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.