Multiplex communications – Communication over free space – Signaling for performing battery saving
Reexamination Certificate
1998-06-19
2001-10-30
Marcelo, Melvin (Department: 2663)
Multiplex communications
Communication over free space
Signaling for performing battery saving
C370S349000
Reexamination Certificate
active
06310865
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high-speed wireless access device, in particular, to a high-speed wireless access device, which reduces power consumption during the stand-by mode of wireless modems in a terminal in a wireless asynchronous transfer mode (ATM).
2. Description of the Related Art
There are several access systems like that illustrated in
FIG. 9
in which wireless terminal access to a hard-wired LAN or public phone network via base stations(BS). Typical TDMA(Time Division Multiple Access)/TDD(Time Division Duplex) systems thereof are exemplified by the configuration illustrated in
FIG. 10 and a
corresponding distributed system in FIG.
11
. In a TDMA/TDD system as exemplified by
FIG. 10
, every mobile terminal (MT) performs timing synchronization by using a preamble signal at the beginning of a frame added by a base station. Then a mobile terminal determines whether or not a packet addressed to that particular mobile terminal exists by checking a control packet following the preamble signal. If the packet signal to such a mobile terminal exists, this packet signal is loaded from any one of the packets (P
1
. . . Pk) following a control packet. In corresponding distributed systems exemplified by
FIG. 12
also signal data is loaded after determining that a received packet is addressed to a mobile terminal. However, there is a difference between the two systems, namely, a TDMA/TDD system requires for only one preamble signal to be added to the beginning of a downlink frame from a base station, while corresponding distributed systems should add respective preamble signals to the beginning of each of packets from and to a mobile terminal. Accordingly, although corresponding distributed systems allow an “ad hoc” communication between mobile terminals, effective data transmission throughput may be decreased depending on data length.
When multimedia services including such image signals as MPEG2 images are provided by access systems such as those as mentioned above, the data transmission rate in a wireless section should be not less than 20 M bps. This is due to existing overheads in regard to preamble signals and the transmission rights control. In addition, if mobile operations are to be considered, wireless transmission systems should allow high-speed multimedia transmission under a multipath fading condition. Accordingly, there are devised systems which prohibit receiving of long-delayed multipath signals by using sharp directional antennas, or equalizing systems which remove an intercede interference by multipath. Sharp directional antennas have an overwhelming advantage for communication between fixed stations, but if any one of the stations is a mobile terminal, complicated and troublesome antenna control is required, hence equalizing systems are currently considered to be advantageous. An appropriate equalizing system against multipath fading may be a system which uses a decision feedback equalizer (DFE) as a on-linear equalizer or a maximum likelihood system estimation type of equalizer, or their combination. However, each system has disadvantages of requiring large-scaled circuits and much power consumption. For example, under some conditions with a delay spread indicating a spreading state of multipath about 20 ns maximum, if high-speed transmission with a symbol rate of 25M symbols/sec is performed, an equalizer should have not less than 10 taps, which results in a large-scaled circuit.
FIG. 8
illustrates a block diagram of a DFE as an example equalizer. Further,
FIG. 13
illustrates an example of the structure of a preamble when using an equalizer. In
FIG. 8
, reference numeral
301
denotes a feed-forward type filter. Similarly,
302
denoting a feedback filter,
303
denoting a discriminator,
304
denoting a parameter estimator, and
305
and
306
denoting adders. Further in
FIG. 13
, there are four preamble signals, namely preamble signal
211
for automatic gain controller (AGC), preamble signal
212
for phase synchronization, preamble signal
213
for timing extraction, and preamble signal
214
for tap coefficient setting. Reference numeral
215
denotes a unique word (UW) in order to identify the beginning of data, and reference numeral
220
denotes data item.
In
FIG. 8
, feedback filter
302
is operated according to a symbol rate. If, however, feed-forward filter
302
is also operated at such symbol rate, there may be occurred aliases, which cause intercode interference against adjacent symbols of both sides. In this case, intercede interference to such adjacent symbols should be excluded through precise timing extraction by using preamble signal
213
for timing extraction as shown in FIG.
13
. However, since receiving signals are significantly distorted under multipath fading conditions, it is difficult to carry out precise timing extraction. In this connection, modems for hard-wired low-speed data communication commonly use a method for avoiding aliases in which a sampling is performed at a frequency double of a symbol rate (“double frequency sampling”). Samplings may be carried out at the same frequency as that of a symbol rate. In such a case it is sufficient to use half the number of taps of feed-forward filter
301
as compared with the double frequency sampling, thereby allowing circuit size to be minimized. However, sampling at the same frequency as the symbol rate as described above requires preamble signal
213
for timing extraction, and time of existence of such a preamble signal
213
correspondingly shortens data transmission rates, which results in less-effective data transmission throughput.
On the other hand, when sampling at a frequency double of a symbol rate is carried out, advantageously effective data transmission may be obtained, since preamble
213
, for timing extraction, is no longer required. However, large-scaled circuits should be prepared for feed-forward filter
301
, and much power consumption is still required in order to activate feed-forward filter
301
at a double clock frequency (for present example, for instance, at 50 MHz). As discussed above, an equalizer for high-speed transmission has disadvantages of large-scaled circuits and much power consumption. Accordingly, either the TDMA/TDD system or corresponding distributed system consumes much electric power it the equalizer is operated throughout the stand-by mode, and such a system is not suitable for wireless multimedia access services on the assumption that terminals are powered by battery. In addition, when an equalizer is used, preamble
214
, for tap coefficient setting, should be included in addition to preamble
211
for AGC,
212
for phase synchronization and
213
for timing extraction (as described above), thereby preamble length may become longer than that of a low-speed wireless modem. Accordingly, depending on the value of data length
220
, there may be a case that the data transmission becomes significantly less effective and only lower speed data service can be provided.
Wireless modems used for high-speed wireless access systems should satisfy two requirements, namely, providing lower power consumption and securing effective data transmission throughput. Therefore, in regard to wireless modems suitable for high-speed wireless transmission and consideration of mobile operation, there should be provided a means of low-speed demodulation to reduce power consumption during the stand-by mode, as well as high-speed modulation/demodulation including an equalizer to assist operability against multipath fading conditions.
The Japanese Patent Laid-Open Publication No. Hei 2-5633 discloses a means for reducing power consumption as illustrated herein in
FIGS. 14
,
15
and
16
. Shown in
FIG. 14
is a block diagram of a receiver, and in
FIGS. 15 and 16
structures of frames. The receiver has a low-speed clock generator
91
and a high-speed clock generator
92
, by which calling words are received at a low-speed clock during the stand-by mode.
FIG. 15
shows a state, in which a
Marcelo Melvin
McGuireWoods LLP
NEC Corporation
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