Patent Publication Number: US-2009238113-A1

Title: Wireless communications apparatus

Description:
The present invention concerns wireless communication and particularly, but not exclusively, wireless communication in a cellular network including one or more mobile terminals. 
     Technology, and standards and specifications defining technology, for mobile communications networks, undergo improvement and change over time. For the practical operability of a cellular network, it is important not only to meet new requirements and needs, but also to support existing (‘legacy’) systems. Such support should take account of legacy performance criteria such as ease of network access, physical level cell/sector identification, or fast handover between cells/sectors. 
     In a packet based communication network, in accordance with many communications standards, much of the structure of a packet is predetermined to enable receipt of a packet by a communication device in a predictable and practical manner. A preamble is positioned conceptually at the ‘front’ of a packet, and this usually equates to it being positioned at the start of a packet when considered in the time domain. A preamble has a structure which is known to a recipient device and so enables information concerning the packet to be conveyed to the recipient device with ease. The design of a preamble, and the nature of information conveyed in a preamble, is key to the support of easy deployment of a system. Preamble design is furthermore important in effecting fast network entry, including handover, accurate initial synchronisation and channel estimation. A downlink preamble should also meet other requirements, such as supporting higher system throughput, reliable cell (or sector) search performance, and so on. Such considerations will be evident to the skilled reader. 
     It might be straightforward to design a preamble for a newly specified communications system, based on new system requirements. However, support for legacy systems is important, particularly in the context of a considerable history of development in the field of mobile communications. 
     The present invention is herein described in the context of a WirelessMAN-OFDMA system for an advanced interface (IEEE 802.16m IMT-advanced) while fully supporting legacy systems. The 802.16m specification aims to meet IMT-advanced requirements for future mobile communication systems. However, it is a requirement of that specification that a system designed in accordance therewith must also support all of its legacy systems. This presents new challenges for system design and deployment in 802.16-WiMAX. 
     Even though legacy systems using such a specification are intended to support up to 20 MHz bandwidth (BW), in reality a legacy system might only support up to 10 MHz BW, in accordance with the current WiMAX profile. However, there is a need for a system to support up to 20 MHz BW to meet advanced performance requirements. A preamble should be designed for the 20 MHz BW, while also supporting the legacy 10 MHz BW. Most technical challenges associated with this need are presented on downlink transmission. 
     A system specified in accordance with WiMAX is required to support three sector antenna configurations. Therefore, the preamble defined for use by the system is required to provide three segments for sector transmissions. Each segment must cover full bandwidth for effective network initiation, synchronisation and channel estimation. Consequently, the downlink can be divided into a three segment structure, and includes a preamble which begins the transmission. The subcarriers used for the preamble are divided into three carrier sets. There are three possible groups, each consisting of one of the defined carrier sets, which may be used by any of the segments. 
     The first symbol of the downlink transmission is the preamble; there are three types of preamble carrier sets, which are defined by allocation of different subcarriers to each one of them. The subcarriers are modulated using a boosted BPSK modulation with a specific Pseudo-Noise (PN) code. Each segment uses a preamble composed of a carrier set out of the three available carrier sets. The preamble carrier sets can be defined as 
       PreambleCarrierSet n   =n+ 3 k    (1) 
     where n is the number of the preamble carrier set index, and k is a running index. 
     Each segment uses a preamble composed of a carrier set selected from the three available carrier sets, on the basis of:
         Segment 0 uses preamble carrier set 0   Segment 1 uses preamble carrier set 1   Segment 2 uses preamble carrier set 2       

     Therefore, each segment eventually modulates each third subcarrier. This is illustrated in  FIG. 1 . As seen in the figure, each segment occupies one third of the subcarriers but covers the whole bandwidth. 
     The architecture of a preamble in accordance with this, for an OFDMA system with 10 MHz bandwidth, is illustrated by way of example in  FIG. 2 . As illustrated,  FIG. 2  shows a packet having a preamble defined across substantially the full extent of the available 10 MHz bandwidth, followed by a mapping portion and then data bursts. As described above, the 10 MHz bandwidth is for a legacy system. Subcarriers at the upper and lower bounds of the bandwidth are defined as guard subcarriers, i.e. they are held to zero, to reduce the impact of collision between bandwidth allocation on a spectrum. The three segments discussed above are defined in the illustrated preamble. 
       FIG. 3  illustrates the preamble according to this legacy system in further detail. As can be seen by the solid, broken and dot-dash lines, which correspond to the segments set out in  FIG. 2 , the three segments are initialised across the full spread of the bandwidth. 
     For a system to support a 20 MHz bandwidth and also to support the 10 MHz legacy system, it is desirable that:
         the preamble should perform well on synchronisation for both new systems and legacy systems;   the preamble should provide full channel estimation for both systems;   the preamble should keep PAPR as low as possible; and   it should not require more resources of a pseudorandom number (PN) generation resource       

     One possible approach to this would be to concatenate two legacy preambles across the newly widened bandwidth. Such an arrangement is illustrated in  FIG. 4 . However, this is problematic, as the result is not a single preamble. The guard intervals defined at the bounds of the legacy preamble cause a region of zeros substantially at the centre point of the resultant preamble. This prevents consistency, and also is undesirable for the purposes of channel estimation as channel information is unavailable within the gap. 
     Similarly, as illustrated in  FIG. 5 , a legacy preamble could be embedded centrally of a wider bandwidth, with the additional portions of bandwidth populated to construct a suitable extended preamble. However, the same problem arises. Gaps in the resultant extended preamble cause inconsistencies, and limit channel estimation. 
     According to an aspect of the invention, there is provided a method of determining a preamble for use in a packet based communication system, on the basis of a preamble for use in a legacy packet based communication system, the method comprising extracting from said legacy preamble a pilot symbol sequence in frequency of said legacy preamble, replicating said pilot symbol sequence, and concatenating said pilot symbol sequence and said replicated pilot symbol sequence. 
     According to another aspect of the invention, there is provided a preamble for use in a packet based communication system is defined on the basis of a preamble for use in a legacy packet based communication system. The preamble is constructed by extracting from the legacy preamble a pilot symbol sequence in frequency of the legacy preamble, replicating the pilot symbol sequence, and concatenating the pilot symbol sequence and the replicated pilot symbol sequence. 
    
    
     
       The invention will be further characterised, exemplified and explained, with the aid of the following description of specific embodiments, in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a graph in frequency of a preamble in accordance with the prior art; 
         FIG. 2  is a schematic diagram of a packet employing the preamble of  FIG. 1 ; 
         FIG. 3  is a graph in frequency of the preamble in accordance with the prior art, in further detail; 
         FIG. 4  is a graph in frequency of an extended preamble in accordance with the prior art; 
         FIG. 5  is a further graph in frequency of an extended preamble in accordance with the prior art; 
         FIG. 6  is a flow diagram of a method in accordance with a first embodiment of the invention, for devising a preamble from a legacy preamble; 
         FIG. 7  is a graph in frequency of a preamble devised by the method of  FIG. 6 ; 
         FIG. 8  is a flow diagram of a method in accordance with a second embodiment of the invention, for devising a preamble from a legacy preamble; 
         FIG. 9  is a graph in frequency of a preamble devised by the method of  FIG. 8 ; 
         FIG. 10  is a graph in frequency of a preamble in accordance with a third embodiment of the invention; 
         FIG. 11  is a schematic diagram of a packet employing the preamble of  FIG. 10 ; 
         FIG. 12  is a graph of correlation properties for a simulation of a legacy preamble; 
         FIG. 13  is a graph of correlation properties for a simulation of a preamble in accordance with a specific embodiment of the invention of 20 MHz BW with FFT size 2048; and 
         FIG. 14  is a graph of correlation properties for a simulation of a legacy preamble of 10 MHz BW with FFT size 1024. 
     
    
    
       FIG. 6  illustrates the structure of a method of constructing a preamble over an extended bandwidth, from a legacy preamble, in accordance with a first specific embodiment of the invention. Construction of a preamble in accordance with the first specific embodiment uses the legacy preamble in its construction. In order to generate this preamble, the method of  FIG. 6  is used. This follows the following steps:
         Based on the existing/legacy preamble, abstract all of the pilots (the active data) of the preamble (guard subcarriers extracted);   Separate the abstracted preamble into two parts (without DC)   Assign the two parts to either side of the abstracted preamble   Add new guard subcarriers on both ends to form the preamble       
     With this design, the preamble in accordance with the specific embodiment maintains good correlation, and is capable of supporting both legacy systems and those in accordance with the specific embodiment of the invention, without a need for modification of such legacy systems. The resultant preamble is of a construction as shown in  FIG. 7 . 
     To ensure that the preamble is of optimal effectiveness, PAPR should be considered. To this end, two windows are inserted, on each side of the abstracted preamble as shown in  FIG. 7 . The purpose of preamble optimisation with the control window(s) is to minimise both PAPR and the impact on legacy systems: such legacy systems will, as a result of the preamble structure as described, now lack a guard interval in the frequency domain. 
     The design of the PAPR control window or windows can use a search engine to identify a random sequence for minimising the value of PAPR of the preamble. The size of the control window or windows should be as small as possible in order that the randomisation property of the designed preamble is the same as for an existing/legacy preamble. 
     Considering the preamble designs set out above, the process set out in  FIG. 6  can be augmented into the process illustrated in  FIG. 8 . 
     Assuming the legacy preamble sequence (S L ) can be expressed by 
         S   L   ={G   l   , D   l ,0 , D   r   , G   r }  (2) 
     where
 
G l , G r  represent left- and right-side guard subcarriers respectively;
 
D l , D r  represent left- and right-side preamble data subcarriers respectively; and
 
DC: central zero ‘0’.
 
     It is further assumed that the size of each of these is:
         sz gl -size of G l      sz gr -size of G r      sz dl -size of D l      sz dr -size of D r  
 
and, sz fft  is FFT-size.
       

     Then, 
         sz   gl   +sz   gr   +sz   dl   +sz   dr +1 =sz   fft    
     Accordingly, for the preamble in accordance with the specific embodiment, and assuming that the reverse sequence of D l  and D r  are  D   l  and  D   r , respectively, the designed preamble sequence (S D ) can be expressed by: 
         S   D   ={G   l     D     ,  D     r   , C   W     l     , D   l ,0 , D   r   , C   W     r     ,  D     l   , G   r     D   }  (3) 
     where 
     G l     D   , G r     D    represent the left- and right-side guard subcarriers for the preamble, respectively; and 
     C W     l   , C W     r    represent the left- and right-side PAPR control window respectively 
     It is further assumed that the size of each of these is:
         sz gl     D   -size of G l     D        sz gr     D   -size of G r     D        sz rdl -size of  D   l      sz rdr -size of  D   r      sz cwl -size of C W     l        sz cwr -size of C W     r    
 
and, sz fft     D    is FFT-size of designed preamble.
       

     Then, 
         sz   gl     D     +sz   rdr   +sz   cwl   +sz   dl   +sz   dr   +sz   cwr   +sz   rdl   +sz   gr     D   +1 =sz   fft     D      
     From the above expression, it can be seen that, if sz rdr  or sz rdl  is equivalent to sz dl  or sz dr , the PAPR control window is feasible to design. However, if sz rdr  or sz rdl  is much greater than sz dl  or sz dr , the control window size could be too large for randomisation without considering other preamble sequences. In this case, the present embodiment offers preamble sequences which are specific to a new communications system but have a legacy preamble embedded therein. This is more convenient for network operation. 
     The procedures set out in (2) and (3) above are also illustrated in  FIG. 8 . Not all embodiments of the invention need have the legacy spectrum placed in the middle of the new spectrum. As shown in  FIGS. 10 and 11 , the 20 MHz band offers a convenient opportunity to concatenate two 10 MHz bands for legacy system support side by side. In that case, two concatenated legacy preambles can be used for access by two legacy devices. However, for consistency with the approach described above, a PAPR control window should be provided, as illustrated in  FIG. 4 . This offers a convenient opportunity to have one legacy system with 10 MHz on one side of the spectrum. 
     By inserting PAPR control windows, the resulting preamble should be inversely utilized to reassign the subcarriers to each segment. It is also practical to be used with a small bandwidth preamble embedded in a long preamble, as demonstrated in  FIG. 9 . 
     In a cellular application, there might be many preambles for different cells, and/or different segments. Here, as an example, one set of legacy preambles in one cell for three segments, is presented as, 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 Segment 0 
                 0xA6F294537B285E1844677D133E4D53CCB1F182DE00489E53E6B6E77065C7EE7D0ADBEAF 
               
               
                 Segment 1 
                 0x52849D8F020EA6583032917F36E8B62DFD18AD4D77A7D2D8EC2D4F20CC0C75B7D4DF708 
               
               
                 Segment 2 
                 0xD27B00C70A8AA2C036ADD4E99D047A376B363FEDC287B8FD1A7794818C5873ECD0D3D56 
               
               
                   
               
            
           
         
       
     
     This is a preamble for OFDMA with FFT-size of 1024 (10 MHz BW). Based on this preamble and the subcarrier assignment described in  FIG. 1 , the preamble has 284 subcarriers across the whole frequency band on each segment for channel estimation, timing synchronisation and frequency synchronisation. Each segment presents the same synchronisation property as shown in  FIG. 12 . 
     As described previously, the present preamble fully supports the legacy system and the system in accordance with the present embodiment with 20 MHz bandwidth as shown in  FIGS. 13 and 14 . It will be noted that the legacy 10 MHz BW has no guard subcarriers with the designed preamble but it still achieved the same correlation property as its legacy preamble. 
     For channel estimation, the 10 MHz bandwidth utilises all available subcarriers: 852 subcarriers are used for legacy channel estimation. Because there are no guard subcarriers in the 10 MHz, the remaining subcarriers are employed for channel estimation in accordance with the present embodiment. Then, with a 20 MHz bandwidth with the present embodiment, it can be seen that the subcarriers are consistently span the whole band for channel estimation. 
     Regarding PAPR, the legacy preamble with FFT-size of 1024 has a 6.8610 dB low PAPR design. With the preamble as described herein, with FFT size of 2048 without PAPR control window insertion (referred to in  FIG. 5 ), the PAPR is relatively high (9.7715 dB). By including a PAPR control window, PAPR can be controlled and a measurement is shown in the following table: 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Designed preamble for new system 
               
               
                   
                 (legacy support) 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 With PAPR control 
               
               
                   
                 Legacy 
                 No PAPR 
                 window (diff. size) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 preamble 
                 control 
                 16 
                 32 
                 64 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 PAPR (dB) 
                 6.8 
                 9.8 
                 8.3 
                 7.6 
                 7.2 
               
               
                   
               
            
           
         
       
     
     In another case, different from the scheme shown in the above table, adopting the preamble design described above to support two concatenated legacy systems as shown in  FIGS. 10 and 11 , the new preamble with no control window has a relatively high PAPR level of 9.9 dB. However, with by using a PAPR control window, such an arrangement can achieve PAPR of 6.9 dB, which is fairly close to the achieved PAPR of legacy systems. Various methods of determining the appropriate content of the PAPR control window exist, and the selection of one of these is not a central part of this disclosure. 
     With the practical example presented here, it can be demonstrated that the proposed mechanism is capable of resulting in a preamble to meet requirements for both legacy and new systems in terms of channel estimation, timing synchronisation, frequency synchronisation and PAPR control. 
     While the invention has been described with reference to certain specific embodiments, the reader will appreciate that the invention is not limited thereto. In particular, the invention should be considered as being defined in the claims appended hereto, with reference to the aforementioned description (but not limited thereto) and accompanying drawings.