  to receive the guard interval  212  of the first symbol  210  of the incoming signal  200 .  
         [0025]     Next, in step  406 , the detecting unit  360  controls the switch  330  to switch to the second antenna  320  before a predetermined time point  24  so as to detect the signal strength of the second antenna  320 . In a preferred embodiment, the detecting unit  360  controls the switch  330  to switch to the second antenna  320  when the first antenna  310  receives the front boundary of the guard interval  212  of the first symbol  210 , i.e., at the time point  22 .  
         [0026]     In step  408 , the detecting unit  360  calculates a second energy value energy — 2 according to the signal strength of the second antenna  320 .  
         [0027]     In step  410 , the detecting unit  360  then detects the signal strength of the first antenna  310  after the predetermined time point  24 . In practical implementations, switching between circuits or signal transitions may cause delays, so the detecting unit  360  can control the switch  330  to switch back to the first antenna  310  at a short period before the predetermined time point  24  so as to detect the signal strength of the first antenna  310 . Similarly, the detecting unit  360  calculates a first energy value energy — 1 according to the signal strength of the first antenna  310  in step  412 .  
         [0028]     In an embodiment, the AGC of the tuner  340  maintains the same gain property in step  406  and step  410  in order to improve the accuracy of signal strength measured by the detecting unit  360 .  
         [0029]     In an embodiment, the first and the second energy values (energy — 1, energy — 2) correspond to the cyclic prefix and the guard interval of the symbol, respectively. The guard interval of each symbol is the cyclic prefix of the useful duration.  
         [0030]     In step  414 , the detecting unit  360  compares the first energy value energy — 1 and the second energy value energy — 2 to determine whether to change to the second antenna  320  to receive the useful duration  224  of the following second symbol  220 . In an embodiment, the switch  330  switches the antennas during the guard interval of the symbol so as to ensure the receipt of the useful durations of the symbols. In other words, the receipt of the useful durations of the symbols is guaranteed by inhibiting switching antennas during the useful duration of the symbols. In practice, the detecting unit  360  can employ various criteria to determine whether to change to the second antenna  320  in step  414 . For example, in a first embodiment of the present invention, if the second energy value energy — 2 is greater than the first energy value energy — 1, the detecting unit  360  controls the switch  330  to switch to the second antenna  320  before a time point  28 . In a second embodiment of the present invention, the detecting unit  360  controls the switch  330  to switch to the second antenna  320  only if the second energy value energy — 2 exceeds the first energy value energy — 1 by a specific amount, such as  3  dB. In a third embodiment of the present invention, the detecting unit  360  does not only compare the second energy value energy — 2 and the first energy value energy — 1, but also determines if the bit error rate (BER) of the processing/processed signal of the demodulator  350  reaches a predetermined threshold value. If the second energy value energy — 2 is greater than the first energy value energy — 1 or exceeds the first energy value energy — 1 by a specific amount, and the BER of the processing/processed signal of the demodulator  350  reaches the threshold value, then the signal strength of the second antenna  320  is stronger than that of the first antenna  310  and the signal quality of the first antenna  310  is poor. In this situation, the detecting unit  360  controls the switch  330  to switch to the second antenna  320  before the time point  28 .  
         [0031]     Please note that in the above steps  406  and  410 , the detecting unit  360  of the present invention can respectively detect the overall signal strength of the two antennas in the same time period, or detect the average signal strength of the two antennas in different time periods.  
         [0032]     Additionally, in the foregoing step  410 , the detecting unit  360  measures the signal strength of the first antenna  310  after the predetermined time point  24 . This is only an embodiment of the resent invention and does not limit other implementations of the present invention. For example, if the length of the guard interval of the symbol of the incoming signal is enough, the detecting unit  360 , in step  410 , can successively detect the signal strength of the second antenna  320  and the first antenna  310  during the guard interval  212  of the first symbol  210 . In fact, the detection period of the signal strength of the first antenna  310  can cross the boundary between the guard interval  212  and the useful duration  214  of the first symbol  210 , i.e., the time point  24 . In other words, the present invention allows other embodiments having the feature that the signal strength of the second antenna  320  is detected during the guard interval of a symbol.  
         [0033]     In practical implementations, the detecting unit  360  can perform a plurality of detections of signal strength to the first antenna  310  and the second antenna  320 , respectively, in several symbol periods, and then employ the overall or average signal strength of each antenna as the first energy value energy — 1 and the second energy value energy — 2 to perform the comparison in step  414 .  
         [0034]     As is well known in the art, the DVB-T standard defines three transmission channels: 8 MHz, 7 MHz and 6 MHz. The frame structure, sub-carrier numbers and channel coding are substantially the same between DVB-T systems with different channels. The difference between channels of different bandwidths is the duration Tu of the useful duration of the symbol. Therefore, the foregoing receiver and antenna diversity method of the present invention can be applied in different DVB systems or different OFDM systems.  
         [0035]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.