Abstract:
A transmission method for use in a multi-hop wireless communication system is provided. The system includes a base station, a relay station and mobile stations. The method transmits using a downlink transmission frame that has a first zone for transmitting radio signals from the base station to the relay station, a second zone for transmitting radio signals from the relay station to a mobile station, and a third zone provided between the first zone and the second zone for transmitting radio signals from the base station directly to a mobile station.

Description:
RELATED APPLICATION 
     This application is a continuation application of pending U.S. patent application Ser. No. 11/840,570 filed Aug. 17, 2007; which claims foreign priority benefits under 35 U.S.C. §119 of United Kingdom Application No. GB 0616477.6, filed on Aug. 18, 2006, entitled “Communication Systems”. 
    
    
     TECHNICAL FIELD 
     This invention relates in general to communication systems, and more particularly to a frame structure for a multihop communication system. 
     OVERVIEW 
     Currently there exists interest in the use of multihop techniques in packet based radio and other communication systems, where it is purported that such techniques will enable both extension in coverage range and increase in system capacity (throughput). 
     In a multi-hop communication system, communication signals are sent in a communication direction along a communication path (C) from a source apparatus to a destination apparatus via one or more intermediate apparatuses.  FIG. 4  illustrates a single-cell two-hop wireless communication system comprising a base station BS (known in the context of 3G communication systems as “node-B” NB) a relay node RN (also known as a relay station RS) and a user equipment UE (also known as mobile station MS). In the case where signals are being transmitted on the downlink (DL) from a base station to a destination user equipment (UE) via the relay node (RN), the base station comprises the source station (S) and the user equipment comprises the destination station (D). In the case where communication signals are being transmitted on the uplink (UL) from a user equipment (UE), via the relay node, to the base station, the user equipment comprises the source station and the base station comprises the destination station. The relay node is an example of an intermediate apparatus (I) and comprises: a receiver, operable to receive data from the source apparatus; and a transmitter, operable to transmit this data, or a derivative thereof, to the destination apparatus. 
     Simple analogue repeaters or digital repeaters have been used as relays to improve or provide coverage in dead spots. They can either operate in a different transmission frequency band from the source station to prevent interference between the source transmission and the repeater transmission, or they can operate at a time when there is no transmission from the source station. 
       FIGS. 5   a  and  5   b  illustrate a number of applications for relay stations. For fixed infrastructure, the coverage provided by a relay station may be “in-fill” to allow access to the communication network for mobile stations which may otherwise be in the shadow of other objects or otherwise unable to receive a signal of sufficient strength from the base station despite being within the normal range of the base station. “Range extension” is also shown, in which a relay station allows access when a mobile station is outside the normal data transmission range of a base station. One example of in-fill shown at the top right of  FIG. 5   b  is positioning of a nomadic relay station to allow penetration of coverage within a building that could be above, at, or below ground level. 
     Other applications are nomadic relay stations which are brought into effect for temporary cover, providing access during events or emergencies/disasters. A final application shown in the bottom right of  FIG. 5   a  provides access to a network using a relay positioned on a vehicle. 
     Relays may also be used in conjunction with advanced transmission techniques to enhance gain of the communications system as explained below. 
     It is known that the occurrence of propagation loss, or “pathloss”, due to the scattering or absorption of a radio communication as it travels through space, causes the strength of a signal to diminish. Factors which influence the pathloss between a transmitter and a receiver include: transmitter antenna height, receiver antenna height, carrier frequency, clutter type (urban, sub-urban, rural), details of morphology such as height, density, separation, terrain type (hilly, flat). The pathloss L (dB) between a transmitter and a receiver can be modeled by:
 
 L=b+ 10 n  log  d   (A)
 
Where d (meters) is the transmitter-receiver separation, b(db) and n are the pathloss parameters and the absolute pathloss is given by l=10 (L/10) .
 
     The sum of the absolute path losses experienced over the indirect link SI+ID may be less than the pathloss experienced over the direct link SD. In other words it is possible for:
 
 L ( SI )+ L ( ID )&lt; L ( SD )  (B)
 
     Splitting a single transmission link into two shorter transmission segments therefore exploits the non-linear relationship between pathloss verses distance. From a simple theoretical analysis of the pathloss using equation (A), it can be appreciated that a reduction in the overall pathloss (and therefore an improvement, or gain, in signal strength and thus data throughput) can be achieved if a signal is sent from a source apparatus to a destination apparatus via an intermediate apparatus (e.g. relay node), rather than being sent directly from the source apparatus to the destination apparatus. If implemented appropriately, multi-hop communication systems can allow for a reduction in the transmit power of transmitters which facilitate wireless transmissions, leading to a reduction in interference levels as well as decreasing exposure to electromagnetic emissions. Alternatively, the reduction in overall pathloss can be exploited to improve the received signal quality at the receiver without an increase in the overall radiated transmission power required to convey the signal. 
     Multi-hop systems are suitable for use with multi-carrier transmission. In a multi-carrier transmission system, such as FDM (frequency division multiplex), OFDM (orthogonal frequency division multiplex) or DMT (discrete multi-tone), a single data stream is modulated onto N parallel sub-carriers, each sub-carrier signal having its own frequency range. This allows the total bandwidth (i.e. the amount of data to be sent in a given time interval) to be divided over a plurality of sub-carriers thereby increasing the duration of each data symbol. Since each sub-carrier has a lower information rate, multi-carrier systems benefit from enhanced immunity to channel induced distortion compared with single carrier systems. This is made possible by ensuring that the transmission rate and hence bandwidth of each subcarrier is less than the coherence bandwidth of the channel. As a result, the channel distortion experienced on a signal subcarrier is frequency independent and can hence be corrected by a simple phase and amplitude correction factor. Thus the channel distortion correction entity within a multicarrier receiver can be of significantly lower complexity of its counterpart within a single carrier receiver when the system bandwidth is in excess of the coherence bandwidth of the channel. 
     Orthogonal frequency division multiplexing (OFDM) is a modulation technique that is based on FDM. An OFDM system uses a plurality of sub-carrier frequencies which are orthogonal in a mathematical sense so that the sub-carriers&#39; spectra may overlap without interference due to the fact they are mutually independent. The orthogonality of OFDM systems removes the need for guard band frequencies and thereby increases the spectral efficiency of the system. OFDM has been proposed and adopted for many wireless systems. It is currently used in Asymmetric Digital Subscriber Line (ADSL) connections, in some wireless LAN applications (such as WiFi devices based on the IEEE 802.11a/g standard), and in wireless MAN applications such as WiMAX (based on the IEEE 802.16 standard). OFDM is often used in conjunction with channel coding, an error correction technique, to create coded orthogonal FDM or COFDM. COFDM is now widely used in digital telecommunications systems to improve the performance of an OFDM based system in a multipath environment where variations in the channel distortion can be seen across both subcarriers in the frequency domain and symbols in the time domain. The system has found use in video and audio broadcasting, such as DVB and DAB, as well as certain types of computer networking technology. 
     In an OFDM system, a block of N modulated parallel data source signals is mapped to N orthogonal parallel sub-carriers by using an Inverse Discrete or Fast Fourier Transform algorithm (IDFT/IFFT) to form a signal known as an “OFDM symbol” in the time domain at the transmitter. Thus, an “OFDM symbol” is the composite signal of all N sub-carrier signals. An OFDM symbol can be represented mathematically as: 
                       x   ⁡     (   t   )       =       1     N       ⁢       ∑     n   =   0       N   -   1       ⁢       c   n     ·     ⅇ     j2   ⁢           ⁢   π   ⁢           ⁢   n   ⁢           ⁢     Δ   ⁢   ft                 ,     0   ≤   t   ≤     T   s               (   1   )               
where Δf is the sub-carrier separation in Hz, Ts=1/Δf is symbol time interval in seconds, and c n  are the modulated source signals. The sub-carrier vector in (1) onto which each of the source signals is modulated cεC n , c=(c 0 , c 1  . . . c N-1 ) is a vector of N constellation symbols from a finite constellation. At the receiver, the received time-domain signal is transformed back to frequency domain by applying Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) algorithm.
 
     OFDMA (Orthogonal Frequency Division Multiple Access) is a multiple access variant of OFDM. It works by assigning a subset of sub-carriers, to an individual user. This allows simultaneous transmission from several users leading to better spectral efficiency. However, there is still the issue of allowing bi-directional communication, that is, in the uplink and download directions, without interference. 
     In order to enable bi-directional communication between two nodes, two well known different approaches exist for duplexing the two (forward or download and reverse or uplink) communication links to overcome the physical limitation that a device cannot simultaneously transmit and receive on the same resource medium. The first, frequency division duplexing (FDD), involves operating the two links simultaneously but on different frequency bands by subdividing the transmission medium into two distinct bands, one for forward link and the other for reverse link communications. The second, time division duplexing (TDD), involves operating the two links on the same frequency band, but subdividing the access to the medium in time so that only the forward or the reverse link will be utilizing the medium at any one point in time. Both approaches (TDD &amp; FDD) have their relative merits and are both well used techniques for single hop wired and wireless communication systems. For example the IEEE 802.16 standard incorporates both an FDD and TDD mode. As an example,  FIG. 6  illustrates the single hop TDD frame structure used in the OFDMA physical layer mode of the IEEE 802.16 standard (WiMAX). 
     Each frame is divided into DL and UL subframes, each being a discrete transmission interval. They are separated by Transmit/Receive and Receive/Transmit Transition Guard interval (TTG and RTG respectively). Each DL subframe starts with a preamble followed by the Frame Control Header (FCH), the DL-MAP, and the UL-MAP. 
     The FCH contains the DL Frame Prefix (DLFP) to specify the burst profile and the length of the DL-MAP. The DLFP is a data structure transmitted at the beginning of each frame and contains information regarding the current frame; it is mapped to the FCH. 
     Simultaneous DL allocations can be broadcast, multicast and unicast and they can also include an allocation for another BS rather than a serving BS. Simultaneous ULs can be data allocations and ranging or bandwidth requests. 
     SUMMARY OF EXAMPLE EMBODIMENTS 
     In accordance with one embodiment of the present invention, a transmission method for use in a two-hop wireless communication system is provided. The system includes a source apparatus, a destination apparatus and an intermediate apparatus. The source apparatus is operable to transmit information along two links forming a communication path extending from the source apparatus to the destination apparatus via the intermediate apparatus. The intermediate apparatus is operable to receive information from the source apparatus and to transmit the received information to the destination apparatus. The system has access to a time-frequency format for use in assigning available transmission frequency bandwidth during a discrete transmission interval, said format defining a plurality of transmission windows within such an interval. Each window occupies a different part of that interval and has a frequency bandwidth profile within said available transmission frequency bandwidth over its part of that interval. Furthermore, each said window is assignable for such a transmission interval to said source or intermediate apparatus for use in transmission. The transmission method for use in this system includes employing said format to transmit information along the path as two successive transmission signals, link by link, said signals being transmitted using different transmission windows of a particular such transmission interval. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and its advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a frame structure; 
         FIG. 2  shows node activity within each zone; 
         FIG. 3  shows an example of zone usage within one cell; 
         FIG. 4  shows a single-cell two-hop wireless communication system; 
         FIGS. 5   a  and  5   b  show applications of relay stations; and 
         FIG. 6  shows a single hop TDD frame structure used in the OFDMA physical layer mode of the IEEE 802.16 standard. 
     
    
    
     DETAILED DESCRIPTION 
     When a node is required to support two independent links to two different nodes, e.g. a relay station communicating with a base station and a mobile, the existing TDD or FDD frame structures require some modification in order to make realization of the relay practical. Particular embodiments of the invention provide a frame structure (format) for a multihop communication system that is an extension of the standard TDD frame structure. 
     One proposed frame structure is designed for the case that the control information originating from the head node that controls the overall medium access is receivable by all subordinate nodes operating in the network. It is further designed in a manner that enables legacy single hop TDD mobile devices that have no knowledge of a relay station to operate within the new relaying enabled system. 
     If control information is not receivable from the head node (or source apparatus) then an extra frame period is required for two-hop transmission. This is because control information sent by the source apparatus to the intermediate apparatus cannot then be received by the destination apparatus in the same frame. The destination apparatus (especially a legacy apparatus) will be designed to receive such control information at the beginning of the frame and therefore an extra frame period is required for the intermediate apparatus to transmit the control information on to the source at the beginning of the frame (in the preamble) and then transmit the data. Thus a frame latency of 1 is incurred. 
     An example frame structure is shown in  FIG. 1 . It is composed of a number of transmission and reception zones for both the downlink and uplink sub-frames. The zone types are either: 
     
         
         B Broadcast of control related information such as: synchronization sequences, commands, information and details of the structure or layout of the frame. 
         C Dedicated control information that is transmitted in a non-broadcast zone (i.e. either to individual or a group of receivers) 
         T Dedicated user-data (transport) transmission
 
The 9 different zones identified in  FIG. 1  are described in Table 1.
 
       
    
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Description of the zones. 
               
             
          
           
               
                 Zone 
                   
                   
               
               
                 Number 
                 Label 
                 Description 
               
               
                   
               
               
                 1 
                 P 
                 Preamble or synchronization sequence transmissions  
               
               
                   
                   
                 for cell identification 
               
               
                 2 
                 MAP 
                 Frame format description (zone boundaries, allocations 
               
               
                   
                   
                 within the zones, etc) 
               
               
                 3 
                 BS-RS/ 
                 BS to RS transmission zone. Can also be used for BS  
               
               
                   
                 BS-MS 
                 to MS transmission if spatial division multiple access  
               
               
                   
                   
                 is supported (i.e. the same transmission resource can  
               
               
                   
                   
                 be used to communicate with more than one entity) 
               
               
                 4 
                 BS-MS 
                 BS to MS transmission zone. RS is not active during  
               
               
                   
                   
                 this period, it is processing any received information  
               
               
                   
                   
                 and turning around prior to transmission. 
               
               
                 5 
                 BS-MS/ 
                 RS to MS transmission zone. Can also be used by the  
               
               
                   
                 RS-MS 
                 BS to transmitted to MSs that do not experience  
               
               
                   
                   
                 significant levels of interference from RS  
               
               
                   
                   
                 transmissions. 
               
               
                 6 
                 MS-BS/ 
                 MS control information transmission zone. Information 
               
               
                   
                 MS-RS 
                 can be received by both the RS and the BS. Control 
               
               
                   
                   
                 information can be information or requests from the  
               
               
                   
                   
                 MS. 
               
               
                 7 
                 MS-BS/ 
                 MS to RS transmission zone. Can also be used by MSs 
               
               
                   
                 MS-RS 
                 who do not cause interference to the RS to transmit to  
               
               
                   
                   
                 the BS. 
               
               
                 8 
                 MS-BS 
                 MS to BS transmission zone. RS is not actively 
               
               
                   
                   
                 transmitting or receiving during this period; it is 
               
               
                   
                   
                 processing any received information prior to turning 
               
               
                   
                   
                 around. 
               
               
                 9 
                 RS-BS/ 
                 RS to BS transmission zone. Can also be used for MS  
               
               
                   
                 MS-BS 
                 to BS transmission if spatial division multiple access  
               
               
                   
                   
                 is supported (i.e. the same transmission resource can  
               
               
                   
                   
                 be used to communicate with more than one entity) 
               
               
                   
               
             
          
         
       
     
       FIG. 2  illustrates a preferred operation of the BS, RS and MS in terms of its activity within each of the zones described in Table 1. 
       FIG. 3  indicates one particular realization of the proposed frame structure in terms of how different user types may be allocated for transmission or reception within the various zone types. 
     In this case there are five link types identified (A-E), as illustrated in  FIG. 3 . A description of the zones that are used in this example is given in Table 2. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Description of example of zone usage within one cell. 
               
             
          
           
               
                   
                 DL Zone 
                 UL Zone 
                   
               
               
                 Link 
                 Usage 
                 Usage 
                 Comments 
               
               
                   
               
               
                 (A) 
                 (1), (2), 
                 (6), (7) 
                 MS and RS are spatially separated and  
               
               
                   
                 (5) 
                   
                 therefore significant interference isolation  
               
               
                   
                   
                   
                 exists. User does not support SDMA. 
               
               
                 (B) 
                 (1), (2), 
                 (6), (9) 
                 MS and RS are spatially separated and  
               
               
                   
                 (3) 
                   
                 therefore significant interference isolation  
               
               
                   
                   
                   
                 exists. User does support SDMA. 
               
               
                 (C) 
                 (1), (2), 
                 (6), (7), 
                 RS receives data in (3) and (7) and then  
               
               
                   
                 (3), (5) 
                 (9) 
                 transmits in (5) and (9) thereby enabling  
               
               
                   
                   
                   
                 in-frame relaying. 
               
               
                 (D) 
                 (1), (2), 
                 (6), (7) 
                 MS communicates with BS via RS. 
               
               
                   
                 (5) 
                   
                 Transmission to the RS happens at the  
               
               
                   
                   
                   
                 beginning of the UL subframe (7) to allow  
               
               
                   
                   
                   
                 sufficient RS relay processing time. 
               
               
                 (E) 
                 (1), (2), 
                 (6), (8) 
                 MSs that communicate directly with the  
               
               
                   
                 (4) 
                   
                 BS that are not isolated from the RS use  
               
               
                   
                   
                   
                 zones (4) &amp; (8) to prevent RS interference  
               
               
                   
                   
                   
                 from impairing link performance. 
               
               
                   
               
             
          
         
       
     
     One of the advantages of adopting the proposed frame structure of invention embodiments is that the BS can make use of all of the transmission resource all of the time to communicate with the RS and MS nodes in the network. This is enabled by reusing the transmission resource used on the RS to MS link for BS to MS communications. In order to effect this, and prevent such a reuse approach from causing excess interference, the BS must ensure that the users it communicates within this reuse zone (i.e. zones (5) &amp; (9)) are sufficiently isolated from the users communicating with the RS. Thus, the BS essentially requires a mechanism to decide whether the users with which it communicates should be in the reuse zone (i.e. zones (5) &amp; (9)) or the normal zone (i.e. zones (4) &amp; (8)). 
     There are numerous algorithms that can be used to form such a mechanism. A few examples are listed below:
     1. Ask the MS to perform a carrier-to-interference-plus-noise (CINR) measurement on the BS transmission during the reuse zone and during the normal zone. If the CINR is much higher in the normal zone then allocate the user to the normal zone. If the CINR is similar, then allocate the user to the reuse zone.   2. Start with all users in the normal zone. If the normal zone becomes fully loaded and cannot accommodate more users without the risk of imposing a reduction on the quality of service new and existing users will experience, then identify candidate users to move from the normal zone to the reuse zone. If subsequently the reported CINR for a user communicating with the BS in the reuse zone falls below a particular threshold then move that to the normal zone.
 
In summary, benefits of particular embodiments may include:
       Enabling the construction and operation of simple, low cost relays that do not need to generate any control information or perform scheduling   Maximize spectral efficiency by ensuring that the BS does not have any time in the frame when it is idle   Minimize latency by enabling two-hop relaying to occur within one frame   Enable the system to potentially provide transparent operation to a legacy single-hop TDD users   The possibility to further improve spectral efficiency through using SDMA based techniques to enable the same transmission resource (frequency &amp; time) to be used between the BS and the RSs and MSs within a cell.   Provide a mechanism to enable reuse of the RS-MS communication zone by the BS to communicate directly with MSs that will not cause a degradation in RS-MS link performance by performing such communications.   
       

     Embodiments of the present invention may be implemented in hardware, or as software modules running on one or more processors, or on a combination thereof. That is, those skilled in the art will appreciate that a microprocessor or digital signal processor (DSP) may be used in practice to implement some or all of the functionality of a transmitter embodying the present invention. The invention may also be embodied as one or more device or apparatus programs (e.g. computer programs and computer program products) for carrying out part or all of any of the methods described herein. Such programs embodying the present invention may be stored on computer-readable media, or could, for example, be in the form of one or more signals. Such signals may be data signals downloadable from an Internet website, or provided on a carrier signal, or in any other form. 
     Although the present invention has been described with several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformations, and modifications as fall within the scope of the appended claims.