Transmit diversity method and system

Details Transmit diversity method and system Transmit diversity method and system

2001-11-06

2003-06-24

Chin, Stephen (Department: 2634)

Pulse or digital communications

Transmitters

Plural diversity

C375S260000

Reexamination Certificate

active

06584161

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a transmit diversity method and system for a wireless communication system, such as the Universal Mobile Telecommunications System (UMTS) comprising a transmitting element and at least one receiver.
BACKGROUND OF THE INVENTION
Wideband Code Division Multiple Access (WCDMA) has been chosen as the radio technology for the paired bands of the UMTS. Consequently, WCDMA is the common radio technology standard for third-generation wide-area mobile communications. WCDMA has been designed for high-speed data services and, more particularly, Internet-based packet-data offering up to 2 Mbps in indoor environments and over 384 kbps for wide-area.
The WCDMA concept is based on a new channel structure for all layers built on technologies such as packet-data channels and service multiplexing. The new concept also includes pilot symbols and a time-slotted structure which has led to the provision of adaptive antenna arrays which direct antenna beams at users to provide maximum range and minimum interference. This is also crucial when implementing wideband technology where limited radio spectrum is available.
The uplink capacity of the proposed WCDMA systems can be enhanced by various techniques including multi-antenna reception and multi-user detection or interference cancellation. Techniques that increase the downlink capacity have not been developed with the same intensity. However, the capacity demand imposed by the projected data services (e.g. Internet) burdens more heavily the downlink channel. Hence, it is important to find techniques that improve the capacity of the downlink channel.
Bearing in mind the strict complexity requirements of terminals, and the characteristics of the downlink channel, the provision of multiple receive antennas is not a desired solution to the downlink capacity problem. Therefore, alternative solutions have been proposed suggesting that multiple antennas or transmit diversity at the base station will increase downlink capacity with only minor increase of complexity in terminal implementation.
According to the WCDMA system, a transmit diversity concept is under consideration which is mainly focused on the closed-loop (feedback) mode.
FIG. 1
shows an example of such a feedback mode for a downlink transmission between a base station (BS)
10
and a mobile terminal or mobile station (MS)
20
. In particular, the BS
10
comprises two antennas A
1
and A
2
, and the MS
20
is arranged to estimate the channel on the basis of two transmission signals received from the two antennas A
1
and A
2
. Then, the MS
20
feeds back the discretized channel estimate to the BS. Naturally, it is desired to develope a robust and low-delay feedback signaling concept.
In WCDMA, three modes are suggested for the closed-loop concept which is optimized for two antennas. In the feedback (FB) mode
1
(also referred to as Selective Transmit Diversity (STD)), one bit per time slot is used to signal the “best” antenna from each terminal. The remaining closed-loop FB modes
2
and
3
provide a slower feedback link, where feedback weights used for controlling the antennas A
1
and A
2
are modified after two or four 0.625 ms slots, respectively. In this case, the antennas A
1
and A
2
are co-phased so that transmitted signals sum up coherently in the MS
20
, to thereby provide the best performance with low mobility “low multipath” environments.
FIG. 2
shows a table indicating characteristic parameters of the above FB modes
1
to
3
. In particular, N
FB
designates the number of feedback bits per time slot, N
W
the number of bits per feedback signaling word, Na the number of feedback bits for controlling an amplification or power at the antennas A
1
and A
2
, and Np the number of feedback bits for controlling a phase difference between the antennas A
1
and A
2
. As can be gathered from the table of
FIG. 2
, one bit is fed back per time slot in each of the FB modes
1
to
3
.
In the FB mode
1
(i.e. STD), the bit length of the feedback signaling word is one bit, which leads to an update rate of 1600/s (i.e. an update is performed at the BS
10
in every time slot). The feedback bit rate is 1600 bps and the feedback signaling word is used for controlling the power supplied to the antennas A
1
and A
2
.
In the FB mode
2
, the feedback signaling word comprises two bits, which leads to an update rate of 800/s, since an update is performed after both feedback bits have been received, i.e. after two time slots. The feedback-signaling word is only used for controlling the phase difference between the two antennas A
1
and A
2
.
In the FB mode
3
, the bit length of the feedback signaling word is four, such that an update rate of 400/s is obtained, i.e. an update is performed every four time slots. In particular, one bit of the feedback signaling word is used for controlling the amplification (power) at the antennas A
1
and A
2
, and three bits are used for controlling their phase difference.
FIG. 3A
shows a table indicating the feedback power control performed in the FB mode
1
or STD. Here, the MS
20
has to estimate the antenna with the smallest path loss. To this effect, the MS
20
estimates the channel power of all “competing antennas”, and determines the one with the highest power. The required channel estimates are obtained e.g. from a common pilot channel transmitted with a known power from each antenna. The table in
FIG. 3A
shows the relationship between the feedback value and the power P
A1
supplied to the antenna A
1
and the power P
A2
supplied to the antenna A
2
. Accordingly, one of the two antennas A
1
and A
2
is selected at the BS
10
in response to the feedback signaling value.
It is to be noted that the FB mode
1
may be implemented in an analog manner in the beam domain. In this case, the MS
20
signals to the BS
10
whether to rotate channel symbols transmitted from the antenna A
2
by 180°. In this case, the BS
10
transmits simultaneously from both antennas A
1
and A
2
. Thus, the phase difference between the antennas A
1
and A
2
is switched between 0° and 180° in response to the feedback value.
The other FB modes
2
and
3
relate to a feedback concept referred to as Transmission Antenna Array (T×AA), in which the MS
20
transmits estimated and quantized channel parameters to the BS
10
which then weights the transmitted signals accordingly.
FIG. 3B
shows the feedback control performed in the FB mode
2
. In the FB mode
2
, only a phase weight feedback value comprising two bits is fed back to the BS
10
. The phase difference indicated in the table of
FIG. 3B
defines the phase difference (in degree) between the antennas A
1
and A
2
, which is to be established by the BS
10
in order to obtain an optimum coherence at the MS
20
.
FIG. 3C
shows the feedback control of the FB mode
3
, wherein one bit, i.e. amplification bit, of the feedback signaling word is used for controlling the power of the antennas A
1
and A
2
, and the other three bits, i.e. phase bits, are used for controlling the phase difference between the antennas A
1
and A
2
. The left-hand table indicates the power control based on the amplification bit, wherein the power P
A1
and P
A2
supplied to the antennas A
1
and A
2
, respectively, is switched between 20% and 80% of a predetermined value. The right-hand table shows the feedback control based on the three phase bits, wherein the phase difference can be quantified into eight different phase difference values to be established by the BS
10
in order to obtain an optimum coherence in the MS
20
.
As regards the table of
FIG. 2
, it is to be noted that an equal power is applied to the antennas A
1
and A
2
in each case where Na=0. Furthermore, the antennas A
1
and A
2
are uniquely defined by their respective pilot codes of the CCPCH (Common Control Physical Channel) of the UMTS. The derived amplitude and phase applied to the antennas A
1
and A
2
is called a weight and the set of weights is grouped into a weight vector. Specifically, the w

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