Abstract:
A diversity switch combiner for use in systems for receiving multi-carrier wideband signals is arranged to determine, during a guard interval, which antenna provides the strongest carrier, for each of a plurality of carrier frequencies. A switch is then operated to select, for the subsequent symbol, that antenna which provides the greatest number of strongest carriers.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to antenna diversity receivers, particularly those suitable for use for wideband radio reception and more particularly for multi-carrier systems. 
   2. Description of the Prior Art 
   Antenna diversity receivers use multiple antennas to overcome signal quality degradation caused by multipath fading. If the antennas are arranged such that their outputs fade independently, then the signals from the antennas can be combined to produce a signal with higher quality since it is unlikely that both antennas (branches) will simultaneously be in a deep fade. This allows the receiver to be used in areas with lower signal strengths or to provide higher signal quality and reliability within the normal system coverage area. 
   A common form of diversity combiner is a switch combiner, in which only one complete receiver is needed. The receiver is switched between the antennas and makes a judgment as to which antenna provides the strongest signal. Numerous schemes for doing this exist, but it is believed that none of them address suitable strategies for wideband channels. In all cases, switch combining performs less well than selection combining, in which two receivers are available so that the performance of both antennas can be simultaneously monitored, but a switch is used to select the signal from only one of them at a time. Maximal ratio combining (MRC) involves using, simultaneously, a plurality of receivers each operating on a signal from a respective antenna, and using signal processing to combine the outputs of the receivers. This gives better performance than either switch combining or selection combining, but is somewhat more expensive. 
   In a wideband fading channel, the bandwidth of the transmitted signal is wider than the coherence bandwidth of the channel (see S. R. Saunders, “Antennas and Propagation for Wireless Communication Systems”, John Wiley &amp; Sons, ISBN 0471986097, July 1999, for precise definitions). This implies that different parts of the received signal bandwidth will be faded to different extents, so the choice of the best antenna is not clear. A conventional switch combiner could make a decision based on the total power available over the whole signal bandwidth, by performing a vector sum of the respective channel outputs of the receiver filter.  FIG. 1  shows (curve A) that this yields only minor diversity gain when the delay spread is large, i.e. when there are significant delayed versions of the signal arriving at the receiver due to multipath echoes. The simulations in this figure assume two identical vertical antennas, with a mobile speed of 20 km/hr. The system simulated represents the ITU-T ISDB-T digital broadcasting standard. Diversity gain is referenced to the power required to achieve a bit error rate of 2×10 −4 . Curve B shows that the results when selection combining is used instead of switch combining are not significantly better. 
   Choosing a single antenna, based on whichever criteria, and using this for the reception of the whole ISDB-T bandwidth can lead to significant degradation in performance. Mostly, this will be due to the fact that somewhere within the signal bandwidth there will be a deep null, so although at some carriers within the bandwidth there may be excellent diversity gain, there is none achieved at other carriers, with the resultant diversity gain essentially an average across the bandwidth. 
   Given that delay spread has been shown to produce this significant performance degradation, it would be attractive to have a combining technique which avoids this problem, but without the expense of MRC systems, and preferably using only one receiver. 
   Accordingly, it would be desirable to provide a switch diversity combiner which has improved performance in high delay-spread environments. 
   SUMMARY OF THE INVENTION 
   Aspects of the present invention are set out in the accompanying claims. 
   According to a further aspect of the invention, the received channel state is estimated at several frequencies across the signal bandwidth, with the number of estimation frequencies being sufficient to adequately represent the variations in the channel state. The channel may be estimated according to a number of conventional approaches. One example is that in the ISDB-T standard there are known symbols, known as scattered pilots, transmitted at various times and at various frequencies. The receiver compares the expected symbols to the data received and estimates the channel by comparison. 
   Once the channel has been estimated at all of the chosen frequencies for one antenna, the results are stored, the switch is changed to the other antenna and the process is repeated. The channel values at each of the sample frequencies are compared to those for the first antenna, and the antenna for which the biggest number of samples are larger than those from the other antenna is selected for subsequent reception. 
   The time taken to monitor the two antennas ready for the decision should be as small as possible, and the subsequent reception period should be small compared with the channel coherence time for best results. 
   If desired, selection combining could be used instead of switch combining, so that the quality estimates for the respective antennas can be obtained simultaneously; however, it is currently felt that it is unlikely the performance improvement would justify the extra expense of this approach. 
   The invention is particularly applicable to multi-carrier signals which are transmitted in the form of symbols comprising a guard period followed by a useful part of the symbol, the guard period corresponding to the end of the useful part. In this case, the quality estimation is preferably performed during the guard period, so that the antenna switching can be carried out without causing a significant deterioration of performance. 
   The combiner is preferably located between the antennas and the receiver, and thus conveys RF signals to the receiver. Alternatively, the combiner could be located within the receiver, e.g. in the IF section, although in this case separate versions of the circuits prior to the combiner would have to be provided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An arrangement embodying the invention will now be described by way of example with reference to the accompanying drawings, in which: 
       FIG. 1  is a graph illustrating diversity gain at different delay spreads for various switch and selection combiner arrangements; and 
       FIG. 2  schematically illustrates a receiver system according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 2 , an embodiment of the invention is shown. The receiver system  2 , which is intended for receiving OFDM (Orthogonal Frequency Division Multiplex) signals, particularly ISDB-T signals, includes an antenna section  4 , a switch combining section  6  (including a switch  20  and a switch control means  22 ) and a receiver circuit  8  which includes means for converting the received signal to baseband. 
   In the antenna section  2 , two antennas, A 1  and A 2 , are arranged so that their outputs fade substantially independently. The outputs are coupled to a switch  20  of the switch combining section  6 . The switch  20  is operable to couple each output to the receiver circuit  8 . 
   The receiver circuit  8  shown in  FIG. 2  is of conventional structure, and includes an RF tuner front end  82 , which receives the signals from the antennas via the switch  20 . The output of the front end is delivered to a down converter and IF amplifier  84 , which supplies its output to an IF-to-baseband converter  86 . The baseband signals from the converter  86  are sent to an FFT and channel estimation block  88 , which generates the OFDM carrier signals. These are demodulated by a demodulator  94 . The baseband signals are also delivered to a symbol synchronisation circuit  90 , for synchronising the operation of the FFT and channel estimation block  88 , and to a sample clock and frequency synchronisation circuit  92  which synchronises the operations of the down converter and IF amplifier  84  and the IF-to-baseband converter  86 . 
   This is merely one example of a number of different types of receiver circuits which could be employed in the system of  FIG. 2 . In alternative arrangements, the switch  20  could instead be provided within the receiver circuit  8 , for example between the down converter and IF amplifier  84  and the IF-to-baseband converter  86 , if the circuits preceding the converter  86  are duplicated. 
   The baseband signals derived by the receiver circuit  8  represent channel amplitudes for each carrier within the signal bandwidth. These signals are continuously fed to the conventional demodulator circuit  94 . 
   The RF input to the receiver circuit  20  is fed by the electronically controlled switch  20  which selects one of the two antennas (A 1  or A 2 ) depending on the value of a control voltage produced by the switch control means  22 . During the symbol guard interval, antenna A 1  is initially selected, and the carrier amplitudes derived by the FF 1  and channel estimation block  88  are stored in a first memory  62  of the switch control means  22 . The switch state is then changed to select antenna A 2  and the carrier amplitudes are stored in a further memory  64  of the switch control means  22 . A bank  66  of comparators compares the amplitudes of corresponding carriers for the two branches, yielding a positive output voltage if a carrier from antenna A 1  is of a higher amplitude than the same carrier from antenna A 2 , and yielding a negative output voltage otherwise. The output voltages from all the comparators are summed by a summing circuit  68 , yielding a positive output voltage if the majority of the comparators indicate that the antenna A 1  amplitudes exceed those from antenna A 2 , and a negative output voltage otherwise. A threshold circuit  70  produces a fixed positive output voltage for any positive input, and a fixed negative output for any negative input. These voltages match the control voltages necessary to place the switch  20  in either the antenna A 1  or antenna A 2  state respectively. This state is held for the duration of a transmitted symbol, following which another guard interval occurs and the whole process is repeated. 
   This arrangement uses signal strength for estimating the quality of the respective channels. There are various other known ways of estimating signal quality. For example, the distances of the carrier outputs from the correct positions for the carrier constellation can be measured. It is not necessary to use all the carriers for quality estimation, although the carriers which are used should be spread throughout the frequency spectrum of the signal. It is possible to base the quality estimation on pilot carriers, by comparing their actual values with the known values they should adopt in a clean, noise-free system. Alternatively, spectrum estimation based on a limited number of samples could be used. 
   The decision steps implemented by the voting and switching logic of the switch combining section  6  are as follows: 
   During each guard interval:
         Switch to antenna A 1     Store quality estimates (e.g. amplitudes) of carriers for antenna A 1     Switch to antenna A 2     Store quality estimates (e.g. amplitudes) of carriers for antenna A 2     Compare stored values   Select branch with biggest number of wins       

   Subsequently:
         Hold switch state for remainder of guard interval and sample duration.       

   If the quality estimation takes longer than the duration of the guard interval, the switching could nevertheless occur during the useful part of the signal. Alternatively, if the quality estimation takes too long to be considered of value for the current symbol, the switch state could instead be set for the useful part of the next symbol (after first altering the state during the next guard interval for obtaining further quality estimates). It is not necessary to repeat the process regularly. Instead the process could be triggered by a detected deterioration in quality. 
   The above embodiment could be modified by having one or more further receiver circuits  8  to provide a selection combining arrangement, so that channel estimation for the respective antennas can be carried out simultaneously. 
   Curve C in  FIG. 1  shows the diversity gain performance for the switch combining arrangement, which is significantly larger than the conventional switch combiner, and is almost identical to that of a selection combining arrangement (curve D) operating using the voting technique described above. 
   The approach can easily be generalised to more than two branches by switching to each branch in turn, storing the sample values, and comparing all the stored values after checking all branches. The branch which has the greatest number of samples with the largest signal strength will be selected. 
   In order to resolve tied votes, where multiple branches have the same number of ‘wins’, the most recently examined branch should be selected since the channel will have changed least between this evaluation and the subsequent reception period. 
   In a more sophisticated approach, each branch may be awarded a score for each sample frequency, which score increases with the signal quality (e.g. strength) estimated at that sample frequency, and decreases with the elapsed time between the sampling instant and the reception period. For example, if antenna A 1  measurements are made before antenna A 2  measurements, the antenna A 1  quality estimates can be weighted to make them of apparently lower quality. The branch with the highest score is deemed to have won the vote for that sample frequency. Again, the branch with the greatest number of votes is selected. 
   It is envisaged that the receiver circuit which is used to generate the main receiver output is also used for obtaining the measurements for the quality estimates, but this is not essential. 
   The diversity system proposed is applicable to any wideband radio system, using any number of antennas. It is particularly relevant to applications at user terminals where power consumption, size and cost are particularly critical, whereas base stations will usually implement diversity combiners which use one receiver circuit per branch. 
   Particular systems which are applicable are:
         ISDB-T   DAB   DVB   UMTS   cdma2000