Testing in an RF communication system

A test method in an RF communication system. In the communication system, at least one RF communication channel lies about a known carrier frequency in an RF communication band. According to the method, a diagnostic signal is introduced into a circuit to be tested, such as a linear power amplifier, the diagnostic signal having an RF diagnostic frequency lying within the communication band. The diagnostic signal is down converted to an intermediate frequency for diagnostic purposes so that DSP is simplified. The diagnostic frequency is selected based on the known carrier frequency of the communication channel so that the communication channel is not down converted as an image band with the diagnostic signal.

The present invention relates to testing in an RF communication system, in
 particular in a mobile communication network where a base station
 communicates with a plurality of mobile stations (for example mobile
 telephones) using a plurality of communication channels within a
 communication band.
 In the following, the present invention is discussed in the context of a
 transmitter at a base station which operates to transmit data to one or
 more mobile stations, each of which is equipped with appropriate receiving
 circuitry. It will be apparent however that the mobile stations are also
 equipped with transmission circuitry and that the base station is equipped
 with appropriate reception circuitry and that therefore the aspects of the
 invention discussed herein may be applied either in base stations or
 mobile stations as appropriate.
 In addition, the invention is discussed herein in the context of wide band
 code-division multiple access (W-CDMA) communication. According to W-CDMA
 communication, data to be transmitted is generated in the form of a
 modulation signal which is modulated onto a selected carrier frequency at
 the transmitter. The carrier frequency with its modulation forms a
 transmission channel of a predetermined bandwidth centred about the
 selected carrier frequency. A predetermined transmission band is allocated
 for each transmitter within which the carrier frequencies and consequent
 transmission channels have to lie. A controller within the transmitter
 governs selection of appropriate carrier frequencies for transmission. The
 bandwidth of the transmission band and the bandwidth of the modulation
 signal for each transmission channel is governed by appropriate Standards.
 According to one Standard, a transmission band has a bandwidth of 60 MHz
 lying between 2.11 GHz and 2.17 GHz and the bandwidth of the modulation
 signal is 5 MHz.
 There are many occasions when it is required to test transmission
 circuitry. One example would be a set-up or initialise phase of
 transmission circuitry required for processing the communication channels
 prior to transmission. Another example would be to monitor operation of
 the transmission circuitry during normal use. Yet a further example might
 be to diagnose faults in the transmission circuitry. For these and other
 similar test purposes, it is known to use a diagnostic signal which is
 introduced into the transmission circuitry, processed by the transmission
 circuitry and then further processed according to a diagnostic or test
 procedure.
 It is clearly advantageous if the diagnostic signal lies at a frequency
 which is close to the normal operational frequency of the transmission
 circuitry. Thus, it is advantageous if the diagnostic signal lies at a
 frequency within the chosen transmission band within which the transmitter
 operates. However, for RF frequencies at the level exemplified above, or
 more generally in the range for example of 400 MHz to 2.4 GHz, it is
 difficult to carry out diagnostic or test procedures on a diagnostic
 signal because the frequency is too high for normal digital signal
 processing equipment to operate.
 It is an object of the present invention to overcome these difficulties by
 allowing diagnostic or test procedures to be conducted at an intermediate
 frequency which is significantly lower than the RF frequencies utilised
 for transmission.
 One problem which arises in attempting to achieve this objective is that a
 diagnostic signal which lies at a frequency within the transmission band
 cannot simply be down converted to a lower intermediate frequency. The
 reason for this is described with reference to FIG. 1. FIG. 1 is a
 diagrammatic frequency chart with the break in the horizontal axis
 indicating a change in frequency range from a low frequency of the order
 of 100 KHz or a few MHz to a high frequency range of the order of several
 hundred MHz to GHz. CB represents a communication band and LO represents a
 local oscillator signal which is used to down convert the communication
 band CB. The frequency difference A between the local oscillator signal LO
 and the communication band CB is selectable according to the desired
 intermediate frequency after down conversion. However, existing down
 conversion circuits down convert not only the communication band but also
 a so-called image band IB which is located a similar frequency distance A
 to the other side of the local oscillator signal LO. Thus, as the arrows
 in FIG. 1 indicate, not only the desired communication band, but also an
 undesired image band are simultaneously down converted to form the down
 converted band DB at the lower intermediate frequency.
 Thus, if a diagnostic signal is placed within a communication band, an
 attempt to down convert it runs a risk of down converting a useful
 transmission channel which happens to lie in the image band during down
 conversion.
 According to one aspect of the present invention there is provided a test
 method in an RF communication system in which at least one RF
 communication channel lies about a known carrier frequency in an RF
 communication band, the method comprising:
 introducing into circuitry to be tested a diagnostic signal at an RF
 diagnostic frequency lying within the communication band, and
 down converting the diagnostic signal to an intermediate frequency for
 diagnostic purposes after processing by the circuitry to be tested,
 wherein the diagnostic frequency is selected based on the known carrier
 frequency of the at least one communication channel such that the
 communication channel is not down converted as an image band with the
 diagnostic signal.
 The method is particularly applicable when operated at a transmitter when
 the diagnostic signal is introduced into transmission circuitry. In the
 example described herein, the transmission circuitry comprises a linear
 power amplifier which processes the at least one communication channel
 prior to transmission. The invention is particularly useful in this
 context because the linearity of a linearised power amplifier is designed
 to be optimized around the operational communication band. In that
 context, the diagnostic signal is removed from the communication band
 prior to transmission.
 The test method can however also be operated at a receiver in a diagnostic
 environment wherein the diagnostic signal is received at receiving
 circuitry to be tested which also receives the at least one communication
 channel.
 In the context of a diagnostic signal for a linear power amplifier, it is
 likely that a single frequency or pure tone will be used.
 In the context of a diagnostic signal for receiving circuitry, it is likely
 that test data will be transmitted using an operational transmission
 channel of a predetermined bandwidth.
 According to another aspect, the invention provides a transmitter for an RF
 communication system comprising:
 transmission circuitry for processing communication channels within an RF
 transmission band prior to transmission;
 a controller for selecting the carrier frequency of at least one
 communication channel for transmission, the carrier frequency lying within
 an RF communication band, said controller also being operable to select a
 diagnostic frequency within the RF communication band for introducing a
 diagnostic signal at the diagnostic frequency into the transmission
 circuitry for test purposes; and
 means for down converting the diagnostic signal to an intermediate
 frequency for diagnostic purposes after processing by the transmission
 circuitry wherein the diagnostic frequency is selected based on the known
 carrier frequency of the at least one communication channel such that the
 communication channel is not down converted with the diagnostic signal.
 Preferably the controller also selects the frequency of a local oscillator
 signal used to down convert the diagnostic signal. This could however be
 selected by another part of the circuitry or could be preset. It is
 desirable however that it varies in dependence on the diagnostic frequency
 so that the downconverted intermediate frequency is fixed (being the
 difference between the diagnostic frequency and the local oscillator
 frequency).
 When the diagnostic signal is used for a linear power amplifier, the
 transmitter can include circuitry for cancelling the diagnostic signal
 prior to transmission.
 In a mobile communication environment, the transmitter includes means for
 modulating the selected carrier frequency with a modulation signal
 representing data to be transmitted. As already mentioned, the invention
 is particularly appropriate for use in a wide band CDMA environment.
 According to a further aspect of the invention there is provided a test
 method in an RF communication system in which at least two RF
 communication channels respectively lying about known carrier frequencies
 are present in an RF communication band, the method comprising:
 downconverting one of said RF communication channels to an intermediate
 frequency for diagnostic purposes using a downconverting signal, where the
 frequency of the downconverting signal is selected based on the known
 carrier frequencies of the communication channels such that the other
 communication channel is not downconverted with the downconverted
 communication channel as an image frequency.

The present invention will be described in the context of a mobile
 communication environment as illustrated diagrammatically in FIG. 2. A
 base station BTS can communicate with a plurality of mobile stations MS
 via RF communication channels CC. The base station and mobile stations
 transmit data using transmission channels based on preselected
 transmission carrier frequencies onto which data to be transmitted is
 modulated.
 FIG. 3 is a circuit diagram of the relevant circuitry within a transmitter
 which can be located either at the base station BTS or within each mobile
 station MS. The circuitry comprises a data generator 2 which generates a
 modulation signal fm having a predetermined bandwidth, for example 5 MHz,
 and which represents the data to be transmitted. A mixer 4 receives the
 modulation signal fm and a carrier frequency fc generated by a first
 oscillator 6. The first oscillator 6 is controlled by a controller 8. The
 controller 8 also controls a second oscillator 10 which generates a signal
 at a diagnostic frequency fp and a third oscillator 12 which generates a
 local oscillator signal LO. A linear power amplifier 14 receives the
 transmission channel fTX which comprises the carrier frequency fc and the
 modulation signal fm. The output of the amplifier 14 is supplied via a
 coupler 16 to a cancellation circuit 18 and finally to an antenna 20 for
 transmission.
 As is known in the art, suitable filters are included. The signal fp
 generated by the second oscillator 10 acts as a pilot tone for the linear
 power amplifier 14. Part of the output of the linear power amplifier 14 is
 extracted at the coupler 16 and supplied to a down conversion circuit 22
 which receives the local oscillator signal LO. The down conversion circuit
 22 uses the local oscillator signal LO to generate a diagnostic signal 24
 at a low intermediate frequency. The diagnostic signal 24 is supplied to a
 digital signal processor 26 for carrying out diagnostic procedures. The
 cancellation circuit 18 removes the pilot tone from the transmission
 signal fTX prior to transmission at the antenna 20.
 The controller receives carrier frequency data from a central system on
 line 28. As illustrated in FIG. 4, this data is used to control the
 frequency of the pilot tone by controlling the frequency of the
 oscillation signal generated by the second oscillator 10. The controller 8
 selects an appropriate carrier frequency fc for generating the
 transmission signal fTX in line with the predetermined set up for that
 particular base station or mobile station. In order to do this, the
 controller holds information about the bandwidth of the transmission band
 and the upper and lowermost frequencies of the transmission band, together
 with information about the transmission channels which are allowed to be
 used within that band. Control logic 28 analyses this information and
 determines an appropriate frequency for the pilot tone such that, when the
 pilot tone is down converted, no other transmission channel in use lies in
 the image band during down conversion. The appropriate selection made by
 the control logic 28 is supplied to a frequency synthesizer 30 which
 controls the second oscillator 10 accordingly.
 The frequency of the local oscillator signal LO may also be selected by the
 controller 8 through control of the third oscillator 12 in order to down
 convert the pilot signal to a predefined intermediate frequency. This has
 the advantage that it allows the intermediate frequency to be fixed so
 that filters such as filter 29 and other circuitry may be set up to
 operate at a predefined frequency. It also simplifies operation of the
 digital signal processor 24 to receive a signal at a fixed, predefined
 frequency.
 In the described example, selection of the frequencies is carried out by
 the controller 8 in line with the following:
 fc lies in the range 2110 MHz to 2170 MHz
 each transmission channel has a bandwidth of 5 MHz (fm=2.5 MHz)
 fp=fc.+-.(fm+fx), where fx lies for example in the range of 100 KHz to 2.5
 MHz for a W-CDMA system
 fLO=fp.+-.fy, where fy is chosen according to the processing capability of
 the DSP 26 lies for example in the range of 100 KHz to 1.25 MHz in a
 W-CDMA system
 FIG. 5 illustrates one example of selections of the frequencies fc, fp and
 fLO. In FIG. 5, two transmission channels fTX1 and fTX2 are illustrated,
 each having a bandwidth of 5 MHz and a carrier frequency fc. FR denotes
 the frequency range within which fp and fco are selected according to the
 above described criteria. The down converted diagnostic signal is
 designated 24 and is surrounded by a dotted line denoting the pass band of
 the baseband filter. It will be appreciated that the frequency of the down
 converted diagnostic signal 24 is the difference frequency fy used to
 generate the downconverting signal fLO for the downconversion circuit 22.
 Thus, in the described example, a linearised power amplifier 14 uses a
 narrow band pilot tone for calibration purposes. The transmit channel fTX
 can be any of a number of channels inside a wide frequency band (60 MHz as
 described in the example herein). The calibration tone is placed inside
 the frequency band for transmission, but cannot overlap the transmission
 channel itself. Thus, the location of the calibration tone is not fixed,
 but instead is placed at a frequency which is free from transmission and
 which has no transmit channel at its image frequency in down conversion,
 using the available information about the transmit frequency.
 Selection of the down converting oscillator frequency fLO can be used for
 another purpose. That is, the transmission channel fTX itself can be down
 converted after the linear power amplifier 14 for diagnostic purposes.
 Thus, consider the situation where it is needed to down convert a single
 transmit channel at an RF frequency (e.g. in the range 2.11.fwdarw.2.17
 GHz) of bandwidth 5 MHz into an intermediate frequency of 7.5 MHz for
 diagnostic procedures. In addition, consider the case where two transmit
 channels can be simultaneously present inside a sub band within the
 transmission band, the sub band having a width of 20 MHz and the
 transmission band having a width of 60 MHz. FIGS. 6a to 6d illustrate some
 of the possible combinations of the transmit channels and the frequency of
 the oscillator signal LO used for down conversion.
 In each case, the local oscillator signal LO is chosen in such a way that
 there is no other transmit channel at the image frequency of down
 conversion. As already described, the image frequency lies at the same
 frequency distance (.DELTA. in FIG. 1) on the other side of the local
 oscillator signal as the down converted channel. In each of FIGS. 6a to
 6d, the transmit channel to be down converted is shown shaded. The image
 frequency in each case is shown as a dotted line. In each case it can
 readily be seen that the image frequency does not overlap the other
 transmission channel which is present in each case. It will be noted that
 FIGS. 6a and 6b illustrate high injection for the down conversion circuit
 22. That is the local oscillator signal is at a higher frequency than the
 channel to be down converted. FIGS. 6c and 6d represent low injection,
 that is the local oscillator signal is at a frequency lower than that of
 the channel to be down converted. In each case, the image frequency lies
 on the other side of the local oscillator signal with respect to the
 channel to be down converted.
 Thus according to this use of the oscillator signal LO, knowledge of a well
 defined radio environment is used to implement simple and efficient
 downconversion by arranging the location of the downconverting local
 oscillator signal suitably. The frequency of the signal fLO involved in
 downconversion is controlled in such a way that the image rejection in
 downconversion is maximised without pre-filtering. The transmitter uses
 the information about the transmit frequencies which is necessary for
 determining transmission channels. This information is employed to
 determine where it is best to place the downconverting signal fLo in order
 to avoid converting down a strong frequency component, that is another
 transmit channel, as an image frequency.