Abstract:
The invention relates to a method and device for performing channel simulation, the device comprising a set of channel simulation units ( 200  to  214 ) for simulating a radio channel, each unit comprising radio frequency parts ( 200 A to  214 A) and baseband parts ( 200 B to  214 B). In the solution of the invention, the baseband parts of several different units ( 200  to  214 ) are arranged to simulate the same channel.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and a device implementing the method for simulating a radio channel. The invention relates especially to implementing a multi-path radio channel simulator. 
     BACKGROUND OF THE INVENTION 
     One essential problem in radio systems is the rapid variation of the properties of a radio channel with time. This relates especially to mobile systems, in which at least one of the participants in a connection is often mobile. The attenuation and impulse response of the radio channel then vary within a wide phase and amplitude range even thousands of times per second. The phenomenon is random by nature, so mathematically it can be described by statistical means. The phenomenon complicates the design of radio connections and the used devices. 
     There are many reasons for the variation in a radio channel. When transmitting a radio frequency signal from a transmitter to a receiver in a radio channel, the signal propagates along one or more paths, in each of which the phase and amplitude of the signal vary, which causes fades of different lengths and strengths in the signal. In addition, noise and interference from other transmitters also disturbs the radio connection. 
     A radio channel can be tested either under actual conditions or using a simulator that simulates the actual conditions. Tests conducted in actual conditions are difficult, because tests taking place outdoors, for instance, are affected for example by the weather and season that change all the time. Even measurements taken in the same place produce a different result at different times. In addition, a test conducted in one environment (city A) does not fully apply to a second corresponding environment (city B). It is also usually not possible to test the worst possible situation under actual conditions. 
     However, with a device simulating a radio channel, it is possible to very freely simulate a desired type of a radio channel between two radio devices in such a manner that the radio devices operate at their natural transmission rates, just like in an actual operating situation.  FIG. 1  illustrates an example of a device for simulating a radio channel. The figure shows a first set of devices  100  to  108  and a second set of devices  110  to  118 , and a channel simulator  120 . The first set of transmitters  100  to  108  can comprise mobile phones, for instance, that through their antenna connectors are connected to the inputs of the channel simulator  120 . The second set of devices  110  to  118  can in turn be receivers of base station equipment that are connected to the outputs of the channel simulator. The number of the first and second devices need not be the same. In the example of the figure, there are five devices in each set. 
     A channel simulator typically comprises several channel elements that are capable of simulating and modelling a desired channel type. The channel simulator of  FIG. 1  comprises eight elements. Each element comprises both a radio frequency part and a baseband part. A signal can be fed to the input of the channel simulator either in radio frequency or baseband format. In the latter case, the radio frequency parts of the channel elements are bypassed. In the radio frequency part, a signal is converted to baseband, and the resulting baseband signal is forwarded to the baseband parts, in which the impact of the channel fade is added to the signal. 
     In prior-art solutions, the channel element forms a fixed unit. As in the case of  FIG. 1 , there may be situations during simulation, in which not all channel elements are used, because there are fewer channels to simulate than the device has capacity for. 
     BRIEF DESCRIPTION OF THE INVENTION 
     One object of the invention is thus to implement a method and a device implementing the method in such a manner that the capacity of a channel simulator can be utilized optimally in different situations and that the simulator can easily be updated. This is achieved by a method for simulating a radio channel, in which the radio channel is simulated by means of channel elements that comprise a radio frequency part and a baseband part and in which a signal of one radio frequency part is processed in more than one baseband part. 
     The invention also relates to a device for performing channel simulation, which comprises a set of means for simulating a radio channel, each means comprising radio frequency parts and baseband parts. In the device of the invention, the baseband parts of several different means are arranged to simulate the same channel. 
     Preferred embodiments of the invention are disclosed in the dependent claims. 
     The invention is based on the idea that each baseband part is capable of connecting the baseband part inputs and outputs to adjacent baseband parts. In a digital radio channel simulator, the channel is modelled in the baseband parts with a FIR (Finite Impulse Response) filter that forms a convolution between the channel model and the input signal in such a manner that the signal that is delayed by different delays is weighted by channel coefficients, i.e. tap coefficients, and the weighted signal components are summed. The channel coefficients are altered to correspond to the behaviour of an actual channel. By enabling a flexible distribution of the baseband parts, multiple signal propagation paths in the channel can be simulated, if necessary. Multiple propagation paths require multiple FIR taps, and by combining different baseband parts in the solution according to the preferred embodiments of the invention, it is possible to achieve a higher number of taps than in prior-art solutions. 
     When the simulation to be performed is defined in a channel simulator, i.e. its parameters, such as the number of channels to simulate, the number and connections of input and output signals, are fed in, a control unit of the channel simulator optimises the use of the simulator equipment on the basis of the parameters. If all radio frequency units of the equipment are not needed during the simulation, the baseband units corresponding to them can be utilized during the simulation. The control unit controls the connections of the input and output signals of the baseband units so that several baseband parts simulate the same channel, and thus, the full capacity of the equipment is utilized. 
     In a preferred embodiment of the invention, the baseband part is divided into two separate modules, an interface module that comprises the input and output parts of the baseband part and a digital module that comprises the component needed for the actual channel modelling, such as the FIR filter. By thus dividing the baseband part into two different modules, significant advantages are gained in the maintenance and updatability of the device. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will now be described in more detail by means of preferred embodiments and with reference to the attached drawings, in which 
         FIG. 1  shows the general structure of a channel simulator that was already described above, 
         FIG. 2  illustrates in more detail an example of the structure of a channel simulator, 
         FIG. 3  is an example of the structure of a baseband part, 
         FIG. 4  is a flow chart showing an example of a solution of an embodiment, and 
         FIGS. 5A and 5B  illustrate examples of different connections. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Let us examine the channel simulator of  FIG. 2 . The simulator comprises eight channel elements  200  to  214 , each of which is made up of a radio frequency part  200 A to  214 A and baseband part  200 B to  214 B. Each radio frequency part comprises a radio frequency input signal from a transmitter and an output signal to a receiver  200 C to  214 C. The simulator further comprises a local oscillator divider  216  that receives as input one or more radio frequency local oscillator signals  218 . The divider  216  divides a suitable radio frequency signal  220  to  234  for each radio frequency unit  200 A to  214 A. 
     In the radio frequency units  200 A to  214 A, the signals from the transmitter are converted to baseband for instance by multiplying them by a local oscillator signal, after which the baseband signal  200 D to  214 D is forwarded to the baseband units. A baseband signal  200 E to  214 E arrives from the baseband units to the radio frequency units through the simulated channel and is converted back to radio frequency in the radio frequency units and transmitted to the receiver. 
     In the baseband units  200 B to  214 B, the impact of the channel fade is added to the signal. This is typically done by FIR filters. The desired form of the channel is achieved by adjusting the tap coefficients of the FIR filter. The channel simulator comprises a simulator control unit (SCU)  236  that controls the tap coefficients of the FIR filters of the baseband units by means of a control bus  238 . The simulator control unit also controls the operation of the entire simulator by means of a control bus  240 . Information on simulation parameters, such as frequency parameters, gains and the like, are transmitted prior to the simulation over the control bus to the different parts of the device. 
     The channel simulator further comprises control means  242  that control the operation of the entire simulator. The control means are preferably implemented by means of a processor or computer and suitable software. The processor can naturally be replaced by a programmable logic made up of separate components. The control means further comprise interface equipment, such as a display and keyboard, by means of which the simulation parameters can be entered into the device. The parameters typically comprise the number of transmitters, the number of receivers, the number of channels to be simulated and their properties. The control means  242  control the simulator through the simulation control unit  236 . The simulation control unit  236  also comprises an input and output  244  of a synchronization signal, by means of which several channel simulators can be synchronized. Thus, several devices can be connected parallel to implement a wide simulation. 
     The baseband units of the channel simulator also comprise connections between each other. A signal coming from the radio frequency unit that has not yet passed through the FIR filter is connected from each baseband unit to the adjacent baseband units, preferably to the inputs of the FIR filters in these units. These connections are illustrated in  FIG. 2  by connections  248  to  260 . Further, from each baseband unit a FIR filter output signal is connected to the adjacent baseband units, preferably to be summed in the outputs of the FIR filters in these units. These connections are illustrated in  FIG. 2  by connections  262  to  274 . 
     The channel simulator can also operate directly on baseband, in which case conversions to and from baseband are not necessary in the radio frequency units. A radio-frequency, analogue or digital signal can be fed as an input signal to the channel simulator. 
       FIG. 3  illustrates the structure of a baseband unit according to a preferred embodiment of the invention. The baseband unit  202 B of  FIG. 2  is used as an example. The baseband unit is divided into two separate modules, i.e. a interface module  300  that comprises the input and output parts of the baseband part and a digital module  302  that comprises the components required in the actual channel modelling. An analogue  304  or digital  306  transmitter signal arrives at the interface module as input from the radio frequency unit. The signal comprises separate I-branch and Q-branch signals. Analogue inputs  304  are forwarded through low-pass filters  308  and  310  to analogue-to-digital converters  312 . 
     The digital I and Q signals  314  are next forwarded to a multiplexer  316  in the digital module  302 . Digital-format I and Q signals  248  to  250  arrive as other inputs to the multiplexer from the adjacent baseband units. Correspondingly, the I and Q signals are forwarded as outputs  248  to  250  to the adjacent baseband units. 
     The multiplexed I and Q signals are FIR filtered in a known manner, whereby the impact of the channel is added to the signals. The I and Q signals are first forwarded to a set of delay elements  318  to  324 , the delay of each of the elements being separately settable. The signals that have been delayed in different ways are forwarded from the delay elements to complex FIR filter elements  326  to  332 . The control bus  238  from the simulation control unit sets the tap coefficients of the FIR elements, the control bus being transmitted to the FIR elements as control data  336  through a bus adapter  334 . The outputs of the FIR elements are summed in adders  338  to  344 , to which the outputs  262  of the FIR elements of the adjacent baseband units are also brought for summing through a multiplexer  346 . The sum total  264  is taken onward to the adjacent baseband elements. The sum is also forwarded to the interface module  300  and from there on either directly in digital format out  348  or through digital-to-analogue converters  350  and low-pass filters  352 ,  354  in analogue format  256  out to the radio frequency unit. 
     In a preferred embodiment of the invention, the control bus  240  from the simulation control unit controls the multiplexers  316  and  346 , by means of which connections between different baseband units are adjusted. 
     In a preferred embodiment of the invention, the channel simulation means of the baseband units are thus divided so that the simulation means of several different baseband parts can simulate the same channel. Let us examine the flow chart shown in  FIG. 4  that illustrates the method steps of an embodiment. In this example, it is assumed that the device structure is modular, i.e. the configuration of the device can be altered between different types of simulations to suit each simulation. 
     In step  400 , current is switched on to the device. After this, in step  402 , the control means  242  of the device check the current configuration of the device. This way, the control means know what the configuration of the device is, i.e. of which modules the device is constructed. In step  404 , simulation parameters are received from the user. This is preferably done by means of suitable interface software, a display and a keyboard. The simulation parameters typically comprise the number of transmitters, the number of receivers, the number of channels to be simulated and their properties. The number of transmitters and receivers is not always the same when a test environment comprises transmission or reception diversity, for instance, or possible interfering transmitters. 
     In step  406 , the control means  242  define, on the basis of the parameters, the connections between the channel elements of the device and transmit the necessary commands to the channel elements by means of the simulation control unit  236  and bus  240 . If it turns out, for instance, that there are fewer channels to be simulated than there are channel elements in the device, the system knows that all radio frequency units will not be used and the baseband units corresponding to the unused ones can then be connected parallel to the baseband units corresponding to the used radio frequency units to assist in the channel simulation. Which baseband unit is connected where is selected on the basis of the simulation parameters. For instance, if there is one radio channel to simulate, whose modelling requires several FIR taps, several baseband units can be allocated for this calculation. The channels whose simulation is simple require no extra calculation capacity. 
     In step  408 , the control means  242  transmit, prior to simulation, information on the simulation parameters, such as frequency parameters and gains, to different parts of the device by means of the simulation control unit  236  and along the control bus  240 . In step  410 , simulation is performed according to the given parameters. The simulation control unit  236  controls the FIR filter taps by means of the bus  238 . 
       FIGS. 5A and 5B  illustrate two examples of different connections in the same equipment but with different simulation parameters.  FIG. 5A  shows a situation where two radio frequency units  200 A and  214 A are used. From the first radio frequency unit  200 A, the signal is connected to five baseband units  200 B to  208 B. From the second radio frequency unit  214 A, the signal is connected to three baseband units  210 B to  214 B. From the baseband units, the output signal is taken back to the radio frequency units. To clarify the figure, the receiving radio frequency units  500 A and  514 A are drawn separate. 
       FIG. 5B  shows a situation where two radio frequency units  200 A and  214 A are used as signal sources and four radio frequency units  500 A,  506 A,  508 A and  514 A are used as signal receivers. In this example, baseband units are connected in a versatile manner between different transmitter/receiver radio units. This way, one and the same physical configuration provides versatile simulation environments in such a manner that the equipment can be efficiently utilized. 
     Even though the invention has been explained in the above with reference to examples in accordance with the accompanying drawings, it is apparent that the invention is not restricted to them but can be modified in many ways within the scope of the attached claims.