Interpretation of a received signal

Details Interpretation of a received signal Interpretation of a received signal

1999-12-29

2003-06-17

Chin, Wellington (Department: 2665)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S252000, C370S347000, C375S267000, C375S347000, C455S132000

Reexamination Certificate

active

06580701

ABSTRACT:

BACKGROUND OF THE INVENTION
FIG. 1
shows transmission of a signal from a sender to a receiver in a telecommunication system. The information to be transmitted is conveyed over a transmission channel, such as a radio channel, modulated into a form suitable for the channel. Known methods of modulation include amplitude modulation, where the information is contained in the signal amplitude, frequency modulation, where the information is included in the signal frequency, and phase modulation, where the information is included in the signal phase. Non-ideal features of the transmission channel, such as signal reflections, noise and interference caused by other connections, cause changes in the signal containing the information, which is why the signal perceived by the receiver is never an exact copy of the signal sent by the sender. Information to be sent in digital systems can be made to better withstand non-ideal features of the transmission path with the aid of channel coding. At the receiving end the receiver will correct the received signal with a channel corrector based on channel characteristics which it knows and it will undo the modulation used on the transmission channel as well as the channel coding.
Besides attenuation of the amplitude, a sent signal will broaden on the transmission channel both at frequency level and time level. The information included in the signal by some modulation method will hereby also change. Broadening of the signal is caused especially in radio systems mainly by multipath propagation, which is shown in FIG.
2
. In the figure a signal is examined which travels from the base transceiver station BTS to a mobile station MS in a mobile station system. The signal travels from the base transceiver station along a straight route, the length of which is L
straight
. In addition, the mobile station perceives two beams, which are reflected from an obstacle and the route lengths of which are L
refl1
and L
refl2
respectively. The mobile station receives the signal conveyed by the reflected beam
1
after a delay &Dgr;T
1
=(L
refl1
−L
straight
) c and the signal conveyed by beam
2
after a delay &Dgr;T
2
=(L
refl2
−L
straigh
)/c later than the signal which propagated straight (c=speed of light). Thus, the receiver perceives the sent signal as three signals arriving at slightly different times and from different directions and summing up as one, which causes overlapping of symbols sent in succession, that is, Inter-Symbol Interference ISI.
Besides multipath propagation, inter-symbol interference is caused by the modulation methods used. E.g. in a Gaussian Minimum Shift Keying method (GMSK) used in a GSM system, changes between successive signals are smoothed to save the frequency band of the radio channel in such a way that the effect of an individual symbol will extend over the time of three symbol periods. Since the effect is on the signal phase, it will cause a non-linear component in the inter-symbol interference. The GMSK method is described more closely e.g. in the GSM 05.04 standard published by the ETSI (ETSI=European Telecommunications Standards Institute).
In order to correct changes caused by the channel, there must be sufficiently accurate knowledge of channel characteristics at the receiving end. Known channel estimation methods are the use of a Training Period TP and blind channel estimation. In blind channel estimation, an estimate of channel characteristics is maintained by defining from the received signal the statistically most likely transmitted signal. If the signal reconstructed from the received signal with the aid of estimated channel characteristics is not probable or even possible, the estimate of channel characteristics is changed.
In channel estimation methods using a training period, the idea is to include such a training period in the transmitted signal, the contents of which are known to the receiver. By comparing the received and distorted training period, which has travelled through the channel, with the training period which it knows and which was sent to the channel, the receiver will obtain information on channel characteristics. Based on the information obtained the receiver may correct any distortions caused by the channel also from such other transmitted information conveyed in other parts of the burst which the receiver does not know beforehand.
FIG. 3
shows how a training period is located in a burst for use in digital radio communication. In the figure the training period is located in the middle part of the burst, whereby the average distance of information bits from the training period is minimised. A first half-burst containing information to be transmitted is located before the training period, and a second half-burst containing information is located after the training period. In addition, at the ends of the burst there are also tails needed for perceiving the ends of the burst and a safety time used for preventing overlapping of successive bursts.
FIG. 4
shows the occurrence of interference caused to one another by simultaneous connections. In the figure, three mobile stations MS
1
, MS
2
and MS
3
communicate with base transceiver stations BTS
1
, BTS
2
and BTS
3
. The signal received by base transceiver station BTS
1
contains a signal S
1
sent by mobile station MS
1
and shown by a solid line, the strength of which depends on the transmission power used by mobile station MS
1
, on fading on the radio path between mobile station MS
1
and base transceiver station BTS
1
and on the antenna's sensitivity in the direction of arrival of the beam. Typically, radio path fading is smaller the closer the mobile station is located to the base transceiver station. Besides signal S
1
, the signal received by the base transceiver station contains signal components I
21
and I
31
resulting from signals sent by mobile stations MS
2
and MS
3
. The receiver perceives signals S
1
, I
21
and I
31
as a straight beam but also as several reflections coming from different directions, which are not however shown in the figure for the sake of simplicity. Components I
21
and I
31
will cause interference in the reception, unless they can be filtered away from the signal received from the base transceiver station. Correspondingly, the signal sent by mobile station MS
1
causes in the signals received by base transceiver stations BTS
2
and BTS
3
signal components I
12
and I
13
which may cause interference in receptions. Components of a similar kind will also occur in the signals received by the mobile stations from the base transceiver stations.
If signal components I
21
and I
31
are on the same channel as signal S
1
, they can not be removed by filtering. Also signals which are on some other channels than the same channel may cause interference. Since e.g. in systems using FDM frequency division such channels which are beside each other at the frequency level are always slightly overlapping due to an optimally efficient use of the frequency spectrum, interference will also be caused in the reception by signals on the adjacent channel. Similarly, when using CDM code division, connections using codes which resemble each other too much will cause interference to each other. However, so-called adjacent channel interference caused by signals on other channels are considerably smaller than the interference caused by equally powerful signals on the same channel.
Thus the magnitude of interference caused by connections to one another depends on the channels used by the connections, on the geographical location of the connections and on the transmission power used. These may be affected by such systematic channel allocation to different cells which takes interference into account, by transmission power control and by averaging of the interference experienced by the different connections.
Besides by the methods mentioned above, connection interference can be reduced by making use of the fact that the desired signal and the interfering signal typically arrive at the receiver from

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