GB2394869A

GB2394869A - A time slot is segmented into a plurality of OFDM symbols, the guard interval of one of the symbols being different to the guard interval of another

Info

Publication number: GB2394869A
Application number: GB0229508A
Authority: GB
Inventors: Przemyslaw Jan Czerepinski
Original assignee: Siemens AG; Roke Manor Research Ltd
Current assignee: Siemens AG; Roke Manor Research Ltd
Priority date: 2002-10-31
Filing date: 2002-12-19
Publication date: 2004-05-05
Anticipated expiration: 2022-12-19
Also published as: GB0225313D0; GB2394869B; GB0229508D0

Abstract

A UTRAN compatible timeslot containing K samples is segmented into N OFDM symbols. Each of the symbols comprises a useful period of Ku useful samples and a guard period of Kg,i guard samples. Since the number of samples in each timeslot is not divisible by the number of symbols, the number of useful samples per symbol can only be kept constant if the number of guard samples per symbol is allowed to vary throughout the timeslot. The length of the guard period is allowed to vary subject to the constraint S i=0 <N-1> Kg,i =K - NKu.

Description

Multiple Carrier Radio-Communication System This invention relates to

telecommunication systems which use multi-carrier transmissions and has particular (but not exclusive) application to systems which 5 employ Orthogonal Frequency Division Multiplexing (OFDM) techniques.

In modern mobile communication systems, data is transmitted in discrete frames or time slots. These times slots will contain a number of transmitted symbols.

Communication systems which employ new techniques cannot be designed 10 unrestrictedly, but have to be integrated into existing systems; that is to say there are a number of parameters which are fixed and cannot practically be varied by the designer. Any new technique or associated structure thereof should be intelligently designed so that they map into existing structures of current systems. In particular, OFDM enhancements for UTRAN (Universal Terrestrial Radio Access Network) 15 have to be compatible with the UTRAN temporal structure. There are problems arising in segmenting a UTRAN-compatible transmission interval, T. into an integer number of OFDM symbols N. It is desirable that in performing such segmentation, the following requirements are met: a) the sampling clock frequency, also known as the chipping frequency, Fs, needs to 20 be retained and gives time interval of Ts = 11Fs b) the useful symbol duration Tu = K,,*Ts is such that KU is an integer power of two.

With the system chipping frequency, Fs' there is an integer number of samples K available for segmentation in the time interval T. where K = T*Fs and is not in general 25 divisible by N. The standard solution to the above problem is to compromise the sampling frequency Fs. A new sampling frequency, Fs,' is selected such that T*Fs' is divisible by N. Unfortunately this requires an extra clock, whose frequency is not simply related to 30 the original system clock, which increases the hardware complexity and the equipment costs.

It is an object of the invention to overcome this problem and provide a technique whereby new systems can be implemented using existing system structure in an effective and robust manner.

S The invention will be described by way of example and with reference to the following figures of which: Figures la and b show respectively representations of a time frame, and symbols which make up the time frame respectively.

Figure 2 shows an example of the invention showing flexible segmentation of a transmission interval into multicarrier symbols.

Figure 3 shows an example of the temporal structure of an OFDM mode within a 15 0.6671ls timeslot according to the example given.

Figure la shows a UTRAN temporal unit 1 of duration T. which is divided into a number of symbols 2. This is achieved in prior art by dividing T into N sub-units of

equal length T/N where N is the number of OFDM symbols. Each symbol is of fixed 20 duration and contains a guard period 3 of duration Tg and the useful symbol period 4 of duration Tu. The values of Tg and Tu are fixed; that is, they do not vary from one OFDM symbol to another.

Where the existing UTRAN temporal structure has to be divided into a number of 25 OFDM symbols, the problem arises in that the UTRAN-specific transmission interval is not necessarily divisible into an integer number of symbols given the existing sampling frequency.

The invention overcomes the problem by keeping the useful symbol period, Tu, fixed, 30 but allows the guard period Tg to be varied. In this way the total symbol duration is variable. This allows an integer number of OFDM symbols to fit perfectly into existing time slot structure (duration) of current systems.

General Example This will now be explained with reference to figure 2. The transmission interval T consists of K samples. The sampling rate is equal to Fs such that K = T*Fs is an integer. Also given is the number of multicarrier symbols N to be mapped onto the 5 transmission interval; K is not in general divisible by N. Every multicarrier symbol consists of a number of contiguous 'useful' samples, Ku, and a number of 'guard interval' samples, Kg. The guard interval samples may be contiguous, but may also be split into prefix samples, preceding the useful symbol samples and the postfix samples, following the useful symbol samples. Every multicarrier symbol contains the 10 same number of useful samples Ku, where NKU< K. The selection of KU depends on system design objectives and is outside the scope of this invention. For example, it may be chosen as an integer power of 2. The duration of the guard interval is made flexible. This enables a simple solution to the segmentation problem stated above, without a need for an additional sampling frequency.

An ith multicarrier symbol within the transmission interval is to comprise KU useful samples and Kg, guard interval samples, where i varies between O and N-1. With the value of Ku set, the task is now to allocate the remaining K-NKU samples to N guard intervals, one guard interval per multicarrier symbol. The number K-NKU may, but 20 does not have to be divisible by N. The essence of this invention is to allow a flexible split of guard interval durations Kg,i, subject to the constraint: N -I Kg, = K-NKU =0 An example flexible segmentation of a transmission interval into multicarrier symbols 25 is shown in figure 2 where K=62, N=6, KU=8' Kg,,3, Kg,=2, Kg 2=2, Kg,3=5, Kg,4=O, Kg,5=2. Specific Example We consider below the case where T is equal to the timeslot duration of 0. 667 ms for 30 maximum compatibility with the existing UTRAN temporal structure. However,

preserving specific timeslots in the OFDM mode is not the only alternative in the scope of the invention; it can be implemented with other possibilities such as T=2 me.

There are K= 5120 samples in a 0.667 ms transmission interval given the sampling rate of Fs = 7.68 MHz. Suppose that the transmission interval is to be segmented into 5 N= 9 OFDM symbols and that the number of useful samples per OFDM symbol is equal to KU = 512. There are 512 samples remaining (5120-9*512) to be allocated to the guard intervals Kg where i varies between 0 and 8. One example allocation is as follows: K J56 for i = 0 go" 157 for i=1..8 10 The resulting temporal structure is shown in figure 3. The oth symbol differs from the remaining symbols by a single guard interval sample. The guard interval may be split arbitrarily into prefix and postfix fields if required.

It would be clear to the skilled person that it would be advantageous for the control of 15 such systems that the variation was performed in a predetermined way. Additionally, in the example, the guard interval is varied by one, however the invention is not limited to such a variation.

Claims

1. In a multi-carrier transmission system having a sampling frequency Fs, a 5 method of inserting an integer number of symbols N into a transmission interval of fixed duration T. and having an integer number of samples K available for segmentation, and wherein said symbols comprise a guard period Tg and a useful symbol duration Tu, where said guard period varies within the transmission interval in pre-set way.

2. A method as claimed in claim 1 wherein K is not divisible by N.

3. A method as claimed in claims 1 or 2 wherein said variation is one sampling period duration.

4. A method as claimed in claims 1 to 3 wherein said symbols are OFDM symbols.

5. A method as claimed in claim 4 wherein said transmission interval is a 20 WCDMA-specific time slot.

6. A method as claimed in claim 4 wherein said transmission interval is an HSDPA TTI specific time slot.

25

7. A method as claimed in any preceding claim wherein Tu is an integer power of 2.