Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-09-22
2003-12-23
Bost, Dwayne (Department: 2681)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S208000, C455S436000
Reexamination Certificate
active
06667961
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to communication systems, and in particular, to a method and apparatus for performing a handoff.
2. Description of the Related Art
In a mobile communication environment, as a user moves from the coverage area of one base station to the coverage area of another base station, a handoff must occur to transition the communication link from one base station to the next. Handoff determinations are typically made based on signal strength measurements by mobile stations of pilot signals transmitted by respective base stations. If the measured pilot signal strength of the present base station falls below a threshold, the mobile station transmits a pilot strength measurement message (PSMM) which is forwarded to a transceiver and selector bank (TSB) of a base station controller (BSC). The base station controller then determines what type of handoff should be performed.
Handoffs are generally classified into two types. The first type is known as a soft handoff. For a soft handoff, a mobile station MS simultaneously maintains connection with two or more base stations (i.e. make before break). That is, as the mobile moves from its current cell (source cell) to the next cell (target cell), a traffic channel is simultaneously maintained with both cells. A soft handoff usually occurs when a mobile station travels from one cell to another cell served by the same BSC, where the base station of the second cell uses the same frequency assignment as the first.
The second type of handoff, hard handoff, is an abrupt handoff in which the mobile station is not controlled simultaneously by two or more base stations. Unlike the soft handoff, the call link connected to the mobile station is not continuously maintained, it is instead, cut-off from a base station located in a source cell and then re-established in a very short time frame with a base station from a target cell (i.e. break before make).
A conventional handoff in a mobile communication system will be described with reference to FIG.
1
.
Before describing a conventional handoff procedure, terms used herein will be defined.
Mode 0 (BS transmission mode 0 and MS reception mode 0): A BS normally transmits data for the entire frame period and an MS receives the data.
Mode 1 (BS transmission mode 1 and MS reception mode 1): The BS transmits data for a part of the frame period and the MS received the data.
Mode 2 (BS transmission mode 2 and MS reception mode 2): During part of the frame period where the BS is not transmitting data, the MS searches for an adjacent BS.
A guard time required to transit from mode 1 to mode 2 is called a and a guard time required to transit from mode 2 to mode 1 or mode 0 is called b.
First frame: The first frame transmitted to the MS by the BS upon request for a handoff.
Second frame: A frame following the first frame.
Slotted Mode (Compressed Mode): An operation mode of the BS in which the BS divides a frame period into time slots and transmits data only in selected slots. A data transmission period is called an action period in the slotted mode and a non-data transmission period is called a non-action period in the slotted mode.
FIGS. 1A and 1B
illustratively depict a conventional handoff in slotted mode 1 and in slotted mode 2, respectively.
Referring to
FIG. 1A
, a BS communicates with an MS in mode 0 in step
100
. Mode 0 is a transmission scheme in which data at a transmission rate RD is spread by a layer-m orthogonal code and transmitted for a frame period T. Upon require for a handoff, the BS doubles the data transmission rate, spreads data by a layer-(m−1) orthogonal code for the first half of the frame period for transmission, and transmits no data for the last half of the frame period, in steps
110
and
120
. Therefore, the MS receives the data from the BS for the first half of the frame period at the doubled data transmission rate and searches for an adjacent target BS to which a handoff will occur for the last half frame period. Then at steps
130
and
140
, the BS transmits data spread by the layer-(m−1) orthogonal code at the doubled data transmission rate for the first half frame period and then transmits no data for the last half frame period. Once again, the MS receives the data from the BS for the first half frame period and then searches for the adjacent BS to which a handoff will occur for the last half frame period.
As stated above, upon require for a handoff, the BS transmits data for the first half of the first and second frame period, and the BS does not transmit any data to the MS in the last half of the first and second frame periods, to allow the MS to search for an adjacent BS, in slotted mode.
Now referring to
FIG. 1B
, the BS communicates with the MS in mode 0 in step
200
. Mode 0 is a transmission scheme in which data at the transmission rate RD is spread by the layer-m orthogonal code and transmitted for the frame period T. Upon require for a handoff, the BS doubles the data transmission rate, spreads data by the layer-(m−1) orthogonal code for the first half of the first frame period for transmission in step
210
, and transmits no data for the last half of the first frame period
220
. T
Therefore, the MS receives the data from the BS for the first half of the first frame period
210
and searches for an adjacent target BS in last half of the first frame and first half of the second frame period
220
. Then, in steps
220
and
230
, the BS transmits no data for the first half of second frame period and transmits data spread by the layer-(m—1) orthogonal code at the doubled data transmission rate for the last half of second frame period
230
. That is, upon request for a handoff, the BS transmits data for the first half of the first frame period and the last half of the second frame period, and the MS searches for the adjacent BS in the last half of the first frame period and the first half of the second frame period without receiving data, in slotted mode
2
.
FIG. 2
illustrates orthogonal code layers which have variable spread gains and maintain orthogonality among channels.
Referring to
FIG. 2
, orthogonal codes in the same layer are mutually orthogonal and orthogonal codes in a direct line are not orthogonal. Therefore, either a direct upper layer (m+k) (k=0, 1, 2, . . . ) orthogonal code or a direct lower layer (m−k) (k 0, 1, 2, . . . , m) orthogonal code cannot maintain orthogonality among channels with respect to a layer-m (m=0, 1, 2, . . . ) orthogonal code.
FIGS. 3A and 3B
illustrate orthogonal code layers to describe an upper layer orthogonal code assigning method when a conventional handoff between frequencies is to be implemented. In the drawings, orthogonal codes marked with rectangles (in layer
3
) represent the current handoff candidate (i.e., requiring a handoff) and orthogonal codes marked with oval circles have assigned to channels in current communication.
Referring to
FIGS. 3A and 3B
, it is assumed that while the BS transmits using an orthogonal code 00000000, a handoff occurs. If an orthogonal code 0000 in the direct upper layer is available as shown in
FIG. 3A
, the BS transmits data with use of 0000. However, if the orthogonal code 0000 cannot be assigned due to an orthogonal code 00001111 in current use as shown in
FIG. 3B
, the BS detects an orthogonal code available among other orthogonal codes in the direct upper layer. Recall that orthogonal code 0000 in
FIG. 3B
cannot be assigned because it is in a direct line with 00001111, which it is not orthogonal each other. Then, the BS determines that orthogonal code 0011 is available and is not in a direct line with code 00001111 and transmits data with use of the orthogonal code 0011. In this case, different orthogonal codes may be used in steps
100
and
150
of FIG.
1
A and in steps
200
and
240
of FIG.
1
B. The probability of using a different orthogonal code from an orthogonal code in a previous period is higher in
FIG. 3B
than in
FIG. 3A
because th
Choi Ho-Kyu
Park Su-Won
Yoon Soon-Young
Bost Dwayne
Dilworth & Barrese LLP
Ramos-Feliciano Eliseo
Samsung Electronics Co,. Ltd.
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