Abstract:
In accordance with one embodiment of the present invention, a wireless transmission method for use in a multi-hop wireless communication system includes determining for a particular transmission at least one measure of the expected link characteristics for at least two links of a communication path, and for each of those links, configuring a particular transmission window by setting its shape and/or a transmission format to be used in that window in dependence upon the measure for that link only. Furthermore, the method includes, during that particular transmission, transmitting information along those links using, for each of those links, the particular transmission window for that link.

Description:
RELATED APPLICATION  
       [0001]     This application claims foreign priority benefits under 35 U.S.C. § 119 of United Kingdom Application No. GB 0616471.9, filed on Aug. 18, 2006, entitled “Communication Systems”.  
       CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0002]     This application relates to the following applications, each of which is incorporated herein by reference: 
        COMMUNICATION SYSTEMS, Attorney Docket 017071.0126, application Ser. No. ______, filed Aug. 17, 2007 and currently pending;     COMMUNICATION SYSTEMS, Attorney Docket 017071.0127, application Ser. No. ______, filed Aug. 17, 2007 and currently pending;     COMMUNICATION SYSTEMS, Attorney Docket 017071.0128, application Ser. No. ______, filed Aug. 17, 2007 and currently pending;     COMMUNICATION SYSTEMS, Attorney Docket 017071.0129, application Ser. No. ______, filed Aug. 17, 2007 and currently pending;     COMMUNICATION SYSTEMS, Attorney Docket 017071.0130, application Ser. No. ______, filed Aug. 17, 2007 and currently pending;     COMMUNICATION SYSTEMS, Attorney Docket 017071.0131, application Ser. No. ______, filed Aug. 17, 2007 and currently pending;     COMMUNICATION SYSTEMS, United Kingdom Application No. GB 0616478.4, filed on Aug. 18, 2006;     COMMUNICATION SYSTEMS, United Kingdom Application No. GB 0616475.0, filed on Aug. 18, 2006; and     COMMUNICATION SYSTEMS, United Kingdom Application No. GB 0616476.8, filed on Aug. 18, 2006.        
 
     
    
     TECHNICAL FIELD  
       [0012]     This invention relates in general to communication systems, and more particularly to burst profile re-dimensioning schemes.  
         [0000]     Overview  
         [0013]     Currently there exists significant 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).  
         [0014]     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. 8  illustrates a single-cell two-hop wireless communication system comprising a base station BS (known in the context of 3 G 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.  
         [0015]     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.  
         [0016]      FIG. 9  illustrates 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. 9  is positioning of a nomadic relay station to allow penetration of coverage within a building that could be above, at, or below ground level.  
         [0017]     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. 9  provides access to a network using a relay positioned on a vehicle.  
         [0018]     Relays may also be used in conjunction with advanced transmission techniques to enhance gain of the communications system as explained below.  
         [0019]     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) 
 
         [0020]     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) .  
         [0021]     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) 
 
         [0022]     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.  
         [0023]     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.  
         [0024]     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.  
         [0025]     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     j2π   ⁢           ⁢   n   ⁢           ⁢   Δ   ⁢           ⁢     f   ⁢   t                 ,     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. 
 
         [0026]     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.  
         [0027]     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.  
         [0028]     As an example,  FIG. 10  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.  
         [0029]     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.  
         [0030]     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  
       [0031]     In accordance with one embodiment of the present invention, a wireless transmission method for use in a multi-hop wireless communication system is provided. The system includes at least three communication apparatuses, one said communication apparatus comprising a source apparatus, another said communication apparatus comprising a destination apparatus and the other said communication apparatus comprising an intermediate apparatus. Said source apparatus is operable to transmit information in a transmission window in a communication direction along a communication path extending from the source apparatus to the destination apparatus via the intermediate apparatus. The transmission window has a shape defined by a transmission frequency bandwidth profile over a transmission interval, and the communication path includes a series of two or more consecutive links from the source apparatus to the destination apparatus. The intermediate apparatus is operable to receive information from a previous communication apparatus in said communication direction and to transmit the received information in such a transmission window to a subsequent apparatus in said communication direction. The wireless transmission method for use in such a multi-hop wireless communication system includes determining for a particular transmission at least one measure of the expected link characteristics for at least two links of the communication path, and for each of those links, configuring a particular transmission window by setting its shape and/or a transmission format to be used in that window in dependence upon the measure for that link only. Furthermore, the method includes, during that particular transmission, transmitting information along those links using, for each of those links, the particular transmission window for that link.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     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:  
         [0033]      FIG. 1  shows wireless relay scenarios in WiMAX;  
         [0034]      FIG. 2  shows an example of permutation zones in one OFDMA frame;  
         [0035]      FIG. 3  shows an example of burst allocation;  
         [0036]      FIG. 4  shows an example of allocating bursts into PUSC (Partial Utilized Subchannlization) permutation zone in an OFDMA TDD frame;  
         [0037]      FIG. 5  shows the burst error rate for different dimension of a burst with fixed payload;  
         [0038]      FIG. 6  shows a flow chat of centralized burst profile re-dimensioning scheme (BPRS);  
         [0039]      FIG. 7  shows the flow chat of distributed BPRS;  
         [0040]      FIG. 8  shows a single-cell two-hop wireless communication system;  
         [0041]      FIG. 9  shows applications of relay stations; and  
         [0042]      FIG. 10  shows a single hop TDD frame structure used in the OFDMA physical layer mode of the IEEE 802.16 standard.  
     
    
     DETAILED DESCRIPTION  
       [0043]     By using relay in wireless communication systems, both the quality of service (QoS) and the coverage can be improved.  FIG. 1  shows two typical application scenarios of WiMAX relay. In  FIG. 1 . a , by using the relay stations (RS), more areas can be covered. In  FIG. 1 . b , the RS can improve the QoS for mobile stations (MS), which is suffering from the shadow of buildings. In this proposal, the RS  1 # and RS  2 # are called the first-hop RS, and the second-hop RS respectively; the MS 1  # and MS  2 # are called the second-hop MS and the third-hop MS respectively.  
         [0044]     The OFDMA-based (Orthogonal Frequency Division Multiplexing Access) WiMAX divides a signal into sub-channels (i.e. groups of carriers), with each subchannel being allocated to different OFDM (Orthogonal Frequency Division Multiplexing) subscribers. Each MS can be treated separately independent of location, and distance from the base station, interference and power controls. The whole frame will be divided by different permutation zones. Each permutation zone type is appropriate to different radio channel environment and different network/cell deployment. For instance,  FIG. 2  shows an example of permutation zones in one TDD frame.  
         [0045]     Normally, a service connection/flow of a MS will be arranged to using a proper permutation zone by the radio resource management algorithms in terms of QoS demands, radio channel conditions, and other factors. Within a permutation zone, a two-dimensional area of a group of subchannels will be allocated to a service connection of a MS. This two-dimensional allocation is called “burst”. The minimum allocation unit for a burst is slot in WiMAX. A burst may be visualized as a rectangle, which can be represented by N Subchannels ×M OFDM     —     Symbols  ( FIG. 3 ).  
         [0046]      FIG. 4  shows an example of allocating bursts into PUSC (Partial Utilized Subchannlization) permutation zone in an OFDMA TDD frame. To a specific payload of a burst, there are many possible combinations of the value N and M. How to decide the values of N and M is a main part of a burst dimensioning algorithm, which will strongly impact the bit error rate (BER) performance and QoS in WiMAX.  
         [0047]      FIG. 5  shows the burst error rate for different dimension of a burst with fixed payload, which is 4 slots. The channel model is ITU Pedestrian B with a velocity of 3 km/hr. It is clear that different burst size in frequency dimension will result in different burst error rate.  
         [0048]     In WiMAX and other relaying systems, the channel characteristics of radio links over different hops are various. It is desirable to provide schemes for BS or RS to re-dimension the burst to adapt to each hop&#39;s radio environment and capability, thus improving the network performance.  
         [0049]     There are three kinds of burst preferred profile re-dimensioning schemes (BPRSs) in WiMAX relay systems according to invention embodiments. One is centralized BPRS, where the BS performs the most parts of the calculations, and decides the burst profiles for all hops&#39; communication, one is distributed BPRS, where the computation of the algorithm is distributed to BS and RSs, and each RS will decide its burst profile, the other is the hybrid BPRS, which is the combination of centralized BPRS and distributed BPRS.  
         [0050]     The centralized BPRS can decrease the capability demands in a RS, thus decreasing the cost of RS. In distributed BPRS, though RSs shall take over some computation from BS, the computation load between BS and RS is balanced, and the signaling between RS and BS can be decreased as well. The hybrid BPRS, where the burst profiles of some hops are decided by the BS, while others are decided by each RS, can suitable to RSs with different capabilities.  
         [0000]     Centralized BPRS:  
         [0051]     There are 6 main steps in centralized BPRS, which also can be illustrated by  FIG. 6 .  
         [0052]     Step one: BS collects the measurements, which is relevant to the burst profile of the link between BS and its first-hop RS (RS  1 # in  FIG. 1   a , and  FIG. 1   b .). These measurements include the channel characteristics, such as mobility of RS. The RS&#39;s mobility can be simply reported by RS, or it can be measured through calculating the correlation factor of channel response, calculating the channel coherent time, or standard deviation of CINR. A bigger value of the correlation factor of channel response or the channel coherent time means higher mobility. A smaller standard deviation of CINR normally means lower mobility.  
         [0053]     Step two: BS decides the type of subchannalization scheme (or permutation zone) for the first-hop radio communication. This type of the subchannelization scheme is depends on the mobility, service type, cell deployment, and other factors of the first-hop RS. For example, AMC subchannelization scheme may be suitable to a fixed RS, and PUSC may be appropriate to multi-sector cell with mobility support. This decision should consider the QoS demands for the first hop.  
         [0054]     Step three: BS decides the burst profiles for the first-hop radio communication. This decision should also consider the QoS demands for the first hop. Because of the limited number of slots in WiMAX frame, and link budget, a service connection in the RS with higher level of services (or higher QoS demands) will have higher priority to allocate an appropriate burst. The dimension of bursts depends on the mobility of RS. For example, a burst for a fixed RS shall have bigger size in frequency dimension rather than time dimension, thus achieving more gain from frequency diversity. On the other hand, a burst for a high mobility RS shall have larger size in time dimension to obtain more time diversity gain.  
         [0000]     Step four: BS collects the measurements, which is relevant to the burst profile for second-hop RS (RS  2 # in  FIG. 1   a ), and the second-hop MSs within the first-hop RS&#39;s coverage. The method of mobility measurement is similar to that in the step one.  
         [0055]     Step five: BS decides the subchannelization schemes, and the burst dimension for the service connections within second-hop communication. This decision should also consider the QoS demands for the first hop. The subchannelization schemes, and the burst dimension, will be decided in terms of the mobility of second-hop RS, or second-hop MSs. For example, AMC subchannelization scheme may be suitable to a fixed RS/MS. A burst for a fixed RS/MS shall have bigger size in frequency dimension rather than time dimension, thus achieving more gain from frequency diversity. On the other hand, a burst for a high mobility RS/MS shall have larger size in time dimension to obtain more time diversity gain. If other relevant burst profile parameters, such as transmission power, coding and modulation schemes, and other factors, need to be changed because the changing of burst dimension and subchannelization scheme, they will be also adjusted by BS as well.  
         [0000]     Step six: BS informs the first-hop RS to adjust the corresponding burst profile in terms of the decision in the step 5. The first-hop RS will re-allocate the burst for the service connections under the instructions by BS.  
         [0000]     Step seven: If the number of hops is more than two, the step four and five will be repeated for the next-hop RSs and MSs until the last-hop MSs receive the relayed data and control information.  
         [0000]     Distributed BPRS:  
         [0056]     There are 6 main steps in distributed BPRS, which also can be explained by  FIG. 7 .  
         [0057]     Step one: BS or RS collects the measurements, which is relevant to the burst profile of the link. These measurements include the channel characteristics, such as mobility of RS and MS. The mobility can be measured through calculating the correlation factor of channel response, calculating the channel coherent time, or standard deviation of CINR. A bigger value of the correlation factor of channel response or the channel coherent time means higher mobility. A smaller standard deviation of CINR normally means lower mobility.  
         [0058]     Step two: BS or RS decides the type of subchannalization scheme (or permutation zone) for its own radio communication. This type of the subchannelization scheme is depends on the mobility, service type, cell deployment, and other factors of the link. For example, AMC subchannelization scheme may be suitable to a fixed or low mobility device, and PUSC may be appropriate to multi-sector cell with mobility support. This decision should consider the QoS demands of the link.  
         [0059]     Step three: BS or RS decides the burst profiles for its own radio communication. This decision should also consider the QoS demands of the link. Because of the limited number of slots in WiMAX frame, and link budget, a service connection in the RS with higher level of services (or higher QoS demands) will have higher priority to allocate an appropriate burst. The dimension of bursts depends on the mobility of RS/MS. For example, a burst for a fixed or low mobility RS/MS shall have bigger size in frequency dimension rather than time dimension, thus achieving more gain from frequency diversity. On the other hand, a burst for a high mobility RS/MS shall have larger size in time dimension to obtain more time diversity gain.  
         [0000]     Step four: BS or RS adjusts the burst profile and sends the data and control information.  
         [0060]     Particular embodiments give schemes for BS or RS re-dimensioning the burst profiles in WiMAX relay system, thus adapting to various conditions of radio channels of each hop. The benefits from this may include:  
         [0061]     1. Through the associated improvement in performance (due to burst profile re-dimensioning to adapt to the radio channel and service types in different hops) to provide an improved OFDMA (such as WiMAX) product;  
         [0062]     2. The proposed method gives schemes to re-dimensioning the burst to adapt to each hop&#39;s radio channel conditions and the type of services.  
         [0063]     3. The proposed re-dimensioning schemes can be centralized, distributed, or the combination of centralized and distributed schemes, thus giving the flexibility to using RSs with various capabilities.  
         [0064]     4. A 2×D burst size controlling method is proposed in this patent. Based on mobility estimation, this 2×D burst size controlling method can improve the bit error rate performance.  
         [0065]     5. If other burst profile parameters, such as transmission power, modulation and coding scheme (MCS) of a burst, and other factors, need to be changed because the changing of burst dimension and subchannelization scheme, they will also be adjusted by BS or RS as well, thus making the communication within each hop adapt to dynamically changing radio channel and services.  
         [0066]     Particular 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.  
         [0067]     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.