Patent Publication Number: US-9414333-B2

Title: System and method for downlink and uplink parameter information transmission in a multi-hop wireless communication system

Description:
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 (throughout). 
     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. 3  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. 
       FIG. 4  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. 4  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. 4  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 modelled 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 IEEE802.11a/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: 
     
       
         
           
             
               
                 
                   
                     
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     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 IEEE802.16 standard incorporates both an FDD and TDD mode. 
     As an example,  FIG. 5  illustrates the single hop TDD frame structure used in the OFDMA physical layer mode of the IEEE802.16 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. 
     Particularly when a relay is controlled in a centralised manner (i.e. not locally) in order to facilitate fast adaptation of the transmitting modem to the channel environment (i.e. adaptation of the modulation &amp; coding, MIMO operation, closed loop power control), it is typically required for the receiver to provide indicators of link quality such as measurement information (CQI) of the receive signal quality and other parameters. The transmitter can then adapt to the current situations. 
     However, the problem is that, as described for example in EP 05257911.7, the signalling bandwidth required can be significantly increased due to the extra links that exist and the fact that all this information has to be conveyed back to the BS (or controlling entity in the general sense). The contents of EP 05257911.7 relating to transmission of link parameters is incorporated herein by way of reference and a copy of this application is filed herewith. To assist in appreciation of the problem, the requirements in the case of a simple 2 hop scenario are illustrated in  FIG. 1 . 
     The invention is defined in the independent claims, to which reference should now be made. Advantageous embodiments are set out in the sub claims. 
    
    
     
       Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  shows CQI signalling requirements in a two-hop network; 
         FIG. 2  shows the CQICH allocation in uplink subframes; 
         FIG. 3  shows a single-cell two-hop system; 
         FIG. 4  shows applications for relay stations; 
         FIG. 5  shows an example TDD frame structure from OFDMA physical layer of the IEEE802.16 standard. 
     
    
    
     For more detail as to frame format, the reader is referred to two further patent applications filed on the same date, by the same applicant, with agent reference numbers P107330GB00 and P107297GB00, as well as earlier applications GB 0616477.6, GB 0616481.8 and GB 0616479.2; each of these describing interrelated inventions proposed by the applicant. The contents of each of these applications are incorporated herein by way of reference and copies of them are filed herewith. 
     Whereas in a single hop network all that would be required is the reporting of the CQI on the DL at the receiver to the BS, it now needs the reporting of the CQI on the DL at the RS, as well as a pair of CQI values for each MS connected to the RS. As in the 802.16 standard, the content of which is incorporated herein by reference, each 4-bit or 6-bit CQI value is mapped separately on to 1 slot (i.e. 48 subcarriers) (note this is a half slot=24 subcarriers for a 3-bit value) using QPSK modulation, this mechanism is inefficient for conveying such large amount of information across a link that might typically be more robust than in the single hop case. Embodiments of the invention therefore provide a very simple signalling solution that reduces the required signalling overhead in the two hop case to a similar level as required in a single hop case by packing the CQI pair on the access links into one channel. 
       FIG. 2  demonstrates how the connection IDs are mapped onto QCI channels (CQICHs). In the access link, (from the subscribers to the relay station) 4 CQICHs are used. In the relay link, 5 are required, the additional CQICH for the CQI value between the base station and relay station in the downlink direction. This last value need not be packed. 
     Embodiments of the invention have two parts. The first is a way of packing CQI values into a CQI channel to reduce overhead. The second part is a new method for CQI channel allocation from the BS for the new links. 
     CQI Reporting 
     Currently the CQI value is mapped to the CQI channel using a simple vector mapping approach rather than the normal FEC applied to data. The reason for using such an approach is the decoding time can be much quicker than that required when performing full FEC based decoding and hence allows the receiver to gain access to the transmitted information much quicker and hence prepare to act on it in a timely manner. 
     This proposal maintains the essence of a robust yet simple to decode modulation process of the CQI value on to the 24 (3-bit CQI) or 48 (4-bit, 6-bit CQI) subcarriers. Instead of using a vector based approach, the two CQI values are simply packed consecutively and then CDMA modulated onto the subcarriers by applying a spreading sequence of appropriate spreading factor. In the case of a pair of 3-bit or 6-bit values the spreading code has a spreading factor of 8 and in the case of a pair of 4-bit values it is 12. Numerous well-known code types can be applied such as Walsh-Hadamard, Gold codes, Kasami sequences or PN codes. 
     The receiver then simply demodulates the CQI value by applying despreading of the 96-bit sequence to get the two CQI values. Consequently, it is possible to convey two CQI values to the BS using the same resource as that required for conveying one value. Whilst the robustness of transmission will obviously be lowered, the combination of QPSK modulation with CDMA modulation of SF=8 combined with the underlying permutation sequence used to map the logical to physical subcarriers should retain a significant degree of coding gain. 
     CQI Channel Allocation 
     CQI channel allocation currently sends a message on a certain connection to indicate to the receiver the CQI channel ID to use for reporting, and how often/when it can use this channel. In the case illustrated in  FIG. 1 , it is possible for the BS to allocate a CQI channel to both the RS and MSs to get them to report their downlink CQI measurement to the BS and RS respectively. However, a new mechanism is required to get the RS to transmit the packed CQI pair available at the RS for each RS-MS link to the BS. 
     In order to do this a new message is sent by the BS to the RS on the RS connection (using its ID) to request transmission of a packed CQICH pair. This message includes the MS CID and CQICH ID that should be used for this packed CQI pair. Both the BS and RS maintain tables that mapped CQICH IDs to MS CIDs so that when the BS receives a CQICH transmission from the RS on a certain channel it knows to expect that it is a packed pair of CQI values from the RS. It is then possible for the BS to support packed CQI pairs from RSs as well as RSs&#39; own CQI report values in the same fast-feedback region that is used for reporting, amongst other values, CQI measurements on CQI channels. 
     The alternative approach to maintaining tables that map channels to CQICH IDs that are consistent between the BS and RS is to use different CDMA codes to indicate the ID of the MS from which the packed reports originated. This gives the RS the freedom to manage the reporting of the pairs on any CQI channel, as the BS can use the unique code to determine to which MS the report relates. 
     SUMMARY OF BENEFITS 
     The key benefits are:
         Enables overhead associated with reporting CQI values in a centrally controlled relaying system to be significantly reduced (halved).   The logical subcarrier modulation mechanism of using a spreading code combined with the underlying OFDMA subcarrier permutation provides a simple mechanism for robustly encoding the packed CQI values (note an alternative would be to modify the existing vector mapping approach).   Does not require the RS to possess complex FEC decoding or encoding hardware (i.e. technique can easily be applied in a low-cost amplify and forward relay, as well as in a full featured relay).   Due to low complexity decoding being available, means the RS can potentially relaying the information in the UL within the same subframe when a frame structure similar to that in P107297GB00 is utilised.       

     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.