Forward link transmission mode for CDMA cellular communications system using steerable and distributed antennas

A DS-CDMA cellular system is provided that employs two forward link transmission modes in parallel. A conventional transmission mode broadcasts a pilot channel that can be used by a conventional mobile station to estimate and detect a dedicated traffic channel. The second transmission mode transmits (via a sectorized or narrow lobe antenna system) all information required by a new generation mobile station to estimate and detect a traffic channel dedicated to that mobile station. Consequently, DS-CDMA system operators can introduce the use of adaptive array antenna systems or distributed antenna systems with new generation mobile stations, without terminating the use of the conventional mobile stations.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
The present invention relates generally to the telecommunications field 
and, in particular, to the use of adaptive antenna arrays or distributed 
antennas in a forward link of a Direct Sequence-Code Division Multiple 
Access (DS-CDMA) cellular communications system. 
2. Description of Related Art 
The next generation of cellular communications systems will be required to 
provide a broad range of services including digital voice, video and data 
in different transmission modes. These systems will require higher bit 
rates and higher received signal power levels, which will result in 
increased interference between users. Consequently, in order to obtain the 
high capacities that will be required of these systems, the interference 
levels will have to be reduced dramatically, and especially in the forward 
links (network to mobile direction). 
Reduced user interference levels in the forward links of these systems can 
be obtained by increased base station antenna sectorization. Adaptive 
antenna arrays can be utilized to form relatively narrow beams and thus 
reduce the size of the user interference areas. For example, adaptive or 
steerable antenna arrays can provide high antenna gains by transmitting 
information to individual users in highly directional, narrow beams or 
lobes. These narrow beams reduce the areas of potential interference. 
Similarly, distributed antenna systems utilize a plurality of antenna 
elements positioned at different locations (e.g., "radio access ports"). 
Consequently, information is transmitted to a user from the closest 
antenna element (port), which also serves to reduce the potential areas of 
user interference. 
In such a sectorized-antenna CDMA system, base station transmits the 
traffic channel data on a code channel for a particular mobile station in 
one antenna lobe. If pilot channel is broadcast over the same area as the 
narrow antenna lobes, then each antenna lobe is treated as a separate 
cell. Consequently, if the mobile station moves into a different antenna 
lobe, which defines a different cell, a handoff of the mobile station 
between the two cells will have to occur. With each pilot channel thus 
defining a cell, increased antenna sectorization results in a situation 
where the mobile stations cross the cell borders more frequently, which 
leads to a larger number of handoffs. Since there is an upper limit to the 
number of handoffs such a system can process efficiently, a design 
trade-off must be made between the amount of antenna sectorization desired 
and user interference that can be accepted. 
Current DS-CDMA systems being developed include those that adhere to the 
IS-95 standard (ANSI J-STD-008) and the European RACE Project R2020 (known 
as the Code Division Testbed or "CODIT"). Both of these types of systems 
use a broadcasted pilot channel in the forward link for two main purposes 
(once the initial connection between a mobile and base station has been 
established): (1) identifying individual cells for Mobile-Assisted 
Handoffs (MAHOs); and (2) facilitating coherent detection of traffic 
channel data by the mobile stations. Essentially, these systems will use 
the same pilot symbols for broadcasted cell identification information and 
facilitating coherent detection by all mobiles in a cell, in order to 
improve system performance without adding excessive pilot symbol overhead. 
For such IS-95 and CODIT systems, the forward link air interfaces have been 
specified so that the mobile stations are required to use QPSK modulation 
with coherent detection. The pilot channel is used to facilitate the 
coherent detection process. Consequently, if the traffic channel is 
transmitted over a different (adaptive) antenna lobe or from a different 
(distributed) antenna element than the pilot channel, the mobile stations 
will experience significant detection errors. These detection errors can 
occur for two reasons: (1) the searching algorithm used in the mobile 
stations will direct the receivers to demodulate rays that contain pilot 
signal energy but no traffic signal energy; and (2) the phase of the pilot 
channel and traffic channel will be different, because they are 
transmitted from different antennas or different sets of antenna elements. 
Consequently, these conditions cause the mobile stations to make erroneous 
channel estimations which lead to corrupted demodulated signals. 
Relatively simple approaches to resolving these problems would be to use 
either non-coherent detection by the mobile station in the forward link, 
or provide dedicated pilot symbols for each traffic channel. However, the 
performance of conventional systems using those approaches would be 
degraded substantially, in comparison to systems using coherent detection 
(at the mobile station) facilitated by broadcasting a common pilot 
channel. Also, newer systems using these approaches would not be 
compatible with those conventional mobile stations that are designed to 
use the broadcasted pilot channel to detect and demodulate incoming 
signals. 
SUMMARY OF THE INVENTION 
A DS-CDMA cellular system is provided that employs two forward link 
transmission modes in parallel. A conventional transmission mode 
broadcasts a pilot channel that can be used by a conventional mobile 
station to estimate and detect a dedicated traffic channel. The second 
transmission mode transmits (via a sectorized antenna system) all 
information required by a new generation mobile station to estimate and 
detect a traffic channel dedicated to that mobile station. 
For systems using conventional broadcast cells, a mode controller selects 
the conventional forward link transmission mode. For sectorized systems 
(e.g., using adaptive arrays or distributed antennas), the common control 
channels utilize the conventional transmission mode. The mobile station 
transmits a signal that indicates whether or not it is equipped to support 
the new transmission mode. The cellular network can then determine, for 
each such mobile station, whether the conventional or new forward link 
transmission mode should be used. Preferably, if a mobile station is 
designed to operate with the new forward link transmission mode, the 
network selects the new transmission mode. If not, the network selects the 
conventional forward link transmission mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the present invention and its advantages are 
best understood by referring to FIGS. 1-3 of the drawings, like numerals 
being used for like and corresponding parts of the various drawings. 
Essentially, for the preferred embodiment of the present invention, a 
DS-CDMA cellular system is provided that employs two forward link 
transmission modes in parallel. A conventional transmission mode 
broadcasts a pilot channel that can be used by a conventional mobile 
station to estimate and detect a dedicated traffic channel. The second 
transmission mode transmits (via a sectorized antenna system) all 
information required by a new generation mobile station to estimate and 
detect a traffic channel dedicated to that mobile station. 
For systems using conventional cells (e.g., broadcasted cells not defined 
by adaptive array or distributed antenna lobes), the conventional forward 
link transmission mode is selected. For sectorized systems (e.g., using 
adaptive arrays or distributed antennas), the common control channels 
(e.g., broadcast control channel, paging channel, and access grant 
channel) utilize the conventional transmission mode. However, at call 
setup, once the initial connection between the mobile station and base 
station has been established, the mobile station transmits a signal 
parameter that indicates whether or not it is equipped to support the new 
(second) transmission mode. The cellular network can then determine, for 
each such mobile station, whether the conventional or new forward link 
transmission mode should be used. Preferably, if a mobile station is 
designed to operate with the new forward link transmission mode, the 
network selects the new transmission mode. If a mobile station does not 
transmit a signal indicating that it can operate with the new forward link 
transmission mode, the network selects the conventional forward link 
transmission mode. 
Specifically, FIG. 1 is a schematic block diagram that illustrates a system 
that can be used to provide parallel forward link transmission modes in a 
cellular communications system, in accordance with a preferred embodiment 
of the present invention. Base station 10 includes a switch 12 that can 
route a bitstream of information, d.sub.i, (e.g., digital data, voice, 
video) to a selected one of two forward link transmission mode units. The 
position of switch 12 is selectively controlled by control signals from a 
transmission mode controller 24 preferably located in base station 10. 
Alternatively, a separate mode controller 24 can be provided for each 
mobile station, either at base station 10 or as a component part of a 
user's mobile station 22. In the exemplary embodiment shown, the dashed 
lines between mode controller 24, switch 12, and mobile station 22 denote 
a flow of control signals therebetween (e.g., via the air interface). 
An output of switch 12 is connected to one of a "new" transmission mode 
signal processor unit 14, or a conventional ("old") transmission mode 
signal processor unit 16. An output of new transmission mode signal 
processor unit 14 is connected to an adaptive antenna array 18. 
Alternatively, a distributed antenna system (e.g., a plurality of radio 
access ports) can be used in place of adaptive array 18. Essentially, each 
directional energy lobe transmitted from adaptive array 18 defines a 
narrow lobe. Similarly, each energy lobe transmitted by an element (radio 
port) of a distributed antenna system also defines a narrow lobe. As such, 
any type of sectorized antenna system, which transmits a relatively narrow 
energy lobe, either fixed or steerable, can fall within the scope of the 
present invention. 
The pilot channel, broadcast channels, and some traffic channels can be 
transmitted over the conventional transmission mode (16) via a broadcast 
antenna 20. The broadcast antenna 20 provides full cell coverage for those 
mobile stations that are equipped to use the conventional forward link 
transmission mode (16). Other traffic channels are transmitted over the 
new transmission mode (14) via the array antenna (or distributed antenna 
element) to the individual mobile stations (e.g., 22), which are equipped 
to operate with the new forward link transmission mode. For flexibility in 
this embodiment, the newer mobile stations can be equipped to operate with 
either the new or old forward link transmission mode. 
Using the new forward link transmission mode (14) with an adaptive antenna 
array (18), each mobile station's traffic channel is transmitted over an 
individual antenna lobe (e.g., a traffic channel lobe). Each such antenna 
lobe is much narrower than the broadcast antenna (20) sector. With a 
distributed antenna system, for example, one of a plurality of fixed 
antenna lobes can be transmitted directionally to a mobile station. With 
an adaptive antenna array, for example, one antenna lobe can be formed and 
directed to an individual mobile station. The formation of such an 
individual antenna lobe can be accomplished with a conventional adaptive 
antenna transmission scheme. 
As described above, transmission mode controller 24 controls the routing of 
forward link traffic signals to either the new transmission mode signal 
processor unit 14 or the conventional transmission mode signal processor 
unit 16. During call setup, an appropriately equipped mobile station 
(e.g., mobile station 22) attempting to make contact with base station 10, 
transmits control signals via air interface link 26 that identify that 
mobile station's "new" transmission mode compatibility. Accordingly, mode 
controller 24 directs switch 12 to route the forward link bitstream, 
d.sub.i, to "new" transmission mode unit 14. Otherwise, if the mobile 
station is not equipped to communicate via the new transmission mode, 
those control signals are not sent to mode controller 24. Consequently, 
mode controller 24 assumes that only the conventional transmission mode 
will be used for this mobile station, and directs switch 12 to route the 
forward link bitstream, d.sub.i, to "old" transmission mode unit 16. 
In addition to determining the presence or absence of mode transmission 
control signals from a mobile station, mode controller 24 can decide 
whether or not to select the new transmission mode, based on other 
criteria. One criterion that can be used by the mode controller for such a 
decision is the type of service requested by the mobile user. For example, 
the mode controller (24) can select the new transmission mode if the bit 
rate of the requested service is higher than a predetermined rate. Or, the 
new transmission mode can be selected based on cell configuration data. 
For example, as described above, the new transmission mode can be selected 
if the cellular system involved defines individual cells using an adaptive 
antenna array or distributed antenna system (e.g., plurality of antenna 
elements or radio access ports). 
If mode controller 24 selects the new transmission mode for an individual 
mobile station, the base station transmits a control message to the mobile 
station via the air interface link 28, which informs the mobile station 
that the new mode has been selected for forward link transmissions to that 
mobile station. 
In the event that a mobile station operating with the new transmission mode 
moves into a cell defined by a different base station, it is possible that 
the different base station is (or is not) capable of operating with the 
new forward link transmission mode. Consequently, it is within the scope 
of the present invention for the system to be capable of switching over 
from one transmission mode to another (new to old or vice versa) during an 
active connection between the mobile and base station. This active 
switching operation can be accomplished, for example, by the base station 
indicating in a handoff message to the mobile station the specific 
transmission mode currently being used, along with timing information 
about when the handoff should occur. If a soft handoff operation is 
occurring, the transmission mode selected can be the same for all of the 
base stations included in the active set of users. Alternatively, 
different base stations can use different transmission modes. 
In a second embodiment of the present invention, if a distributed antenna 
system is in use (as opposed to the adaptive array shown in FIG. 1), the 
broadcast channel and pilot channel can be transmitted from all of the 
distributed antenna elements (e.g., all of the radio access ports) in the 
cell. However, the traffic channel is transmitted preferably from either 
one or a select plurality of antenna elements or radio access ports to an 
appropriately equipped mobile station (22). 
FIG. 2 is a schematic circuit block diagram of a signal processor unit 100 
that can be used to implement the new forward link transmission mode (14) 
shown in FIG. 1, in accordance with the preferred embodiment of the 
present invention. The embodiment shown in FIG. 2 can be for use in a 
known CODIT environment. However, the invention is not intended to be so 
limited and can be used with any appropriate CDMA-type system. For 
example, in accordance with the present invention, an alternative to the 
CODIT-based implementation described directly below (wherein the physical 
data channel and physical control channel are separated) is to 
differentially encode the full traffic channel. In the embodiment 
illustrated by FIG. 2, for the forward link modulation format, the 
physical control channel data is differentially encoded before spreading 
(spread spectrum) with the dedicated spreading code, and then it is QPSK 
modulated for transmission via array 18 to the mobile station (22). The 
forward link physical data channel data is also QPSK modulated for 
transmission via array 18, but the physical data channel data is not 
differentially encoded. By differentially encoding the forward link 
physical control channel data, the signal transmitted on the narrow lobe 
from array 18 can be received and demodulated at the mobile station using 
a differential coherent detection scheme. Consequently, by using this 
differential forward link modulation scheme, a receiving mobile station 
(22) can accurately estimate the channel phase for the incoming 
differentiated data symbols. In other words, by using differential 
encoding for the forward link physical control channel data, the receiver 
in the mobile station (22) does not need a pilot channel as a reference 
for channel estimation. The differentially encoded bitstreams can be 
detected without explicitly estimating the channel phase. Channel 
estimation is accomplished in the differential coherent detection process. 
Therefore, the mobile station's receiver can decode the physical control 
channel data without relying on the pilot channel. The receiver can then 
use the channel estimations from the physical control channel detection to 
successfully detect and decode the QPSK modulated physical data channel. 
Specifically, referring to the embodiment illustrated in FIG. 2, different 
spreading codes in the I and Q channels are used. The physical control 
channel data bits, d.sub.i, to be differentially encoded are input to 
differential encoder 114. The output signal from differential encoder 114 
can be expressed as: 
EQU .delta..sub.i =d.sub.i .multidot..delta..sub.i-1 .delta..sub.i 
.epsilon.{.+-.1} (1) 
The differentially encoded data bits .delta..sub.i .epsilon. {.+-.1} are 
split into two signals and coupled to a respective multiplier (spectrum 
spreader) 116 and 118. In each path, these data bits are multiplied with 
different binary spreading sequences having the respective values 
c.sub.1,j, c.sub.2,j .epsilon. {.+-.1}. 
Each differentially encoded spread signal, .alpha..sub.1,j, 
.alpha..sub.2,j, at the output of the respective spreaders 116 and 118 is 
coupled to a respective pulse shaping lowpass filter 120 and 122. Each 
filter has the impulse response h(t). The output signals from filters 120 
and 122 are coupled to respective multipliers 124 and 126, multiplied with 
two orthogonal carrier sine waves, and algebraically added together by 
adder 128. The modulated signal, x(t), to be transmitted on the forward 
link can be expressed as: 
EQU x(t)=.SIGMA..sub.j (.alpha..sub.1,j .multidot.h(t-jT.sub.c).multidot. cos 
(.omega.t)+.alpha..sub.2,j .multidot.h(t-jT.sub.c).multidot. sin 
(.omega.t)) (2) 
Consequently, using differential encoding in the forward link with a narrow 
lobe transmission, the intended mobile station can utilize an existing 
differential coherent detector to detect two consecutive received data 
symbols and estimate the channel phase by determining the difference 
between the two symbols. In summary, by using differential encoding for 
the traffic channel, the receiver in the mobile station does not need a 
pilot channel as a reference for channel estimation. Channel estimation is 
accomplished using the differential coherent detection process. Therefore, 
the mobile station's receiver can decode the traffic channel without 
depending on the pilot channel. 
FIG. 3 is a schematic circuit block diagram of a conventional signal 
processor unit 200 that can be used for the "old" forward link 
transmission mode unit 16 shown in FIG. 1. For a conventional DS-CDMA 
system, the forward link transmission mode (16) typically employs a QPSK 
modulation scheme. Notably, signal processor unit 200 is similar to signal 
processor unit 100 in FIG. 2, except signal processor unit 200 does not 
differentially encode the incoming data bits. Consequently, the output 
signals of multipliers (spreaders) 216 and 218 are not differentially 
encoded signals. Notably, since the incoming bitstream is not 
differentially encoded, signal processor unit 200 can also be used to 
modulate the physical data control channel data transmitted to a mobile 
station equipped for the new transmission mode. 
In a third embodiment of the present invention, instead of using 
differentially encoded QPSK forward link transmissions for the physical 
control channel data, the new transmission mode unit 14 shown in FIG. 1 
can be used to generate and transmit pilot symbols associated with the 
intended mobile station (22). For example, in an IS-95 type CDMA system, 
known pilot symbols can be inserted into the data bitstream by use of 
time-multiplexing. For example, every nth modulation data symbol is a 
pilot symbol instead of a data-modulated symbol. The mobile station's 
receiver will recognize each such recurring pilot symbol and use it to 
estimate the channel. Generally, M consecutive modulation symbols can be 
replaced with each interval of N modulation symbols. More specifically, 
from a practical standpoint, every second modulation symbol can be 
replaced by such a pilot symbol. Albeit, in an IS-95 type system, using 
every second modulation symbol for a pilot symbol would leave 4.8 kbps for 
user data. However, this technique would be advantageous for relatively 
high bit rate mobile users (e.g., where one mobile user occupies several 
code channels), because only one of those channels would have to carry the 
pilot symbols, which significantly decreases the signaling overhead 
requirements. 
Although a preferred embodiment of the present invention has been 
illustrated in the accompanying Drawings and described in the foregoing 
Detailed Description, it will be understood that the invention is not 
limited to the embodiments disclosed, but is capable of numerous 
rearrangements, modifications and substitutions without departing from the 
spirit of the invention as set forth and defined by the following claims.