WO2000070903A1

WO2000070903A1 - Channel assignment in a spread spectrum cdma communication system

Info

Publication number: WO2000070903A1
Application number: PCT/US2000/013924
Authority: WO
Inventors: Stephen G. Dick; Eldad Zeira
Original assignee: Interdigital Technology Corporation
Priority date: 1999-05-19
Filing date: 2000-05-19
Publication date: 2000-11-23
Also published as: JP2005237027A; EP1998589B1; DK1183892T3; ES2315232T3; NO20015616D0; AU775980B2; EP1998589A3; KR100408982B1; HK1127208A1; CA2371482A1; TW476199B; BR0010635A; HK1044255B; ATE412319T1; MXPA01011773A; JP4249718B2; KR20020014796A; CN1486009B; NO328334B1; CN1351808A

Abstract

A selected user equipment transmits a signature in a selected one of the common packet channel's time slots. The base station, in response to receiving the transmitted signature, selects a currently unused code, if available, out of a plurality of code associated with the access opportunity defined by the selected signature and selected time slot. The base station transmits an acknowledgment signal comprising an identifier of the selected code. The selected user equipment receives the acknowledgment signal. The selected user equipment and the base station communicate using the selected code.

Description

CHANNEL ASSIGNMENT LN A SPREAD SPECTRUM CDMA COMMUNICATION SYSTEM

BACKGROUND

The invention relates generally to resource allocation in a wireless code

division multiple access communication system. More specifically, the invention

relates to assigning uplink and downlink channels in response to access requests of

user equipment.

Figure 1 depicts a wireless spread spectrum Code Division Multiple Access

(CDMA) communication system 18. A base station 20 communicates with user

equipment (UE) 22- 26 in its operating area. In a spread spectrum CDMA system

18, data signals are communicated between UEs 22-26 and the base station 20 over

the same spread bandwidth. Each data signal in the shared bandwidth is spread with

a unique chip code sequence. Upon reception, using a replica of the chip code

sequence, a particular data signal is recovered.

Since signals are distinguished by their chip code sequences (code), separate

dedicated communication channels are created using different codes. Signals from

the base station 20 to the UEs 22-26 are sent on downlink channels and signals from

the UEs 22-26 to the base station 20 are sent on uplink channels. For coherent

detection of downlink transmissions by the UEs 22-26, pilot signals are transmitted

to all the UEs 22-26 within the base station's operating range. The UEs 22-26

condition their receivers based on the pilot signals to enable data reception.

In many CDMA systems, a common packet channel (CPCH) is used for uplink transmissions. A CPCH is capable of carrying packets of data from different

UEs 22-26. Each packet is distinguishable by its code. For detection by the base

station 20, the packets have a preamble which also distinguishes it from other

packets. The CPCH is typically used to carry infrequently communicated data at high

rates.

Figure 2 depicts a CPCH access scheme 28. The CPCH access scheme 28

is time divided into intervals having time slots 30-34, such as 8 time slots proposed

for the Third Generation Mobile Telecommunications System (LMT-2000) . A group

of predetermined signatures 36-40 are assigned to the time slots 30-34 to allow more

than one UE 22-26 to use the same time slot 30-34. A particular signature used

within a particular time slot is referred to as an access opportunity 66-82. For

instance, for each of the 8 time slots in the proposal for LMT-2000, one out of 16

signatures is available to be chosen, resulting in 128 access opportunities. Each

signature 36-40 is preassigned a virtual channel. A virtual channel uniquely defines

operating parameters for both the uplink and downlink, i.e., an uplink spreading

factor and a unique code for the downlink.

Broadcast from the base station 20 to each UE 22-26 is the availability of each

virtual channel over an acknowledge indication channel (AICH). The UE 22-26

monitors the AICH to determine the availability of each virtual channel. Based on

the operating parameters required by the UE 22-26 and the availability of the virtual

channels, the UE determines the access opportunity to select. Upon identifying a

particular access opportunity, the base station 20 sends out an acknowledgment message (ACK) if the corresponding downlink channel is still available. In the

proposal for LMT-2000, the ACK simply repeats the signatures 36-40 associated with

the access attempt. If the downlink channel is not available, a negative acknowledge

(NAK) is sent.

After receiving a corresponding acknowledgment, the UEs 22-26 determine

the proper code to recover communications on the downlink channel based on the

access opportunity 66-82 used to send the UEs' packet. Either stored in the UEs 22-

26 or transmitted on a Base Station's Broadcast Channel is a list of the code assigned

to each access opportunity 66-82. This scheme severely increases the packet

collision probability and therefore the packet delay which is undesirable.

In some situations, monitoring the AICH is not desirable. At a particular

moment, some UEs 22-26 will be operating in a "sleep" mode. In the "sleep" mode,

the UE 22-26 only runs when there is a need to send data. Monitoring the AICH

during "sleep" mode will both reduce battery life and introduce a delay in the

transmission of the first packet. Additionally, when a UE 22-26 borders between

two base station's operating areas, monitoring multiple AICHs further exacerbates

-these drawbacks.

Monitoring creates other problems. It further complicates the UE' s receiving

circuitry, making the UE 22-26 more expensive.

Monitoring results in a suboptimum use of the CPCH. AICH monitoring

provides information when a channel becomes busy. The time at which the channel

becomes free is deduced on a worst case maximum packet length. If a packet is not maximum length, the channel will be idle while the UEs 22-26 are waiting to

transmit. On the other hand, if monitoring is not performed in such a system,

channel availability information is unavailable. The UE 22-26 may randomly choose

a busy virtual channel increasing the packet delay by causing a collision.

Accordingly, it would be desirable to allow the UEs 22-26 to wait a period shorter

than the maximum packet length and provide for some other collision reducing

mechanism.

One technique to reduce the possibility of collision is to raise the number of

codes, for instance to 128 different codes. In the proposal for LMT-2000 the 128

sequences represent approximately half of the sequences available at the base station

20. Accordingly, this solution is undesirable. Additionally, since monitoring the

AICH complicates the UE receiver circuitry increasing its cost, it is undesirable.

Accordingly, an alternate approach to assign virtual channels is desirable.

SUMMARY

A selected user equipment transmits a signature in a selected one of the

-common packet channel's time slots. The base station, in response to receiving the

transmitted signature, selects a currently unused code, if available, out of a plurality

of code associated with the access opportunity defined by the selected signature and

selected time slot. The base station transmits an acknowledgment signal comprising

an identifier of the selected code. The selected user equipment receives the

acknowledgment signal. The selected user equipment and the base station communicate using the selected code.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is an illustration of a typical wireless spread spectrum CDMA

communication system.

Figure 2 is an illustration of a common packet channel access scheme.

Figure 3 is an illustration assigning virtual channels.

Figure 4 is a graph of the probability of a collision versus demand for the

prior art and the virtual channel assignment.

Figure 5 illustrates a simplified base station and user equipment.

Figure 6 is an identifier transmitter circuit.

Figure 7 is an identifier receiver circuit.

Figure 8 is a table of an assignment of Golay sequences.

Figure 9 is a circuit for detecting the Golay sequences of Figure 8.

Figure 10 is an assignment table for a system having physical channels with

two time slots.

DETAILED DESCRIPTION OF THE PREFERRED EMBODLMENTS

The preferred embodiments will be described with reference to the drawing

figures where like numerals represent like elements throughout. Figure 3 illustrates

a virtual channel assignment scheme. Each virtual channel 48-64 is defined by its

operating parameters, such as uplink spreading factor and down link code. Additionally, instead of assigning virtual channels 48-64, the same principles may

be applied to assigning physical channels which are defined by their downlink code.

To reduce the number of used physical channels and increase the power level

of each channel, each physical channel may be multiplexed, such as by using two

time slots. Using two time slots will increase the channel's effective data rate, such

as from 8 Kbps to 16 Kbps. In such a system, the virtual channel 48-64 also defines

which multiplexed signal is assigned to the UE 22-26.

Instead of assigning a single virtual channel for each signature 36-40 as in the

prior art, a set 42-46 of virtual channels are assigned for each grouping 116-120 of

access opportunities. A grouping may contain all of the channels in one group or as

few as 2 or 3 channels. One possible virtual channel grouping may group all virtual

channels with the same data rate for the uplink. For groups having the same uplink

data rate, the UE 22-26 selects an access opportunity out of the groups having the

UE's desired uplink data rate. Another grouping may be formed based on the access

opportunities' signature 36-40. Based on the selected access request and the UE's

priority, one of the virtual channels 48-64 assigned to the group 116-120 associated

-with the access attempt is used for the UE if available. Once the virtual channel is

assigned, it will not be assigned again until the particular UE's transaction is

complete. Additionally, a receiving circuit at the base station 20 with the proper data

rate is assigned to the UE 22-26.

In the prior art system, the UEs 22-26 determine which channel is assigned

to the downlink based on the access opportunity 66-82. Virtual channel assignment transmits a channel identifier 84-88, preferably along with the ACK, indicating

which of the set 42-46 of channels assigned to the group 116-120 is selected. When

all of the virtual channels are in the same group, the identifier 84-88 indicates the

selected virtual channel. If no channel is available out of the set 42-46, a no channel

is available (NAK) identifier is sent. Since more than one virtual channel is

potentially assigned to a particular access attempt, the probability of UE collisions

is reduced.

Figure 4 is a graph 91 depicting the probability of a collision (Collisions)

versus the number of UEs 22-26 requesting access (Demand). As shown, the

collisions using 2 or 3 virtual channels per group (2 states/AP or 3 states/ AP) is

lower than the prior art (AICH monitor) regardless of demand.

Figure 5 illustrates a simplified base station 20 and a UE 22 for use in

implementing channel assignment. The UE 22 has a controller 144 for determining

the code of the uplink and downlink communications. A UE transmitter 140 sends

communications, such as access opportunities and uplink packet signals, to the base

station 20. A UE receiver 142 receives communications, such as ACK messages,

- NAK messages and downlink signals.

The base station 20 has a controller 134 for determining the code of uplink

and downlink communications as well as determining channel availability. A base

station transmitter 136 sends communications, such as ACK messages, NAK

messages and downlink signals, to the UE 22. The base station receiver 138 receives

communications, such as access opportunities and uplink packet signals. Techniques for sending the identifiers are to attach extra bits to the ACK or

to change the phase of the ACK to indicate the selected identifier. For a system

using a single group of virtual channels, the extra bits identify the selected virtual

channel. Circuits for sending the identifiers by phase shifting the ACK are depicted

in Figures 6 and 7. The circuits are capable of sending up to four channel identifiers

without a NAK identifier or three channel identifiers with a NAK identifier. In the

transmitter circuit 92 of Figure 6, the ACK sequence is generated by a sequence

generator 94. The sequence itself is associated with the preamble access opportunity

and is unique to the access attempt. Several such sequences may be transmitted to

several users at the same time. The ACK sequence is passed through a mixer 96

which multiplies the signal with either +1 or -1. The mixed signal is subsequently

passed through another mixer 98 where the signal is mixed with an in-phase carrier

(cos wt) or a quadrature carrier (sin wt). As a result of the two mixers 96, 98, the

transmitted ACK is at one of four phases 0°, 90°, 180° or 270°. Each identifier 84-

88 is preassigned to one of the phases.

The receiver circuit 14 of Figure 7 is used to determine the phase of the ACK

- sent

by the transmitter circuit 92 of Figure 6. The ACK is mixed with both an in-phase

carrier by mixer 100 and a quadrature carrier by mixer 102. Each of the mixed

signals are correlated with a replica of the ACK's sequence by sequence correlators

104, 106. The in-phase and quadrature correlation signals are each negated by

mixers 108, 110 by multiplying the correlation signals by -1. The two correlated signals and the two negated signals are supplied to an identifier circuit 112. The

identifier circuit 112 determines which of the four phased versions of the correlated

signal has the highest magnitude. Since the downlink transmissions from the base

station are synchronized and their phase is known, the identifier circuit 112

determines which identifier 84-88 was sent based on the phase of the ACK. A list

stored either in the UEs 22-26 or transmitted on a Base Station's Broadcast Channel

is used to determine the virtual channel 48-64 associated with the identifier 84-88

and the group 116-120 of the UE's access request. Using the determined virtual

channel 48-64, transmissions sent by the base station 20 using the selected downlink

channel's code are recovered at the UE 22-26.

Another technique for sending the identifier 84-88 is to use the ACK and a

collision resolution signal (CR). After a collision between UEs 22-26 is detected at

a base station 20, in many spread spectrum systems the base station 20 sends a CR

directed to the colliding UEs. The CR has a sequence which is associated with a

specific UE 22 for detection by the UE 22. By inverting the ACK and CR, an

identifier 84-88 is sent to the specific UE 22. An inverted ACK indicates a NAK.

- By inverting the CR, one virtual channel is assigned to +CR a second virtual channel

is assigned to -CR. Accordingly, using the ACK and CR an identifier indicating

either a NAK or one of two channels is sent. Additionally, using a CR with multiple

states, such as three, one of multiple channels is assigned to the CR.

Alternatively, the identifier is sent with a signal using a Golay sequence. A

Golay sequence is constructed out of short sequences, such as X and Y. By inverting the shorter sequences and changing their order many unique longer sequences may

be constructed as shown in table 122 of Figure 8. To reduce the size of the table

122, only half of the possible sequences are shown. By negating each sequence,

another unique Golay sequence results. As shown in Figure 8, each UE 22-26 is

assigned a unique set of Golay sequences, such as 4. For instance, user 0 is assigned

four sequences, the top two sequences and the negation of those sequences. By

assigning each of the Golay sequences a virtual channel, upon reception, the

receiving UE 22-26 determines the code of the downlink transmission.

A Golay sequence detector is shown in Figure 9. The received signal is

correlated with a Golay Correlator 123 and interleaved by an interleaver 124 to

detect the short codes. The arrangement of the short codes for two assigned

sequences within the long codes is shown as Signature 0 and 1. Using mixers 125,

126, the signatures are mixed with the detected short codes. Adders 127, 128 are

coupled to the mixer and also to delay devices 129, 130. The delay devices 129, 130

take the output of the adders 127, 128 and feed them back to those adders 127, 128

for correlation with the next short sequence. The output of each adder 127, 128

- determines the Golay sequence of the received signal.

Figure 10 illustrates an assignment scheme for a system using two time slot

multiplexing for the physical channels. In table 132, each of the sixteen different

signatures is assigned a downlink code and one of two time slots. The selected time

slot is indicated by the transmitted identifier.

Claims

CLAIMSWhat is claimed is:

1. A method for sending data in a spread spectrum code division multiple

access communication system between a selected user equipment out of a plurality

of user equipments and a base station, the system having a common packet channel

defined by a predetermined set of codes, the common packet channel having a

plurality of access opportunities, each defined by a time slot and signature, the

method comprising:

transmitting from the selected user equipment a signature in a selected one of

the common packet channel time slots;

in response to receiving the transmitted signature at the base station, selecting

a currently unused code, if available, out of a plurality of codes associated with the

access opportunity defined by the selected signature and selected time slot;

transmitting from the base station an identifier of the selected code;

receiving the identifier at the selected user equipment; and

communicating data between the selected user equipment and the base station

_ using the selected code.

2. The method of claim 1 wherein the identifier is transmitted along with

an acknowledgment signal.

3. The method of claim 1 wherein the identifier is transmitted by a

acknowledgment signal and a collision resolution signal.

4. The method of claim 1 wherein the identifier indicates an uplink

spreading factor for communications to be sent from the user equipment to the base

station.

5. The method of claim 1 wherein the transmitted signature is selected

from a set of sixteen signatures and the selected one time slot is selected from a set

of eight time slots.

6. The method of claim 2 further comprising if no codes associated with

the access opportunity are available, transmitting a negative acknowledgment signal

indicating that no codes are available.

7. The method of claim 1 wherein the selected code is used to define a

downlink physical channel for downlink communications.

8. The method of claim 7 wherein the downlink physical channel is time

multiplexed into time slots and the identifier indicates a selected one of the time slots

for the downlink communications between the base station and the selected user

equipment.

9. The method of claim 1 wherein the plurality of codes associated with

each access opportunity includes all of the codes available to all of the access

opportunities.

10. The method of claim 1 wherein the plurality of codes associated with

each access opportunity numbers from a range of two codes to all of the codes

available to all of the access opportunities.

11. The method of claim 2 wherein the acknowledgment signal identifier

is indicated by a selected one out of a set of phases of the acknowledgment signal.

12. The method of claim 2 wherein the acknowledgment signal identifier

is attached extra bits.

13. The method of claim 2 wherein the acknowledgment signal identifier

is indicated by a selected one out of a set of Golay sequences.

14. A base station for a spread spectrum code division multiple access

system comprising:

a receiver for receiving access opportunities and packet data over a common

packet channel, the common packet channel defined by a predetermined set of codes

and having a plurality of access opportunities, each access opportunity defined by a time slot and a signature;

a code selection controller associated with said receiver for selecting a

currently unused code from a plurality of codes associated with a received access

opportunity, such that a code, if available, is selected in response to receiving a

signature in one of said time slots from a user equipment from the plurality of codes

associated with the access opportunity defined by the received signature and said one

time slot; and

a transmitter associated with said controller which transmits to the user

equipment an identifier of the selected available code, whereby said transmitter sends

communications to the user equipment encoded with said selected code.

15. The base station of claim 14 wherein the plurality of codes associated

with each access opportunity includes all of the codes of all the access opportunities.

16. The base station of claim 14 further comprising:

a sequence generator for producing a replica of a received access

opportunities' signature;

a first mixer for selectively inverting the signature replica in response to the

code selection; and

a second mixer for selectively mixing the selectively inverted signature with

an in-phase or quadrature phase carrier in response to the code selection to produce

the identifier.

17. A spread spectrum code division multiple access communication

system having a base station and a plurality of user equipments, the system using a

common packet channel for communication, the common packet channel defined by

a predetermined set of codes and having a plurality of access opportunities, each

access opportunity defined by a time slot and signature, the system comprising:

the plurality of user equipments, each having:

means for transmitting a signature in a selected one of the common

packet channel time slots;

means for receiving an identifier, the identifier indicating a selected

code; and

means for communicating with the base station using the selected code;

and

the base station having:

means, in response to receiving the transmitted signature, for

determining the selected code by selecting a currently unused code, if available, out

of a plurality of codes associated with the access opportunity defined by the selected

_ signature and selected time slot; and

means for transmitting the identifier of the selected code.

18. The system of claim 17 wherein the identifier indicates an uplink

station.

19. The system of claim 17 wherein the transmitted signature is selected

of eight time slots.

20. The system of claim 17 wherein the base station further having means,

if no codes associated with the access opportunity are available, for transmitting a

negative acknowledgment signal indicating that no codes are available.

21. The system of claim 17 wherein the selected code is used to define a

downlink physical channel for downlink communications.

22. The system of claim 21 wherein the downlink physical channel is time

for the downlink communications between the base station and the user equipment.

23. The system of claim 17 wherein the plurality of codes associated with

each access opportunity includes all of the codes of all access opportunities.

24. The system of claim 17 wherein the plurality of codes associated with

each access opportunity numbers from a range of two codes to all of the codes of all

access opportunities.

25. The system of claim 17 wherein the identifier is indicated by a selected

one out of a set of phases of an acknowledgment signal.

26. The system of claim 17 wherein the identifier is attached extra bits to

an acknowledgment signal.

27. The system of claim 17 wherein the identifier is indicated by a selected

one out of a set of Golay sequences.