Abstract:
Method and system for data transmission in a wide area mobile network supporting both MIMO User Equipments and non MIMO User Equipments, comprising:
       Allocating in a first signal ( 6   a ) the traffic of non MIMO user Equipments ( 9 ), a first MIMO data stream and a primary pilot channel.   Allocating in a second signal ( 6   b ) a second MIMO data stream and a secondary pilot channel.   Transmitting the first signal ( 6   a ) and the second signal ( 6   b ) with two orthogonal circular polarizations.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention has its application within the telecommunications sector and, especially, in the industrial area engaged in providing Radio Access Networks (RANs) with elements of cellular infrastructures such as Radio Network Controllers (RNCs) and Base Stations (Nodes B) for wireless communications systems. 
         [0002]    More specifically, it relates to wireless communications systems supporting both MIMO and non MIMO variants of HSDPA technologies. 
       BACKGROUND OF THE INVENTION 
     Related Art 
       [0003]    HSDPA (High Speed Downlink Packet Access) is a packet-based data service in the 3rd generation W-CDMA (Wideband Code Division Multiple Access) systems, which provides high-speed data transmission (with different download rates according to the HSDPA technology step e.g. 7.2/10.8/16.2/21.6/28.8 Mbps over a 5 MHz bandwidth) to support multimedia services. 
         [0004]    In order to reach the higher peak rates (28.8 Mbps with 3GPP Release), the MIMO (Multiple Input Multiple Output) feature is used in HSDPA, in which multiple antennas are implemented at both base station (Node B) and mobile terminals (UE: User Equipment). 
         [0005]    The basic MIMO feature as standardised in 3GPP Release 7 is based on two transmitter antennas (at the node B) and two receiving antennas (at the UE) using a common carrier. At the transmitter, the transmitted data is divided into 2 data streams and transmitted through the two antennas using the same radio resource (same time i.e. Transmission Time Interval and HSDPA codes). The two streams of data are recovered by the UE from the signals received via its 2 antennas (Rx Diversity). Thus, the MIMO feature needs support in MIMO-enabled terminals as well as in the network. In order to deploy MIMO and transmit two parallel data streams, two power amplifiers are required per sector (one for each of the two antennas). In order to not use an entire carrier for MIMO only (5 Mhz), it is more efficient and practical to use the same carrier as non MIMO devices e.g. HSDPA legacy terminals to utilise all available capacity. 
         [0006]    MIMO technology is an important step in the evolution of HSDPA, as it provides higher data rates in downlink whilst further improving spectrum efficiency. 
         [0007]    However, there is an important drawback as a consequence of transmitting two independent MIMO data streams through two independent channels (e.g. by using two orthogonal linear polarizations). A non MIMO user equipment equalizes the channel by listening to a pilot signal transmitted together with its data, however unless some special measure is applied, the device is only able to equalize one of the channels (the one over which its data is being transmitted), the second MIMO stream and the second pilot channel becoming both a harmful interference as its effect is not equalized since it is transmitted through a independent channel. 
         [0008]    There are some potential solutions based in Transmit diversity that can reduce the effect of this interference such as STTD (Space Time Transmit Diversity) or CLTD (Closed-Loop Transmit Diversity) in HSDPA. However they are not only generally complex from a system perspective, as they imply serious constraints for upcoming features such as Dual Carrier, but what is worse, in the specific case of HSDPA none of them fully resolves the issue due to the observed lack of support in legacy terminals (i.e. non MIMO user equipments). This follows a well known compromise design taken by WCDMA chipset manufacturers for Advanced Receivers (those using an Equaliser), according to which the equalizer is turned off whenever transmit diversity is used in the system. This results in an unacceptable performance degradation of the service for users with non MIMO terminals. Field trials carried out in the field by mobile network operators have shown that the STTD activation significantly decreases the performance of some categories of HSDPA terminals already in the market (i.e. those with Advanced Receivers) when the terminals are operating in both good and medium radio conditions. Performances in good radio conditions are precisely those that permit reaching the highest peak rates offered by the mobile network operators. 
         [0009]    Thus, there is a need of a solution that allows the implantation of MIMO systems in a network with coexisting non MIMO terminals, without worsening the service received by this legacy terminals. 
       SUMMARY OF THE INVENTION 
       [0010]    The current invention solves the aforementioned problems by disclosing a system and method that allows non MIMO User Equipments to perform a correct equalization thanks to the use of two orthogonal circular polarizations for the two MIMO data streams. 
         [0011]    In a first aspect of the present invention, a method for transmitting data in a wide area mobile network with coexisting MIMO User Equipments and non MIMO User Equipments is disclosed. The network provides at least one radio carrier for traffic allocation, the at least one radio carrier carrying a first signal and a second signal to being able to support MIMO traffic, which comprises a first MIMO data stream associated to a primary pilot channel (e.g. Common Pilot Channel, CPICH, in 3G) and a second MIMO data stream associated to a secondary pilot channel. 
         [0012]    The disclosed method comprises:
       Allocating in the first signal the traffic of non MIMO user Equipments (e.g. non MIMO HSDPA, Release 99, CCH), the first MIMO data stream and the primary pilot channel.   Allocating in the second signal the second MIMO data stream and the secondary pilot channel.   transmitting the first signal using a first circular polarization (Left Handed Circular LHC or Right Handed Circular RHC);   transmitting the second signal using a second circular polarization which is orthogonal to the first circular polarization (Right Handed Circular RHC or Left Handed Circular LHC).       
 
         [0017]    In this way, both signals undergo the same modifications due to diffraction and reflection when received by a non-MIMO terminal, allowing correct equalization of the channel, while for MIMO terminals the signals maintain their independence by using orthogonal polarizations, thus being able to guarantee that, for MIMO terminals, the performance of the network is at least maintained by using the present invention. 
         [0018]    Preferably, the aforementioned circular polarizations are achieved by forming two combined signals, each of which contains both original signals (i.e. the first and second signals), one of which has its phase shifted 90°; and then transmitting the combined signals with two orthogonal linear polarizations, thus resulting in the transmission of the original signals with circular polarization. 
         [0019]    In another aspect of the present invention, a system is disclosed that implements the described method by comprising:
       Allocation means configured to allocate in the first signal the traffic of non MIMO user Equipments, the first MIMO data stream and the primary pilot channel; and in the second the second MIMO data stream and the secondary pilot channel.   Transmission means configured to transmit each signal with a different circular polarization, being these polarizations orthogonal to preserve the independence of the channels.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    For the purpose of aiding the understanding of the characteristics of the invention, according to a preferred practical embodiment thereof and in order to complement this description, the following figures are attached as an integral part thereof, having an illustrative and non-limiting character: 
           [0023]      FIG. 1  shows a diagram of a first preferred embodiment of the method of the invention, which uses a 90° hybrid combiner. 
           [0024]      FIG. 2  shows a diagram of a second preferred embodiment of the method of the invention, which uses a specific implementation of the Virtual Antenna Mapping functional block as described in 3GPP Standard to achieve circular polarization. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The matters defined in this detailed description are provided to assist in a comprehensive understanding of the invention. Accordingly, those of ordinary skill in the art will recognize that variation changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, description of well-known functions and elements are omitted for clarity and conciseness. 
         [0026]    Note that in this text, the term “comprises” and its derivations (such as “comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc. 
         [0027]      FIGS. 1 and 2  show two different preferred embodiments of the system of the invention, according to whether the forming of the combined signals  7   a ,  7   b  is performed before or after the amplification introduced by means of two Power Amplifiers (PA)  5 . 
         [0028]    In  FIG. 1 , the traffic of MIMO UEs  8  and the traffic of non MIMO UEs  9  are allocated in a first signal  6   a  and a second signal  6   b  by the allocation means  1 . After this allocation, the first signal  6   a  comprises:
       non MIMO traffic (e.g. HSDPA, Rel 99, etc)   First MIMO stream   Primary CPICH       
 
         [0032]    while the second signal  6   b  comprises:
       Second MIMO stream   Secondary CPICH       
 
         [0035]    These first signal  6   a  and second signal  6   b , are transmitted with orthogonal circular polarizations by the transmission means  2 , which comprise two PAs  5 , combining means  3 ,  3 ′ and a cross polar antenna  4 . 
         [0036]    In order to circularly polarize the first signal  6   a  and second signal  6   b , they are combined by the combining means  3 ,  3 ′, thus forming two combined signals (first combined signal  7   a  and second combined signal  7   b ). The first combined signal  7   a  comprises the first signal  6   a  plus the second signal  6   b  with a 90° phase shift, while the second combined signal  7   b  comprises the first signal  6   a  with a 90° phase shift plus the second signal  6   b . It is a key point to make sure that any further elements located between the combining means  3 ,  3 ′ and the antenna  4 , maintain the relative phase between the first combined signal  7   a  and the second combined signal  7   b.    
         [0037]    As a result, the first combined signal  7   a  comprises:
       non MIMO traffic   First MIMO stream   Primary CPICH   Second MIMO stream (shifted 90°)   Secondary CPICH (shifted 90°)       
 
         [0043]    and the second combined signal  7   b  comprises:
       non MIMO traffic (shifted 90°)   First MIMO stream (shifted 90°)   Primary CPICH (shifted 90°)   Second MIMO stream   Secondary CPICH       
 
         [0049]    By transmitting the first combined signal  7   a  by means of the cross polar antenna with a first linear polarization, and the second combined signal  7   b  with a second linear polarization, orthogonal to the first linear polarization, the sum of both transmission results in the first signal  6   a  and the second signal  6   b  being transmitted with two orthogonal circular polarizations (i.e. the first signal  6   a  being transmitted with left hand circular polarization, LHC, and the second signal  6   b  with right hand circular polarization, RHC, or viceversa, depending on the sign of the shift). 
         [0050]    In order to have a deeper understanding on how the circular polarization is produced, hereafter you can find the explanation that justifies the methodology proposed. 
         [0051]    Since an Electrical Field ({right arrow over (E)}) is transmitted over two orthogonal antenna arrays with linear polarizations (i.e. +45°, −45°), it can be expressed as the combination of two orthogonal vectors (Ex and Ey): 
         [0000]        {right arrow over (E)}= ( Ex,Ey ) 
         [0052]    wherein
       Ex=Eo·cos(wt+Φ) (antenna array) +45°)   Ey=Eo·cos(wt+Φ) (antenna array) −45°)       
 
         [0055]    wherein Eo is the amplitude of the Electrical Field, w is its frequency, Φ is the phase of Ex and Ey. 
         [0056]    As stated above, the invention proposes to have one of the components 90° shifted, the above formulae resulting as (for the case of shifting Ey):
       Ex=Eo·cos(wt+Φ) (antenna array +45°)   Ey=Eo·cos(wt+pi/2+Φ)       
 
         [0059]    or, in an equivalent manner
       Ey=Eo·sen(wt+Φ) (antenna array −45°)       
 
         [0061]    If the two components are represented in a plane the variation of {right arrow over (E)} in the plane xy versus time, which is actually the {right arrow over (E)} polarization, represents a circumference. LHC or RHC polarization are obtained depending on the antenna array where the 90 a  phase shifted is introduced. 
         [0062]    Non MIMO traffic, both MIMO streams and the primary and secondary CPICH are thus transmitted through both antennas and with the needed phase shifts in order to produce circular polarization. Therefore, regardless of the introduced phase, a non MIMO terminal sees both signals as suffering the same transformations before being received by the UE. Therefore, the second MIMO signal does not become in a harmful interference because it is transmitted through the same channel and the orthoganality with regards to the other non MIMO signals is kept. 
         [0063]    On the other hand, the use of LHC circular polarization and RHC circular polarisation allows a MIMO UE receiving the two MIMO streams through independent channels as they are orthogonal polarisations, decoding each MIMO stream independently. 
         [0064]    Two alternative implementations for the combining means  3 ,  3 ′ are hereby presented:
       A 90° hybrid combiner  3  ( FIG. 1 ), which combines the first and second signals after its amplification by the PAs  5 . The combination is thus performed at a physical level.   Virtual Antenna Mapping means  3 ′ ( FIG. 2 ), which combines the first and second signal at a logical level, before converting the data into electrical signals and before its amplification by the PAs  5 .