Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-09-21
2004-02-03
Tran, Sinh (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S436000, C455S442000, C455S067110
Reexamination Certificate
active
06687500
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for testing the soft handoff functionality of a mobile station, such as cellular telephone acting as the mobile station being handed off between two base stations. More particularly, the present invention relates to an apparatus for testing soft handoff functionality of a Code Division Multi-Access (CDMA) communication system.
2. Description of the Related Art
An explanation of soft handoff and a description of a conventional apparatus to test soft handoff are provided below.
A. Soft Handoff
Soft handoff is a handoff between CDMA channels that have identical frequency assignments. Soft handoff provides diversity of Forward Traffic Channel and Reverse Traffic Channel paths on the boundaries between base stations. A traffic channel is defined as a communication path between a mobile station and a base station used by both a user and signaling traffic, the signaling traffic controlling communication between the mobile station and base station. The term “traffic” implies a forward traffic channel (base station to a mobile station) and a reverse traffic channel (mobile station to base station) functioning as a pair.
Before and during soft handoff, a base station and its neighboring base stations send pilot channel signals on a carrier and a MS (Mobile Station) monitors the signal strength of the pilot channels. The pilot channel is an unmodulated direct-sequence spread spectrum signal transmitted continuously to each CDMA base station. A pilot channel allows a MS to acquire the timing of a forward CDMA channel, provide a phase reference for coherent demodulation, and provide a means for signal strength comparisons between base stations for determining when to handoff. The MS measures the pilot strengths and provides a signal back to the base stations reporting the pilot signal strengths.
FIG. 1A
illustrates soft handoff between two base stations.
FIG. 1A
shows a MS moving away from a Base Station (BS
1
) and approaching a Base Station
2
(BS
2
), while the MS is receiving a traffic channel from BS
1
on the boundary between BS
1
and BS
2
. When the MS detects a pilot signal from BS
2
of sufficient strength, the MS reports the pilot strength to BS
1
. Then BS
1
sends the MS a message telling the MS to receive the traffic channel of BS
1
and the traffic channel of BS
2
. The MS then begins to receive both BS
1
and BS
2
traffic channels simultaneously (providing a diversity of forward link). At this point, both BS
1
and BS
2
are receiving traffic channels from the MS, which allows the transmission of higher quality traffic data to the network (providing diversity of reverse link). As the MS moves farther away from BS
1
, the MS will detect a weak pilot strength and report the weak pilot strength from BS
1
. The MS then begins to receive only the traffic channel from BS
2
. During the entire process, the MS can make handoffs between BS
1
and BS
2
without interrupting communication.
FIG. 1B
illustrates sector-to-sector handoff for a single base station
100
transmitting over different antennas to separate sectors using different sector units
102
and
104
of the single base station
100
. Such sector-to-sector handoff is sometimes referred to as softer handoff.
FIG. 1B
shows the two sectors
102
and
104
located in the single base station
100
for covering adjacent sectors. Softer handoff occurs when the MS is moving on the boundary between a sector
1
controlled by a sector unit
102
and a sector
2
controlled by sector unit
104
.
B. Conventional System To Test Soft Handoff Functionality
Soft handoff is a procedure enabling an interruption-free handoff. Manufacturers of CDMA phones assure communication quality by verifying the soft handoff functionality. The following are requirement specifications for a conventional test apparatus for testing soft handoff functionality of a MS:
Be able to simulate two base stations.
Be able to add Additive White Gaussian Noise (AWGN) to the forward link signal.
Be able to control the level of the forward link signal (including the forward link signal of BS
1
, the forward link signal of BS
2
and (AWGN) correctly and with sufficient resolution.
Be able to exchange messages needed during handoff with the MS.
Be able to display the contents of the pilot strength measurement report sent from the MS.
Be able to define the contents of the system parameter message from the base station (some parameters are used to inform the MS of a condition to monitor pilot strength).
FIG. 2
shows one example of a conventional test system for testing soft handoff. The test system includes two base station simulators (BS
1
)
202
and (BS
2
)
204
, an AWGN generator
206
, a test system controller
200
, couplers
207
,
208
, a duplexer
210
and a power divider
212
. Generally the test system shown in
FIG. 2 and a
(MS)
214
are connected using RF cables. An antenna or aerial interface may be used between the duplexer
210
and MS
214
instead of the RF cables depending on the condition of the test.
1. Base Station Simulators
202
,
204
Two base station simulators
202
and
204
are needed to simulate two base stations. The MS
214
will make a handoff from one base station simulator to another during testing.
FIG. 3
shows a block diagram of a conventional base station simulator as described in more detail below.
2. AWGN Generator
206
The AWGN generator
206
is used to simulate noise and signals sent from other base stations in the field. Some base station simulators have a built-in AWGN generator. The base station simulators in a conventional test system can have built in AWGN generators, so a separate AWGN generator as shown in
FIG. 2
is not needed.
3. Test System Controller
200
In order to execute soft handoff between one base station simulator to the other, the two base stations
202
and
204
need to synchronize with each other. The test system controller
200
is used to control both base station simulators, allowing them to handle call processing or message transmission/receptions to and from the MS
214
being tested. The test system controller
200
also controls the AWGN. The test system controller
200
controls the forward link signal for both of the base station simulators
202
and
204
and the AWGN
206
. The test system controller
200
further provides for displaying of the contents of the pilot strength measurement report message sent from the MS
214
.
4. Couplers
207
,
208
& Duplexer
214
The couplers
207
and
208
are used to combine the forward link signals of the base station simulators, and the AWGN and provides those signals to a duplexer
210
. The duplexer directs the signal from the coupler
208
through either a cable or antenna to a mobile station
214
.
5. Power Divider
212
The reverse link signal from the MS
214
is directed by the duplexer which then feeds the signal received to both base station simulators using the power divider
212
.
6. Conventional Base Station Simulator Circuitry (
FIG. 3
)
FIG. 3
shows the components of a conventional base station simulator, such as either
202
or
204
. The conventional components of the base station simulator includes a transmit (TX) baseband processor
302
which generates a digital forward link signal with a desired indication for amplitude. The digital forward link signal is converted to an analog signal in a digital to analog (D/A) converter
304
. The output of the D/A converter
304
is then provided through a quadurature modulator
306
and upconverter
308
for transmission from the base station simulator. A signal received by the base station simulator shown in
FIG. 3
is downconverted in downconverter
310
, demodulated in quadrature demodulator
312
and converted from an analog to a digital signal in the A/D converter
314
. The output of the A/D converter is provided to a receive (RX) baseband processor
316
which functions to further demodulate the received IF digital, decode the demodulated data and provide the demodulated data
Fliesler Dubb Meyer & Lovejoy LLP
Nguyen Huy
Tran Sinh
LandOfFree
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