GB2538316A

GB2538316A - PRACH signal generation

Info

Publication number: GB2538316A
Application number: GB1508395.9A
Authority: GB
Inventors: Huang Li-Ke; Antoniou Solomos
Original assignee: Aeroflex Ltd
Current assignee: Aeroflex Ltd
Priority date: 2015-05-15
Filing date: 2015-05-15
Publication date: 2016-11-16
Anticipated expiration: 2035-05-15
Also published as: GB201508395D0; GB2538316B

Abstract

The present application relates to a method for generating a combined PRACH time-domain signal containing a plurality of Physical Random Access Channel (PRACH) preamble sequences in a system comprising a first Inverse Fast Fourier Transform (IFFT) unit and a second IFFT unit, the method comprising: generating a first set of PRACH preamble sequences, the first set of PRACH preamble sequences comprising a first plurality of PRACH preamble sequences; generating a second set of PRACH preamble sequences, the second set of PRACH preamble sequences comprising a second plurality of PRACH preamble sequences; converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and adding the respective frequency-domain signals to form a first composite frequency-domain signal; converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal and adding the respective frequency-domain signals to form a second composite frequency-domain signal; converting the first composite frequency-domain signal into a first composite time-domain signal using the first IFFT unit; converting the second composite frequency-domain signal into a second composite time-domain signal using the second IFFT unit; adding the first and second composite time-domain signals to form the combined PRACH time-domain signal containing the first plurality of PRACH preambles and the second plurality of PRACH preambles.

Description

PRACH SIGNAL GENERATION

Technical Field

The present application relates to a method and apparatus for generating a combined PRACH time-domain signal containing a plurality of Physical Random Access Channel (PRACH) preamble sequences.

Background to the Invention

In a wireless communication system in which multiple user devices communicate with a single base station or evolved Node B (eNodeB), the base station communicates with each of the individual user devices to instruct them as to the frequency channel and timeslot that each device is to use for its transmissions. This helps to ensure that there can be no collision or interference between transmissions from two or more individual user devices communicating with the base station since, in principle, each user device transmits in a different frequency channel and a different timeslot than the other user devices.

However, for physical random access channel (PRACH) signals sent to the base station by the user devices, the base station is not able to control the frequency channel and timeslot used by the user devices for their transmissions. Thus, multiple different PRACH signals can arrive simultaneously at the base station, each having a different phase, time delay and amplitude. This can cause problems or unexpected behaviour at the base station.

One difficulty that arises in developing base stations is that it can be difficult to simulate, during manufacture and testing, conditions or situations that will occur when the base station is in active service in the field. This may be due to shortcomings in test equipment that is available. For example, due to hardware limitations some existing test equipment is only able to generate and transmit two simultaneous PRACH signals, which has proved insufficient to simulate the situation described above, in which multiple (more than two) different PRACH signals are received simultaneously by a base station.

Figure 1 is a schematic representation of a prior art system for generating a time-domain signal containing two PRACH preambles. The system, shown generally at 10, includes an FPGA 12 which includes first and second inverse fast Fourier transform (IFFT) units 14, 16. The first and second 1FFT units 14, 16 receive at their inputs frequency-domain representations of respective first and second PRACH preamble sequences, which may be generated, for example, by respective first and second discrete Fourier transform (DFT) units 18, 20, and perform an IFFT on these, outputting respective first and second time-domain versions of the input frequency-domain PRACH preamble sequences.

The time-domain signals output by the first and second IFFT units 14, 16 are output to an adder 18, where they are combined to form a combined PRACH time-domain signal, containing two separate PRACHs.

As will be appreciated from the foregoing description, the combined PRACH time-domain signal generated by the prior art system of Figure 1 can contain only two PRACHs, due to the fact that the FPGA 12 contains only two IFFT units 14, 16.

The capacity of the system of Figure 1 to generate PRACHs simultaneously can be increased by increasing the number of IFFT units available. However, this approach increases the cost and complexity of the system, so is undesirable.

Accordingly, a need exists for a method and system for accurately and cost effectively generating multiple (more than two) simultaneous PRACH signals to be received simultaneously by a base station, to facilitate simulation and testing of the base station.

Summary of Invention

According to a first aspect of the present invention there is provided a method for generating a combined PRACH time-domain signal containing a plurality of Physical Random Access Channel (PRACH) preamble sequences in a system comprising a first Inverse Fast Fourier Transform (LEFT) unit and a second LEFT unit, the method comprising: generating a first set of PRACH preamble sequences, the first set of PRACH preamble sequences comprising a first plurality of PRACH preamble sequences; generating a second set of PRACH preamble sequences, the second set of PRACH preamble sequences comprising a second plurality of PRACH preamble sequences; converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and adding the respective frequency-domain signals to form a first composite frequency-domain signal; converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal and adding the respective frequency-domain signals to form a second composite frequency-domain signal; converting the first composite frequency-domain signal into a first composite time-domain signal using the first IFFT unit; converting the second composite frequency-domain signal into a second composite time-domain signal using the second IFFT unit; adding the first and second composite time-domain signals to form the combined PRACH time-domain signal containing the first plurality of PRACH preamble sequences and the second plurality of PRACH preamble sequences.

The method of the present invention facilitates the use of limited IFFT resources to generate, accurately and cost effectively, multiple simultaneous PRACH signals to be received simultaneously by a base station, to facilitate simulation and testing of the base station.

The method may further comprise performing a frequency conversion on the first composite time-domain signal by mixing the first composite time-domain signal with a first reference signal from a first numerically controlled oscillator and performing a frequency conversion on the second composite time-domain signal by mixing the second composite time-domain signal with a second reference signal from a second numerically controlled oscillator.

The frequency of the second reference signal may be different from the frequency of the first reference signal.

The method may further comprise performing interpolation on the first and second composite time-domain signals.

The method may further comprise adding the first and second composite time-domain signals with a time-domain non-PRACH signal.

The method may further comprise adjusting an amplitude, path delay and phase shift of each of the PRACH preamble sequences of the first and second set of PRACH preamble sequences.

A bandwidth of the first composite time-domain signal may be different from a bandwidth of the second composite time-domain signal.

Converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal may comprise performing a discrete Fourier transform (DFT) on each of the first plurality of PRACH preamble sequences and performing a DFT on each of the second plurality of PRACH preamble sequences.

Alternatively, converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal may comprise retrieving the respective frequency-domain signal for each of the first and second plurality of PRACH preamble sequences from a pre-populated look-up table.

An input of the first IFFT unit or the second IFFT unit may be shared on a time multiplexed basis between the respective first or second composite frequency-domain signal and a nonPRACH frequency domain signal.

According to a second aspect of the invention there is provided an apparatus for generating a combined PRACH time-domain signal containing a plurality of Physical Random Access Channel (PRACH) preamble sequences, the apparatus comprising: a first Inverse Fast Fourier Transform (IFFT) unit; a second IFFT unit, the method comprising: a first PRACH preamble sequence generator configured to generate a first set of PRACH preamble sequences, the first set of PRACH preamble sequences comprising a first plurality of PRACH preamble sequences; a second PRACH preamble sequence generator configured to generate a second set of PRACH preamble sequences, the second set of PRACH preamble sequences comprising a second plurality of PRACH preamble sequences; means for converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal; a first adder for adding the respective frequency-domain signals to form a first composite frequency-domain signal; means for converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal; a second adder for adding the respective frequency-domain signals to form a second composite frequency-domain signal; wherein the first IFFT unit is configured to convert the first composite frequency-domain signal into a first composite time-domain signal, and the second IFFT unit is configured to convert the second composite frequency-domain signal into a second composite time-domain signal, the apparatus further comprising a third adder for adding the first and second composite time-domain signals to form the combined PRACH time-domain signal containing the first plurality of PRACH preamble sequences and the second plurality of PRACH preamble sequences.

The apparatus may further comprise a first mixer for performing a frequency conversion on the first composite time-domain signal by mixing the first composite time-domain signal with a first reference signal from a first numerically controlled oscillator and a second mixer for performing a frequency conversion on the second composite time-domain signal by mixing the second composite time-domain signal with a second reference signal from a second numerically controlled oscillator.

The apparatus may further comprise a first interpolator for performing interpolation on the first composite time-domain signal and a second interpolator for performing interpolation on the second composite time-domain signal The apparatus may further comprise a fourth adder for adding the first and second composite time-domain signals with a time-domain non-PRACH signal.

The apparatus may further comprise a plurality of multipliers for adjusting an amplitude, path delay and phase shift of each of the PRACH preamble sequences of the first and second set of PRACH preamble sequences.

The means for converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal may comprise a first discrete Fourier transform (DFT) unit for performing a DFT on each of the first plurality of PRACH preamble sequences and a second DFT unit for performing a DFT on each of the second plurality of PRACH preamble sequences.

Alternatively, the means for converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal may comprise a pre-populated look-up table from which can be retrieved the respective frequency-domain signal for each of the first and second plurality of PRACH preamble sequences.

The apparatus may further comprise a multiplexer for time multiplexing an input of the first IFFT unit or the second IFFT unit between the respective first or second composite frequency-domain signal and a non-PRACH frequency domain signal.

According to a third aspect of the invention there is provided a method for generating a combined time-domain signal containing a plurality of Physical Random Access Channel (PRACH) preamble sequences and a non-PRACH signal in a system comprising an inverse fast Fourier transform (IFFT) unit, the method comprising: generating a first set of PRA CH preamble sequences, the first set of PRACH preamble sequences comprising a first plurality of PRACH preamble sequences; generating a non-PRACH frequency domain signal; converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and adding the respective frequency-domain signals to form a first composite frequency-domain signal; on a time multiplexed basis: converting the first composite frequency-domain signal into a first composite time-domain signal using the IFFT unit; and converting the non-PRACH frequency domain signal into a nonPRACH time-domain signal; and adding the first composite time-domain signal to the non-PRACH time-domain signal to form the combined PRACH time-domain signal containing the plurality of PRACH preamble sequences and the non-PRACH signal.

According to a fourth aspect of the invention there is provided apparatus for generating a combined time-domain signal containing a plurality of Physical Random Access Channel (PRACH) preamble sequences and a non-PRACH signal, the apparatus comprising: an inverse fast Fourier transform (IFFT) unit; a first PRACH preamble sequence generator for generating a first set of PRACH preamble sequences, the first set of PRACH preamble sequences comprising a first plurality of PRACH preamble sequences; a non-PRACH signal generator for generating a non-PRACH frequency domain signal; means for converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and adding the respective frequency-domain signals to form a first composite frequency-domain signal; a multiplexer for multiplexing an input of the 1FFT unit, wherein the IFFT unit is configured to, on a time multiplexed basis: convert the first composite frequency-domain signal into a first composite time-domain signal using the IFFT unit; and convert the non-PRACH frequency domain signal into a non-PRACH time-domain signal; the apparatus further comprising an adder for adding the first composite time-domain signal to the non-PRACH time-domain signal to form the combined PRACH time-domain signal containing the plurality of PRACH preamble sequences and the non-PRACH signal.

Brief Description of the Drawings

Embodiments of the invention will now be described, strictly by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a schematic representation of a prior art system for generating a time-domain signal containing two PRACH preambles; Figure 2 is a schematic functional block diagram of a system for generating a time-domain signal containing multiple PRACH preambles; Figure 3 is a schematic functional block diagram of an alternative system for generating a time-domain signal containing multiple PRACH preambles; Figure 4 is a schematic functional block diagram of a further alternative system for generating a time-domain signal containing multiple PRACH preambles; and Figure 5 is a schematic functional block diagram of a further alternative system for generating a time-domain signal containing multiple PRACH preambles.

Description of the Embodiments

Referring now to Figure 2, a system for generating a time-domain signal containing two or more PRACH preambles is shown generally at 50. In the system 50 illustrated in Figure 2 up to 32 PRACH preambles can be contained within an output time-domain signal, but it will be appreciated that the system and method described herein can be used to generate a time-domain signal containing more or fewer than 32 PRACH preambles.

As in the system illustrated in Figure 1, the system 50 of Figure 2 includes a signal processing element 52, which in the exemplary embodiment illustrated is an FPGA (Field Programmable Gate Array), but which could equally be an ASIC (Application Specific Integrated Circuit), DSP (Digital Signal Processor) or other signal processing element. The signal processing element 52 includes first and second inverse fast Fourier transform (IFFT) units, labelled in Figure 2 as 54 and 56 respectively. The first and second IFFT units 54, 56 receive at their inputs frequency-domain signals output by first and second adders 58, 60, and output first and second time-domain signals, which may be centred around different frequencies, depending upon the centre frequencies of the input frequency-domain signals.

The system 50 includes a first plurality (in this example 16) of DFT units 621 -6216, each of which receives at its input a different PRACH preamble sequence, and outputs a frequency-domain representation of the input PRACH preamble sequence. An output of each of the first plurality of DFT units 621 -6216 is connected to the first adder 58, which is configured to add the plurality (again, in this example 16) of frequency-domain representations of the input PRACH preamble sequences, and to output a composite signal containing all of the different frequency-domain representations of the input PRACH preamble sequences to the first TFFT unit 54.

The system 50 further includes a second plurality (in this example 16) of DFT units 6217 62s2, each of which receives at its input a different PRACH preamble sequence, and outputs a frequency-domain representation of the input PRACH preamble sequence. An output of each of the second plurality of DFT units 6217 -6232 is connected to the second adder 60, which is configured to add the plurality (again, in this example 16) of frequency-domain representations of the input PRACH preamble sequences, and to output a composite signal containing all of the different frequency-domain representations of the input PRACH preamble sequences to the second [EFT unit 56.

The time-domain signals output by the first and second 1FFT units 54, 56 each undergo an upconversion and interpolation process, as illustrated schematically in Figure 2 by respective processing blocks 64 and 66 of the signal processing element 52. One of the reasons for performing interpolation is to ensure that the sample rates of the signals output by the respective processing blocks 64, 66 (that are subsequently combined by an adder 76, as will be explained below) match. The parameters applied in the processing blocks 64, 66 affect the bandwidth of the signals output by the processing blocks 64 and 66. For example, the PRACH signal may have a bandwidth 20 times smaller than the combined signal output by the adder 76, and could therefore be processed at a proportionally lower sample rate. Thus, it will be appreciated that the signals output by the processing blocks 64 and 66 may have different bandwidths.

Following upconversion and interpolation, the time-domain signals output by processing blocks 64, 66 undergo frequency conversion, by being mixed with reference signals generated by respective first and second numerically controlled oscillators (NCO) 68, 70. The first NCO 68 receives a signal at a user-selectable reference frequency fRAJ, whilst the second NCO 70 receives a signal at a user-selectable reference frequency fRA. 2. b this way the centre frequency of the upconverted and interpolated signals transmitted can be selected by the user. The reference frequency fRA,1 and the reference frequency fRA,2 may be the same frequency, or may be different frequencies.

The frequency converted signals output by the first and second NCOs 68 and 70 are input to first and second delay units 72, 74, where a user-selectable delay Tl, T2 can be introduced. The first and second delay units 72, 74 output delayed time-domain signals to an adder 76, where they are combined to form a combined PRACH time-domain signal, containing 32 separate PRACHs.

Figure 3 is a schematic representation of an alternative system for generating a time-domain signal containing two or more PRACH preambles. This modified system, shown generally at 80, is based on the system 50 of Figure 2, and thus the references numerals used in Figure 2 are re-used in Figure 3 for those elements that are common to both the system 80 of Figure 3 and the system 50 of Figure 2, and, for the sake of clarity and brevity, those elements will not be described again here.

As will be appreciated from Figure 3, the system 80 differs from the system 50 of Figure 2 only in that the system 80 includes a further adder 82, which is configured to receive at a first input the combined PRACH time-domain signal output by the adder 72 and at a second input a non-PRACH time-domain signal containing data and control channels that are required to simulate multiple UEs, and to output a combined time-domain signal, which can then be transmitted by a common transmitter. In this example, the upconversion blocks 64, 66 must match the sample rate of the PRACH signals with that of the non-PRACH signal, to ensure that the combined time-domain output by the adder 82 can be transmitted correctly. In the example of LTE, the typical bandwidth of the combined signal is 1.4 to 2.0 MHz.

Figure 4 is a schematic representation of a further alternative system for generating a time-domain signal containing two or more PRACH preambles. This modified system, shown generally at 90, is based on the system 50 of Figure 2, which has been generalised to refer to M input time-domain PRACH preambles. The reference numerals used in Figure 2 are re-used in Figure 4 for those elements that are common to both the system 90 of Figure 4 and the system 50 of Figure 2, and, for the sake of clarity and brevity, those elements will not be described again here.

As can be seen from Figure 4, the system 90 differs from the system 50 in that it includes a plurality (M) of DFT units 621 -62M, which are divided into a first plurality of DFT units 621 -62,, and a second plurality of DFT units 62n+i -62M. The system 90 also includes a plurality (M) of multiplication units 921 -92M, which each receive, at a first input thereof, the output of one of the respective DFT units 621 -62M. Each of the plurality of multiplication units 921 -9232 receives, at a second input thereof, a respective multiplier signal of the form Anerii+pii. For example, the multiplication unit 921 receives at its second input a multiplier signal AI eln+(pi. The purpose of the multiplier signals is to permit independent variation of the amplitude, path delay and phase of each PRACH preamble, so as to permit simulation of LT mobility, i.e., different UEs at different locations each transmitting a PRACH signal. Thus, the amplitude of a PRACH preamble can be adjusted by adjusting the value A of the multiplier signal input to the appropriate one of the multiplication units 921 -92M, whilst the path delay can be adjusted by adjusting the value T and the phase shift can be adjusted by adjusting the value (p. Thus, in the system 90 shown in Figure 4, up to M frequency-domain PRACH preamble sequences, each with its own amplitude, path delay and phase shift, can be combined into a composite frequency-domain signal before being converted by the processing units of the signal processing element 52 into a composite time-domain signal containing all of the input PRACH preamble sequences.

Figure 5 is schematic representation of a further alternative system for generating a time-domain signal containing two or more PRACH preambles. This modified system, shown generally at 100, is similar to the system 50 of Figure 2, and thus the reference numerals used in Figure 2 are re-used in Figure 5 for those elements that are common to both the system 100 of Figure 5 and the system 50 of Figure 2, and, for the sake of clarity and brevity, those elements will not be described again here.

As can be seen from Figure 5, the system 100 differs from the system 50 of Figure 2 in that it includes only a single IFFT unit 102, which is shared on a time-multiplexed basis between a first plurality of input frequency-domain PRACH sequences, a second plurality of input frequency-domain PRACH sequences and an input frequency-domain nonPRACH signal containing data and control channels other than PRACH signals that are required to simulate multiple UEs, as will be described in detail below. This sharing is possible since the IFFT unit 102 can operate at a higher frequency than the required output sample rate, meaning that there is "spare" capacity in the LEFT block 102 to accommodate the input frequency-domain PRACH signals as well as the input frequency-domain PRACH signal. Because the input frequency-domain MACH signals have a lower bandwidth than the output sample rate, they can be processed by the IFFT unit 102 with a lower sample rate.

To facilitate this time-multiplexed sharing of the IFFT unit 102, the system 100 includes a first multiplexer 104, which selectively transmits signals from the first plurality of input frequency-domain PRACH sequences, the second plurality of input frequency-domain PRACH sequences and the input frequency-domain non-PRACH signal to the IFFT unit 102, and a second multiplexer 106, which selectively transmits signals from the IFFT unit 104 for downstream processing. In order to ensure a continuous stream of samples, a first buffer 108 stores samples of the frequency-domain non-PRACH signal for transmission to the IFFT unit 102, whilst a second buffer 110 stores samples of a time-domain non-PRACH signal output by the IFFT unity 102 for onward transmission to an adder 112, where they are added to the combined time-domain signal output by adder 72, as described above, for transmission by a common transmitter.

Although in the example described above with reference to Figure 5 there is only a single IFFT unit 102, it will be appreciated that two (or more) IFFT units could be shared on a time multiplexed basis between input PRACH and non-PRACH signals in a similar manner. For example, the first and second IFFT units 54, 56 of the system 80 illustrated in Figure 3 could be shared on a time multiplexed basis with the non-PRACH signal shown in Figure 3, using multiplexers similar to the first and second multiplexers 104, 106.

Moreover, it is to be appreciated that the features of the systems described above and illustrated in Figures 2 -5 and disclosed above are not mutually exclusive. Instead, any combination of the features described above and illustrated in Figures 2 -5 could be implemented by those skilled in the relevant art. For example, the time-multiplexed IFFT unit 102 of Figure 5 could be combined with the multiplication units 921 -92m of Figure 4 and with the adder 82 and non-PRACH signal of Figure 3.

In the examples described above and illustrated in Figures 2 to 5, the DFT units 621 to 6232 are illustrated as distinct units which calculate the DFTs of each of the plurality of input time-domain PRACH preamble sequences. However, it is to be appreciated that these distinct DFT units need not be present, but instead the DFTs of each of the plurality of input time-domain PRACH preamble sequences may be retrieved from one or more pre-populated look-up tables containing the DFTs of all possible Zadoff-Chu sequences.

Claims

CLAIMS1. A method for generating a combined PRACH time-domain signal containing a plurality of Physical Random Access Channel (PRACH) preamble sequences in a system comprising a first Inverse Fast Fourier Transform (IFFT) unit and a second IFFT unit, the method comprising: generating a first set of PRACH preamble sequences, the first set of PRACH preamble sequences comprising a first plurality of PRACH preamble sequences; generating a second set of PRACH preamble sequences, the second set of PRACH preamble sequences comprising a second plurality of PRACH preamble sequences; converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and adding the respective frequency-domain signals to form a first composite frequency-domain signal; converting each of the second plurality of PEACH preamble sequences into a respective frequency-domain signal and adding the respective frequency-domain signals to form a second composite frequency-domain signal; converting the first composite frequency-domain signal into a first composite time-domain signal using the first IFFT unit; converting the second composite frequency-domain signal into a second composite time-domain signal using the second IFFT unit; adding the first and second composite time-domain signals to form the combined PEACH time-domain signal containing the first plurality of PEACH preamble sequences and the second plurality of PEACH preamble sequences.
2. A method according to claim 1 further comprising performing a frequency conversion on the first composite time-domain signal by mixing the first composite time-domain signal with a first reference signal from a first numerically controlled oscillator and performing a frequency conversion on the second composite time-domain signal by mixing the second composite time-domain signal with a second reference signal from a second numerically controlled oscillator.
3. A method according to claim 2 wherein the frequency of the second reference signal is different from the frequency of the first reference signal.
4. A method according to any one of the preceding claims further comprising performing interpolation on the first and second composite time-domain signals.
5. A method according to any one of the preceding claims further comprising adding the first and second composite time-domain signals with a time-domain non-PRACH signal.
6. A method according to any one of the preceding claims further comprising adjusting an amplitude, path delay and phase shift of each of the PRACH preamble sequences of the first and second set of PRACH preamble sequences.
7. A method according to any one of the preceding claims wherein a bandwidth of the first composite time-domain signal is different from a bandwidth of the second composite time-domain signal.
8. A method according to any one of the preceding claims wherein converting each of the first plurality of PRA CH preamble sequences into a respective frequency-domain signal and converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal comprises performing a discrete Fourier transform (DFT) on each of the first plurality of PRACH preamble sequences and performing a DFT on each of the second plurality of PRACH preamble sequences.
9. A method according to any one of claims 1 to 7 wherein converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and converting each of the second plurality of PRA CH preamble sequences into a respective frequency-domain signal comprises retrieving the respective frequency-domain signal for each of the first and second plurality of PRA CH preamble sequences from a pre-populated look-up table.
10. A method according to any one of the preceding claims wherein an input of the first IFFT unit or the second IFFT unit is shared on a time multiplexed basis between the respective first or second composite frequency-domain signal and a non-PRACH frequency domain signal.
11. An apparatus for generating a combined PRACH time-domain signal containing a plurality of Physical Random Access Channel (PRACH) preamble sequences, the apparatus comprising: a first Inverse Fast Fourier Transform (IFFT) unit; a second IFFT unit, the method comprising: a first MACH preamble sequence generator configured to generate a first set of PRACH preamble sequences, the first set of PRACH preamble sequences comprising a first plurality of PRACH preamble sequences; a second PRACH preamble sequence generator configured to generate a second set of PRACH preamble sequences, the second set of PRACH preamble sequences comprising a second plurality of PRACH preamble sequences; means for converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal; a first adder for adding the respective frequency-domain signals to form a first composite frequency-domain signal; means for converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal; a second adder for adding the respective frequency-domain signals to form a second composite frequency-domain signal; wherein the first IFFT unit is configured to convert the first composite frequency-domain signal into a first composite time-domain signal, and the second IFFT unit is configured to convert the second composite frequency-domain signal into a second composite time-domain signal, the apparatus further comprising a third adder for adding the first and second composite time-domain signals to form the combined PRACH time-domain signal containing the first plurality of PRACH preamble sequences and the second plurality of PRACH preamble sequences.
12. An apparatus according to claim 11 further comprising a first mixer for performing a frequency conversion on the first composite time-domain signal by mixing the first composite time-domain signal with a first reference signal from a first numerically controlled oscillator and a second mixer for performing a frequency conversion on the second composite time-domain signal by mixing the second composite time-domain signal with a second reference signal from a second numerically controlled oscillator.
13. An apparatus according to claim 12 wherein the frequency of the second reference signal is different from the frequency of the first reference signal.
14. An apparatus according to any one of claims 11 to 13 further comprising a first interpolator for performing interpolation on the first composite time-domain signal and a second interpolator for performing interpolation on the second composite time-domain signal.
15. An apparatus according to any one of claims 11-14 further comprising a fourth adder for adding the first and second composite time-domain signals with a time-domain non-PRACH signal.
16. An apparatus according to any one of claims 11 -15 further comprising a plurality of multipliers for adjusting an amplitude, path delay and phase shift of each of the PRACH preamble sequences of the first and second set of PRACH preamble sequences.
17. An apparatus according to any one of claims 11-16 wherein a bandwidth of the first composite time-domain signal is different from a bandwidth of the second composite time-domain signal.
18. An apparatus according to any one of claims 11-17 wherein We means for converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal comprises a first discrete Fourier transform (DFT) unit for performing a DFT on each of the first plurality of PRACH preamble sequences and a second DFT unit for performing a DFT on each of the second plurality of PRACH preamble sequences.
19. An apparatus according to any one of claims 11 to 17 wherein the means for converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and converting each of the second plurality of PRACH preamble sequences into a respective frequency-domain signal comprises a pre-populated look-up table from which can be retrieved the respective frequency-domain signal for each of the first and second plurality of PRACH preamble sequences.
20. An apparatus according to any one of claims I 1-19 further comprising a multiplexer for time multiplexing an input of the first IFFT unit or the second IFFT unit between the respective first or second composite frequency-domain signal and a non-PRACH frequency domain signal.
21. A method for generating a combined time-domain signal containing a plurality of Physical Random Access Channel (PRACH) preamble sequences and a non-PRACH signal in a system comprising an inverse fast Fourier transform (IFFT) unit, the method comprising: generating a first set of PRACH preamble sequences, the first set of PRACH preamble sequences comprising a first plurality of PRACH preamble sequences; generating a non-PRACH frequency domain signal; converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and adding the respective frequency-domain signals to form a first composite frequency-domain signal; on a time multiplexed basis: converting the first composite frequency-domain signal into a first composite time-domain signal using the IFFT unit; and converting the non-PRACH frequency domain signal into a non-PRACH time-domain signal; and adding the first composite time-domain signal to the non-PRACH time-domain signal to form the combined PRACH time-domain signal containing the plurality of PRACH preamble sequences and the non-PRACH signal.
22. An apparatus for generating a combined time-domain signal containing a plurality of Physical Random Access Channel (PRACH) preamble sequences and a non-PRACH signal, the apparatus comprising: an inverse fast Fourier transform (IFFT) unit; a first PRACH preamble sequence generator for generating a first set of PRACH preamble sequences, the first set of PRACH preamble sequences comprising a first plurality of PRACH preamble sequences; a non-PRACH signal generator for generating a non-PRACH frequency domain signal; means for converting each of the first plurality of PRACH preamble sequences into a respective frequency-domain signal and adding the respective frequency-domain signals to form a first composite frequency-domain signal; a multiplexer for multiplexing an input of the IFFT unit, wherein the IFFT unit is configured to, on a time multiplexed basis: convert the first composite frequency-domain signal into a first composite time-domain signal using the IFFT unit; and convert the non-PRACH frequency domain signal into a non-PRACH time-domain signal; the apparatus further comprising an adder for adding the first composite time-domain signal to the non-PRACH time-domain signal to form the combined PRACH time-domain signal containing the plurality of PRACH preamble sequences and die nonPRACH signal.