Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1999-02-19
2003-04-08
Nguyen, Duc (Department: 2682)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S203000, C370S329000, C370S478000, C455S552100
Reexamination Certificate
active
06545997
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a transmission method and a transmission apparatus for transmitting signals on the basis of an OFDM-TDMA-system and further to a receiving method and a receiving apparatus for receiving signals transmitted by means of such a transmission method.
BACKGROUND OF THE INVENTION
A transmission method and a transmission apparatus for transmitting signals on the basis of a OFDM/TDMA-system are explained relating to
FIGS. 1-4
of the present application. In such a transmission method and apparatus, a plurality of subcarriers
1
being orthogonal to each other can be allocated to a variable number of channels U
0
, U
1
. . . U
9
, each channel U
0
, U
1
. . . U
9
containing a variable number of subcarriers
1
depending on information to be transmitted as shown in
FIGS. 1-4
.
FIG. 1
shows a group of ten frequency channels U
0
, U
1
. . . U
9
. Each frequency channel U
0
, U
1
. . . U
9
can contain a variable number of subcarriers depending on information to be transmitted, as shown for the channels U
0
and U
1
in FIG.
2
. The channel U
0
contains a plurality of subcarriers
1
, and the channel U
1
contains a number of subcarriers
1
different from channel U
0
. In a transmission method and the transmission apparatus for transmitting signals on the basis of a OFDM/TDMA-system, a variable number of subcarriers
1
can be allocated to each channel depending on the amount of information to be transmitted. The channel U
0
shown in
FIG. 2
contains 21 subcarriers
1
, whereas the channel U
1
shown in
FIG. 2
contains only 10 subcarriers
1
. Therefore, the channel U
0
can be transmitted by more than twice the transmission rate of the channel U
0
. On the border of each channel U
0
, U
1
. . . U
9
, a single subcarrier having zero power is placed as guard band
2
to minimize interference to users placed in the adjacent frequency band or to fulfill certain spectrum masks. If the influence of an interference by the band in the neighborhood is small, the guard band
2
need not to be provided, whereas, when the influence is excessive, a plurality of guard bands
2
can be provided.
The subcarriers
1
are generated by orthogonal frequency division multiplex (OFDM) processing. As shown in
FIG. 3
, W(f) indicates a wave form indicating an energy on the frequency axis and B(Hz) indicates the distance between two adjacent subcarriers. The OFDM processing provides for a multi-subcarrier-system, wherein the number of channels which can be multiplexed is not limited by an interference from the other channels and can be freely determined depending on the bandwidth to be allocated. By changing the number of the subcarriers to be allocated to the different channels, it is possible to change the transmission rate or to achieve a variable transmission rate. The subcarriers between the respective channels can be easily separated by means of a filter, thereby making it possible to prevent deterioration of S/N characteristics. Since the OFDM processing is used for the multi-subcarrier modulation, a guard band S is not necessarily needed between different channels, thereby achieving a very high spectral efficiency. Further on, because fast Fourier transformation can be utilized, the necessary processing can be rapid and small.
Further on, the number of channels in each group of channels can be varied, as shown in FIG.
4
. In
FIG. 4
, a group of six channels U
0
, U
1
. . . U
5
is shown. In a OFDM/TDMA-system, the number of channels in a group of channels can be varied within the system frequency band depending on information to be transferred.
In the known and standardized GSM-System, a type of single carrier frequency modulation called GMSK is used. The frequency channels are constant and the spacing (bandwidth) between adjacent frequency channels is 200 kHz. The number of FDMA-channels is 124 and a time division multiple access (TDMA) is used to support the number of parallel connections. The TDMA scheme in the GSM-System is 8 GSM-timeslots within one time frame. The GSM-timeslot length is 576,9 &mgr;s (15/26 ms), as is shown in FIG.
5
. As can be seen in
FIG. 5
, the transmitted GSM-timeslots are not fully occupied by the transmitted burst to reduce interference from adjacent GSM-timeslots if the system is not perfectly synchronized. The guard period is 8,25 bits, which corresponds to 30,5 &mgr;s. The guard period is divided in two parts, wherein one of the parts is located at the beginning of the GSM time slot, and the other part is located at the end of the GSM-timeslot.
A GSM time frame consists of 8 GSM time slots and has therefore a length of 4615,4 &mgr;s, as is shown in FIG.
6
. The GSM-system supports slow frequency hopping, which is explained in FIG.
6
. The shown GSM-timeslot
3
is a receiving timeslot. According to the time division duplex (TDD)-system of the GSM-system, a corresponding transmission GSM-timeslot
4
is transmitted some timeslots later. Further on, the GSM-system makes use of the frequency division duplex (FDD)-system with 45 MHz between uplink and downlink, so that the transmission GSM-timeslot
4
is transmitted in the corresponding uplink frequency band, when the receiving GSM-timeslot
3
had been sent in the uplink frequency band, or vice versa. The next succeeding receiving GSM-timeslot
5
is of course transmitted in the same uplink or downlink frequency band as the preceding GSM-timeslot
3
, but in a different frequency channel, according to the slow frequency hopping. The frequency hopping improves, together with the interleaving procedure, the transmission of the signals in view of the frequency and interference diversity. The usual interleaving depth in the GSM-system is 36,923 ms corresponding to 8×8 GSM-timeslots.
When transmitting signals between a base station and one or more mobile stations, the mobile channel introduces multipath distortion on the signaling wave forms. Both the amplitude and phase are corrupted as the channel characteristics changes because of movements of the mobile station. In order to perform a coherent detection of the transmitted signals, reliable channel estimates are required. This can be obtained by occasionally transmitting known data or so-called pilot symbols. The corresponding receiving side interpolates the channel information derived from the pilot symbols to obtain a channel estimate for equalizing the received data signal. The pilot symbol is thereby known both by the transmitting and the receiving apparatus.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is therefore to provide a transmission method and a transmission apparatus for transmitting signals on the basis of an GSM compatible OFDM-TDMA-system and further a receiving method and a receiving apparatus for receiving such signals, which allow for a reliable channel estimation on the receiving side.
This object is achieved by a transmission method, a transmission apparatus, a receiving method and a receiving apparatus according to the claims.
Advantageous features of the present invention are defined in the respective subclaims.
The transmission method for transmitting signals on the basis of a OFDM/TDMA-system comprises the steps of
allocating a plurality of subcarriers being orthogonal to each other to a variable number of channels, each channel containing a variable number of subcarriers depending on information to be transmitted in said signals,
wherein, for the transmission of said signal in a GSM-system having a constant number of predetermined GSM-frequency channels and a constant number of predetermined GSM-timeslots being grouped in GSM-frames, a number of said subcarriers is allocated corresponding to the bandwidth of said GSM-frequency channels, so that a multiple of one resulting OFDM/TDMA-timeslots matches with one or a multiple of one GSM-timeslot,
wherein a pilot symbol is allocated to every n-th subcarrier, whereby n is an integer and n>1, and transmitting said signals.
The transmission apparatus for transmitting signals on the basis of a OFDM/T
Böhnke Ralf
Izumi Seiichi
Frommer William S.
Frommer & Lawrence & Haug LLP
Nguyen Duc
Simon Darren M.
Sony International (Europe) GmbH
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