CA2619679C - Quadriphase spreading codes in code division multiple access communications - Google Patents

Abstract

Optimal code sequences are generated for use in spreading and de-spreading functions in a code division multiple access (CDMA) communications system. In particular, a family of quadriphase spreading codes is employed that provides a maximal number of spreading codes to achieve a high capacity in the CDMA communications system while at the same time having a minimal peak cross-correlation between any two spreading codes within that family to ensure cross-correlation interference is kept at or below acceptable levels. That optimal quadriphase spreading code family is the S(2) family of four phase code sequences of length L = 2m-1, where m is an integer greater than or equal to 5. The size of the S(2) family of quaternary spreading codes is (L + 2)(L + 1)2, and the maximum cross-correlation is 1 + 4.sqroot.(L +1). The spreading codes are preferably allocated to base stations using specific code subsets of the S(2) family having the same cross-correlation properties of the S(O) and/or S(1) family of codes. Spreading codes are advantageously extended by one or more code symbols as necessary or otherwise desirable. For example, to support variable transmission rate services, it is desirable to employ spreading codes whose length may be expressed as an integer multiple of each spreading factor in the mobile communications system. Since individual spreading codes have a length of 2m-1, one code symbol is added to the generated spreading code.

Description

_ . ., . , ,.~ .,.~ _ . ..

QUADRIPHASE SPREADING CODES IN CODE DIVISION
MULTIPLE ACCESS COMMUNICATIONS

This is a divisional application of Canadian Patent Application Serial No.

2,335,018 filed on June 11, 1999.

FIELD OF THE INVENTION

The present invention relates to spread spectnun communications, and more particularly, to the generation of optimal code sequences used to perform spreading and de-spreading functions in a code division multiple access communication.
It should be understood that the expression "the invention" and the like encompasses the subject-matter of both the parent and divisional applications.

BACKGROUND OF THE INVENTION

A direct sequence spread spectrum (DSSS) system is a wide-band system in which the entire frequency bandwidth of the system is available to each user all the time. A DSSS system employs a spreading signal that expands or "spreads" the bandwidth of the transmitted signal much more than it is required for the transmission of infonmation symbols. The spreading signal is usually called a spreading or scrambling code or sequence. The tenm spreading code is generally adopted for this description.
Different users in a DSSS system are distinguished using the different spreading codes.
This is why DSSS systems are also referred to as Direct Sequence-Code Division Multiple Access (DS-CDMA) systems. In general, spreading codes are usually bi-phase, with elements belonging to the set {+l, -1 }, or polyphase, with elements belonging to the set of complex numbers corresponding to equidistant points on the unit circle in the la complex plane. For example, quadriphase corresponds to four points of unit length from the origin.

In general, there is a trade-off between increasing the number of spreading codes and decreasing interference. The number of spreading codes used to distinguish mobile station users, particularly on the uplink direction from a mobile station to a base , .,,., _ .....,.,M,..õ....- _.

station, should be as large as possible. This is because more spreading codes provide more radio channels so that more mobile stations can communicate at the same time in the same geographic area. But increasing capacity in a CDMA system comes at a cost--interference which reduces the quality of communication for all users.

s SUlViMARY OF THE INVENTION

It is desirable that the amount of correlation between any two of the spreading codes be reduced to minimizP the interference between the mobile stations communicating using those codes. More formally, the maximuin, periodic, cross-correlation between any two spreading codes should be as low as possible.
The periodic cross-correlation, also called even correlation, is equal to a correlation output under the assumption that the data modulation format does not change during the correlation operation. In practice, successive data modulation symbols have random rather than periodic values. Therefore, an odd correlation function better represents the correlation output when a data symbol of interfering signal changes during the correlation operation. While both the even and odd correlation Junctions should be evaluated to obtain an interference measure for any two spreading codes assigned to a pair of mobile stations to dctermine the degree of cross-correlation, odd cross-correlation is difficult to determine theoretically for a given set of spreading codes. Therefore, the even correlation function is used to compare different families or sets of spreading codes to determine an optimal family/set.

The present invention provides an optimal set of spreading codes for use for example in a wideband-CDMA (WCDMA) mobile radio communications system.
Although this set of spreading codes may be employed in synchronized, downlink transmissions from the base station, it is particularly useful in the uplink direction from the mobile station to the base station where the different mobile stations are not mutually synchronized. Thc optimal spreading code family provides a large number of codes thal also have low cross-correlation between spreading codes for all possible time shifis betwecn the different mobile stations. In this way_ the mobile communications system capacity is significantly increased while still providing satisfactory radio comrriunications with minimum interference to/from the other mobile stations.

In a preferred embodiment, the optimal code family is the S(2) family of four-phase code sequences of length L 2 ` - 1, where m is an integer greater than or equal to 5. The codes in the S(2) family are generated by summing modulo-4 three component sequences including a first component quatemary sequence a(n), a second component binary sequence b(n), and a third component binary sequence c(n), where the binary sequences b(ri) and c(n) are multiplied by two before summing. The size of the family, i.e., the number of quaternary spreading codes, is (L + 2)(L + 1) 2 , and the to maximum cross-correlation between any two of the codes is 1+ 4 (L + l) .
The three component sequences may be generated using corresponding linear feedback shift-register generators. The set of (L + 2XL + 1)2 different S(2) sequences is obtained by combining the different component sequences produced by the different initial shift register states: (L + 2) initial states for an a(n) sequence and (L + 1) initial states for ts b(n) and for c(n) sequences.

As an exaniple, the number of S(2) spreading codes having a length (L) of 255 chips is 16,842,752 with a maximum, absolute, even cross-correlation of 65.
Over 16 million uplink spreading codes provides considerable system capacity.
If one assumes that no more that 256 mobile stations will be served in a single base station 20 sector, then 65, 792 code sets may be re-used in the mobile communications svsteni.
This large number of code sets provides considerable flexibility in network planning.
Although 'spreading codes from the S(2) family may be randomly selected and allocated to various users in a CDMA mobile communications system, a preferred example embodiment of the present invention allocates the spreadina codes in 25 accordance with a specific code allocation procedure that achieves more advantaeeous results compared with random code selection_ Assuming the entire mobile communications system employs the S(2) family of codes, specific spreading code subsets of the S(2) family of codes are allocated to each base station (or base station sector). The spreading code subsets have the same cross correlation properties of the S(O) and/or S(l) families of codes and provide reduced interference for mobile stations operating in the same base station (or same base station sector) as compared to randomlv selected codes from the S(2) family of codes.

Capacity is one important aspect of a communications system, but services are also very important. There are certain services provided in mobile communications systems like WCDMA cellular systenis that may require or support more than one data rate. For variable rate and other services, it is desirable to provide spreading codes whose length may be expressed as an integer multiple of each spreading factor in the mobile comtnunications system. The spreading factor corresponds to the number of chips used to spread a single data symbol.
Relatively short spreading codes, whose code period covers one or more data symbols, are desirable in order to support low-complexity, multi-user detection at the CDMA
radio base stations.

One way of implementing multiple data rates is to use those data rates which allow corresponding spreading factors ~SF} to be expressed as SF(k) =
L12' , where L is the length of each spreading codes in the code family and k is a positive integer and varies in proportion to the data rate. Therefore, the spreading code length should be some power of two. Having the spreading code length expressible as an integer niultiple of each possible spreading factor in the system significantly alleviates overall synchronization in the receiver makino it independent of the data rate. In other words, if the spreading code period contains an integer number of data symbols, data frame and data synchronization in the receiver are derived automatically when the receiver despreading sequence is synchronized with tlle incomina signal.
Otherwise.

the data symbol position with respect to the (relatively small) spreading code period fluctuates over the time, i.e., it is different in consecutive spreading code periods. As a result, it is difficult to attach a single data synchronization signal to a spreading code period, and consequently, a separate circuit in addition to a code synchronization circuit 5 must be employed to acquire and track data synchronization.

However, the length (L) of the codes in typical spreading code families is 2'n -1, like the S(2) spreading code family described above. For example, if m=8, the code length is 255. In order to obtain the advantages of optimal high capacity at minimal cross-correlation code interference as well as support the variable data rate applications, the present invention extends the length of each spreading code by a code symbol to make the spreading code length -a power of 2_ In a preferred example embodiment, an additional code symbol is added to the end of each spreading code.
More specifically, the extended spreading code is obtained by adding another code synabol after L symbols of the original (non-extended) code of length L.

In one example embodiment, the added code symbol may be fixed, i.e., have the same value, for all spreading codes in the family. In other example embodiments, the added code symbol has the same value as the first chip in the original spreading code. In the case of quaternary spreading codes like those in the S(2) family, the additional spreading code symbol may have four possible values, i.e., 0, 1, 2, or 3. Preferably, the value of the additional spreading code symbol is selected to optimize the, mutual cross-correlation between the extended spreading codes.
Broadly in one aspect, the invention provides in a direct sequence spread spectrum (DSSS) mobile communications system in which a plurality of mobile radio stations communicate with one or more radio base stations located in corresponding geographic areas over a radio channel, each radio channel corresponding to one of a set of spreading codes, one or more of the radio stations comprising:
a spreading code generator configured to provide quatemary spreading codes from a family of quaternary sequences of length L=2' -l, where m is an integer greater than or equal to 5, having code elements from an alphabet {0, 1, 2, 3), generated by summing 5a modulo-4 three.component sequences including a first component quatemary sequence a, a second component binary sequence b, and a third component binary sequence c, where the component binary sequences b and c are multiplied by 2 before the modulo-4 summing;
a spreader configured to spread an information signal to be transmitted by the mobile radio using one of the quatemary spreading codes to provide a spread signal;
and a de-spreader configured to de-spread a received signal using one of the quatemary spreading codes.

Broadly in one aspect, the invention provides in a code division multiple access (CDMA) mobile communications system in which a plurality of mobile radio stations communicate with one or more radio base stations located in corresponding geographic areas over a radio channel, each radio channel corresponding to one of a set of spreading codes, one or more of the radio stations comprising:
a code generator configured to provide quadriphase spreading codes determined from an S(2) set of quatemary spreading codes having a maximal number of quaternary spreading codes with a minimal cross-correlation and to extend a length of the S(2) quatemary spreading codes to support multi-rate communications in the CDMA mobile communications system;
a spreader configured to spread an information signal to be transmit by the radio station using one of the quadriphase spreading codes allocated to the radio station to provide a spread signal; and a modulator configured to modulate the spread signal onto a radio carrier.

Broadly in one aspect, the invention provides in a code division multiple access (CDMA) communications system in which a plurality of communications.devices communicate using allocated communications channels, each channel corresponding to one of a set of CDMA spreading codes, a method comprising:
generating a family of original codes, each original code having a predetermined length;
and extending the length of original codes from the family of spreading codes by a code symbol to generate a family of CDMA spreading codes by detecting the end of one of the original codes, and adding the code symbol to the end of one of the original codes;

5b wherein the family of original codes has a length of L=2' -1, where m is an integer, and a single code symbol is added to the end of the original code periodically.

Broadly in one aspect, the invention provides in a code division multiple access (CDMA) communications system in which a plurality of communications devices communicate using allocated communications channels, each channel corresponding to:
generating a family of original codes, each original code having a predetermined length;
and extending the length of original codes from the family of spreading codes by a code symbol to generate a family of CDMA spreading codes by detecting the end of one of the original codes, and adding the code symbol to the end of one of the original codes;
wherein the family of original codes are an S(2) family of codes and the code symbol is selected to minimize cross-correlation between CDMA spreading codes.

Broadly in once aspect, the invention provides a CDMA code generator providing CDMA spreading codes, the generator comprising:
one or more feedback shift registers having m stages, where m is an iriteger, where an output of a last stage is fed back to an input of a first stage, the output of the one or more feedback shift registers corresponding to one of a family of codes; and electronic circuitry for adding an additional code symbol to the one code to provide an extended code corresponding to one of the CDMA spreading codes.

Broadly in one aspect, the invention provides in a mobile communications system including plural base stations for communicating with mobile stations and employing spreading codes from a particular spreading code family for radio communications between the mobile stations and the base stations, a method comprising:
allocating a first subset of the particular spreading code family to a first base station;
and allocating a second subset of the particular spreading code family to a second base station;
wherein the spreading codes in the first and second subsets have lower cross-correlation than spreading codes in the particular spreading code family.

5c Broadly in one aspect, the invention provides in a mobile conununications system including base station having plural sectors for communicating with mobile stations and employing spreading codes from a particular spreading code family for radio communications between the mobile stations and the base station, a method comprising:
allocating a first subset of the particular spreading code family to a first base station sector; and allocating a second subset of the particular spreading code family to a second base station sector;
wherein the spreading codes in the first and second subsets have lower cross-correlation than spreading codes in the particular spreading code family.

Broadly in one aspect, the invention provides in a code division multiple access (CDMA) communications system in which a plurality of communications devices communicate using allocated communications channels, each channel corresponding to one of a set of CDMA spreading codes, a method comprising:
generating a family of original S(2) quaternary spreading codes, each original code having a predetermined length; and extending the length of original codes from the family of original S(2) quatemary spreading codes by a code symbol to generate a family of CDMA spreading codes without having to increase the number of the original codes from the family.

Broadly in one aspect, the invention provides a CDMA code generator providing CDMA spreading codes, the generator comprising:
one or more feedback shift registers having m stages, where m is an integer, where an output of a last stage is fed back to an input of a first stage, the output of the one or more feedback shift registers corresponding to one of a family of S(2) quaternary spreading codes of length L=2' -1; and electronic circuitry for adding an additional code symbol to the one S(2) quatemary spreading code to provide an extended S(2) quaternary spreading code corresponding to one of the CDMA spreading codes.

5d BRIEF DESCRIPTION OF'THE DRAWINGS

The foregoing and other objects, features, and advantages of the invention will be apparent from the following description of preferred embodiments as well as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Fig. l is a function block diagram of an example mobile communications system in which the present invention may be advantageously employed;

Fig. 2 is a function block diagram of an example radio station transceiver in which the present invention may be advantageously employed;

Fig. 3 is a function block diagram illustratina additional details of the spreader and modulator blocks shown in Fig. 2;

Fig. 4 illustrates a unit circle diagram illustrating four quadriphase values lo in a complex plane;

Fig. 5 is a flowchart diagram illustrating example procedures for providing a spreading code from an optimal S(2) spreading code family in accordance with the present invention;

Fig. 6 is a schematic diagram illustrating in further detail the code generator shown in Fig. 2;

Fig. 7 is a schematic diagram illustrating an extended spreading code generator in accordance with a fixed extended symbol example embodiment; and Fig. 8 is a schematic diagram illustrating an example extended spreading code generator in accordance with a periodic extended symbol example embodiment;
Fig. 9 is a function block diagram illustrating example procedures in accordance with an extended spreading code embodiment of the present invention; and Fig. 10 is a graph illustrating a performance of the fixed and periodic extended spreading codes.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular embodiments, procedures, techniques, etc., in order to provide a thorough understanding of the present invention.

However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. For example, while the present invention is sometimes described in the context of a mobile radio station using uplink spreading codes, the present invention is equally applicable to other radio stations, e.g., radio base stations, and indeed, to any spread spectrum io communications svsteni. In other instances, detailed descriptions of well-known methods, interfaces, devices, and signaling techniques are omitted so as not to obscure the description of the present invention with unnecessary detail.

The present invention is described in thc context of a universal niobile telecomniunications system (UMTS) 10 shown in Fig. 2. A representative, connection-oriented, e;ctemal core network, shown as a cloud 12 may be for example the Public Switched Telephone Network (PSTN) and/or the intearated Services Digital Network (ISDN). A representative. connectionless-oriented external core network shown as a cloud 14, may be for example the Internet. Both core networks are coupled to eorresponding service nodes 16. The PSTN/ISDN connection-oriented network 12 is connected to a connection-oriented service node shown as a Mobile Switching Center (MSC) node 18 that provides circuit-switched services. In the existing GSM
model, the MSC 18 is connected over an interface A to a Base Station Subsystem (BSS) 22 which in turn is connected to radio base station 23 over interface A'. The Internet connectionless-oriented network 14 is connected to a General Packet Radio Service (GPRS) node 20 tailored to provide packet-switched type services. Each of the core network service nodes 18 and 20 connects to a UMTS Radio Access Network (URAN) 24 over a radio access network (RAN) interface. URAN 24 includes one or more radio network controllers 26. Each RNC 26 is connected to a plurality of base stations (BS) 28 and to any other RNC's in the URAN 24.

In the preferred embodiment, radio access is based upon wideband, Code Division Multiple Access (WCDMA) with individual radio channels allocated using CDMA spreading codes. WCDMA provides wide bandwidth for multimedia services and other high rate demands as well as robust features like diversity handoff and RAKE
receivers to ensure high quality. Each mobile station 24 is assigned its own spreading code in order for a base station 20 to identify transmissions from that particular mobile station as well as for the mobile station to identify transmissions from the base station intended for that mobile station from all of the other transmissions and noise present in the same arca.

A CDMA radio station transceiver 30 in which the present invention may be employed is shown in Fig. 2 in fur,ction block format. Those skilled in the art will appreciate that other radio transceiver functions used in CDMA transceivers not is particularly relevant to the present invention are not shown. In the transmit branch, information bits to be transmit are received by a spreader 32 which spreads those information bits over the available frequency spectrum. (for wideband CDMA
this frequency band could be for example 5 MHz, 10 MHz, 15 MHz or more), in accordance with a spreading code generated by a spreading code generator 40. Controller determines which spreading code should be provided by code generator 40 to spreader 32. The spreading code provided by code generator 40 corresponds to a radio channel in a CDMA conimunications system. Because a very large number of code symbols (sometimes called "chips") may be used to code each information bit, (depending on the current data rate in a variable data rate system such as a WCDNIA

system), the spreading operation considerably increases the data rate thereby expanding the signal bandwidth. The spread signal is provided to a modulator 34 which niodulates the spread signal onto an RI: carrier. An oscillator 42 generatcs an appropriate radio frequency carrier at a frequcncy selected by the controller 44. The modulated RF signal is then filtered and amplified in RF processing block 36 before being transmitted over the radio interface by way of antenna 38.

Similar but reverse operations are carried out in the receive branch of the transceiver 30. An RF signal is received by antenna 38 and filtered in RF
processing block 150. The processed signal is then demodulated to extract the baseband signal from the RF carrier in a demodulator 48 using a suitable RF carrier signal provided by the oscillator 44. The demodulated signal is de-spread in a de-spreader 46 in accordance with a code selected by the controller 44 and gerierated by the code generator 40. The de-spread signal corresponds to the received information bits at baseband which are then typically fui-ther processed- While individual functional blocks are shown in the radio station transceiver 30, those skilled in the art will appreciate that these functions may be performed by individual hardware circuits, by a suitably programmed digital microprocessor, by an application specific integrated circuit (ASIC), and/or by one or more digital signaling processors (DSPs).

Fig. 3 illustrates in schematic form further example details of the spreader 32 and the modulator 34. A similar schematic would apply to the demodulator 48 and the de-spreader 46 with opposite functions in the reverse direction.
Quadrature phase shift keying (QPSK) is used both for the data modulation (performed by spreader 32). and the spreading modulation (perfornied by quadrature modulator 34)..

Fig. 4 illustrates four quadriphase points in the unit circle corresponding to the complex plane defined by a real axis I and an imaginary axis Q. The four quadriphase alphabet values correspond to n 3a 3)r a j- j---- -j-- -j-e 4,e a.e 4 and e 4 where j=~.

The example spreader 32 in Fig. 3 includes two biphase (+/- 1) information streams to be demodulated separately, such as a traffic data stream and a control data stream, which are input to respective multipliers 52 and 54 in order to be spread and IQ multiplexed. The traffic and control data streams are spread by different 5 channelization codes and then mapped to the I and Q branches. Channelization codes are employed to separately identify and distinguish the real and imaginary information streams at the receiver, even if there is imperfect I and Q phase synchronization at the receiver. In the situation where plural traffic and control data streams are to be transmit in parallel from a single mobile user, (e.g., multicode transmissions --intended for very 10 high data rates), plural orthogonal channelization codes are used to make the necessary parallel code channels. The channelization codes can be based on so-called Orthogonal Variable Spreading Factor (OVSF) codes which maintain orthogonality even if different spreading factors are used. The channelization codes are common for all mobile stations.

The I and Q information streams represent the real and imaginary parts of a complex data stream to be transmit over a CDMA radio channel. In the present description, separate real and imaginary information streams and corresponding different channelization codes have been employed to generate a complex signal to be spread using a corresponding radio CDMA spreading code. However, the signal need not be complex. Indeed, the present invention may be employed to spread any type of information signal.

The spreading code generated by spreading code generator 40 is employed by the complex multiplier 60 to spread the complex information signal. The complex multiplier 60 in a QPSK data modulator performs complex multiplication between the complex data stream I + jQ and a complex spreading code (e.g., temporarily allocated to a mobile station) to provide the spread signal output to the modulator 34. Quadrature modulator 34 splits the spread signal into real (I) and imaginary (Q) streams which are processed by a corresponding pulse shaping filter 62, WO 99/66645 PC'T/SE99/01040 64, such as a root-raised cosine filter, and then provided to respective mixers 66 and 68 which also receive in-phase and quadrature versions of the RF carrier. The modulated carrier quadrature signals are summed in summer 70 and output to the RF
processing block 36.

As mentioned above, the number of CDMA spreading codes used to distinguish mobile station users, particularly on the uplink direction from a mobile station to a base station should be as large as possible to allow more mobile stations to communicate at the same time in the same geographic area. On the other hand, the number of spreading codes cannot be too large; othernvise, there is too much interference generated aniong mobile stations to have an acceptable communication.
The present invention provides a set of spreading codes with an optimal balance: a relatively large number of spreading codes with only minimal periodic cross-correlation between any two of the spreading codes in the faniily.

For comparison, parameters of various bi-phase and quadriphase ts spreading code families are shown in Table I below. The alphabet size corresponds to the number of different valucs that each code symbol may assume. For bi-phasc codes, the alphabet size is two; for quadriphase codes the alphabet size is four. The sequence length (L) is the number of code synibols ("chips") in each code and for all code families in the Table I is equal to 2 ' -1, where 'm is a positive integer whose possible values may be restricted depending on how the particular code family is constructed.
The family size (M) is the number of codes in a particular spreading code family. The larger the family size M. the greater the eapacity. The niaximum absolute cross-correlation (C,,,at ) is the maximum, periodic, cross-correlation between any two spreading codes in the spreading code family.

TABLEI
Spreading Alphabet Sequence Length Family Size (M) Max. Abs. Cross-Code Size (p) Length (L) Restriction Corr. (C.) Family Gold 2 2n_I rn=lmod? L+2 I+ 2 L+I
Gold 2 2M_ 1 nr= 2 mod4 L+2 1+2 L+
Gold-like 2 2m - I t = 0 mod 4 L+ 1 i+ 2,F1 + 1 Reciprocal 2 2"' - I nr = 0 mod 2 L + 2 2 L+ I-Gold-like Small 2 2-t _1 ur=0mod2 L+1 I+ L+1 Kasami Large 2 rn = 2 mod 4 ( L+ 2)4-L I 1+ 2 L+ I
Kasam i Large 2 2' -1 m=0mod4 (L+2) L+I-I I + 2L+1 Kasami Very Large 2 ~-) u1=0mod2 (L + 2)2[+l 1+4 L+l Kasami Verv Lar e 4 2u~ _ 1 nr = 0 mod 2 L+~2 L+ I
Kasami g ( ~) ~(1+4 L T I) Family s(o) 4 2n'- 1 None L + 2 1+ L+
Family s(l) 4 2 '_ 1 m>_ 3 (L+2)(L+t) 1 +2 L+ 1 Family - I Y s(z) 4 rn>_ 5 (L+2)(L+I)Z 1+4 L+ l Based on this analysis of various characteristics of these code families, the inventor determined that the S(2) family of spreading codes offers.the optimal cotnproniise between the largest number (M) of spreading codes, (L + 2)(L +
1)2 , and the smallest cross-correlation, 1+4 L+ I.]n other words, for the S(2) spreading s codes, the ratio of the number of spreading codes to the cross-correlation peak is maximized for a given spreading code iength L. The S(1) and S(2) spreading code faniilies are obtained by generalizing from the construction of the S(O) family of l~ -quadriphase spreading codes, The S(2) spreading code family includes the S(l) family, which is the subset of (L + 2XL + l) spreading codes obtained by combining different a(n) and b(n) component sequences. The S(2) and S(l) spreading code families include the S(0) family, which is the subset of (L + 2) spreading codes obtained by different initial.states of an a(n) component sequence shift-register. The S(0) spreading code family has the same number, of spreading codes as the Gold spreading code family, but the S(O) family has a smaller cross-correlation at least by factor of vr2-.

In order to provide a better understanding of the present invention, io construction of the S(2) family of spreading codes is now described. Let h(x) = x' + hi x'"-~ +...+h,,,_,x+ h, where hk , x E Z4 , be a primitive polynomial over Z4 of degrec m, where Z4 is the set of integers 4 0,1,2,3} . i.e., the ring of modulo-4 integers. A list of all the primitive polynomials over Z4 up to degree m= 15 can be found in "On a Recent 4-Phase Sequence Design for CDMA;" Hammons et al., IEICE

ts Trans. Commun:, vol_ E76-B, no. 8, pp. 804813. The,nth=order linear recurrence ar (n) over Z4, defined by h(x) as a,.(n) = h ja,(n-1)-h2a,(n-2)-. ..-ha,(n-m) (mod 4), n>_ m, (1) produces a quaternary sequence of period L = 2' - 1. The above recurrence may be implemented using a shift register with feedback connections.

20 There are L + 2 cyclically distinct sequences which can be obtained from the recurrence defined in equation (1) by choosing an appropriate initial state of the recurrence, i_e., of the shift register. The initial state rr is a vector of m elements.
which can be represented as r, [ar (nr - 1), a, (ln - 2),... ar (0)]=

The L+ 2 initial states ro, ri, r2,... , rL+1 can be chosen according to the following algorithm -M-z ~
Tr - [T(yXm1), 7~Y,x ), ... , T(Yrx), T(Y,~ (-) where ~ q ~n'' I l T(~~ = ~~ + ~- + ~ + + ~- (mod Ir(x), mod 4)J (mod x), (3) and Yo=1= Yi=2, y2=3.

73 = 1- x,Yq = 1- x2.... YL+I = 1- x(niod h(x), niod 4) (4) io The set of sequences la,. (n)j defined by equations (1)-(4) represents the S(O) family of sequences whose paramcters arc given in Tablc I.

The set of sequences { a,. (n)} of length 255 can be generated by a degree 8 primitive polynomial over Z4. The primitive polynomial of degree 8, whicli provides the simplest feedback connections of the corresponding shift-register generator, is as follows:

h(x) =Xx+x5+3X3+x`+2x+1. (5) The S(l) family of sequences {yõ(n)} , u = 0,1,...,(L + 2XL + l) - 1, is the generalization of the S(O) sequence family obtained by combining the quaternary sequences from the set {a,. (n)} , r = 0,1,2,...,(L + 1), with the binary sequences {bs (n)) , s 0,1, 2,..., L, of the same length. The exact algorithm is given by the following relation:

yõ (n) = a, (n) + 2bs(n) (mod 4), n= 0,1,..., L- L. (6) The sequences bs (n) are obtained by a linear recurrence over Z,, defined by the polynomial g(x) = x` + glY`-I+...+ ge_I x + 1 as bs(n)=glbs(n-1)+g2b.,(n-2)+. , .+bs(n-e) (mod 2), n e, (7) where e<ni is a minimum intecer satisfying (3. 2') mod (2' - 1) = 3.

The polynomial g(x) is related to the polynomial h(x) and is obtained from the polynomial g(x)~ , given by lo g(x)(x-x3)(x-(x3)2)x-(x3)ZZ ... x-(,r3)Z~ (mod h(x),mod 2), (8) according to the following relation = g(x) , e < m g(x) [h(x) + g(x) ~ mod 2, e= m (9) For h(x) given by equation (5). the corresponding g(x) is equal to g(x) =xK+x7+xs+.r+1. (10) The set of L + I distinct (but not cyclically distinct) sequences bs (n) is defined by the appropriate initial states of the recurrence defined in equation (7). The L +
I initial states SO,Sj.S,,,..,sL are defined as So = 0, Si = 1, (1 1) C52 = x, 83 = x 2,..., SL = X L-1 (mod h(x), mod 2).

The actual initial state [b-s (m - 1), bs (m - 2),..., bs (0)] is given by the coefficients o f corresponding polynomial 8S defined by equation (11) according to the following notation:

8S =b,.(m- 1)x"'-I +bs(m-2)x"'-2+==-+bS(0) The S(2) family of sequences {z,, (n)}, v= 0, i, 2, ... ,(L + 1)(L + 1)2 1, is a further generalization using the S(0) and S(1) families. It is obtained by combining the sequences from the previouslv defined sets {a,. (n)} and {bs (n)j with an additional set {c, (n)} of L + I binary sequences. according to the following relation:
zõ(n)=a,.(n)+2bt(n)+2c,(n) (mod4), ~o (12) n=0,1,...,L- l.

An enumeration algorithm for the set S(2) can be defined by (/.+'x/.+i) t+i vr=2 +s=2 +l r=0,1.2,...,L+l (13) s=0,1,2,...,L
t=0,1,2,...,L
The sequences c,(n) are obtained by a linear recurrence over Z, , defined by the polynomial f(z) = x" + fix"-I +...+J,-ix + I as c, (n) = . fi c, (n - l )+ fz c, (n - 2)+...+c, (n - e) (nrod 2), n >- e , (14) WO 99166645 PCr/SE99/01040 where e< m is a minimum integer satisfying (s. 2`~ mod (2'" - 1) = 5. The polynomial x is related to the ol nomial h x and is obtained from the ol nomial xf( ~ P Y
(} P Y J~~ }
given by f(x)r =(x - x5 )(x -(x5)2)[x - (x5)22(mod h(x), mod 2),( 15) according to the following relation _ f(X)- f(x) f e<m (16) [h(x) + f(x)]nioat2. e= m For h(x) given by equation (5), the corresponding f(x) is equal to f(x)=xR+x7+x5-1-x4 +1. (17) The set of L + I distinct (but not cyclically distinct) sequences c, (n) is defined by the appropriate initial states of the recurrence (14). These initial states are already defined by equation (11).

The above constructions for the S(2) spreading code family produce quatemary codes with elements belonging to the set 10,1, 2. 31. To obtain coniplex quadriphase spreading codes having a constant envelope, with real and imaginary parts is being bi-phase values, i.e., with elements belonging to the set ~

T ? n Jf j- j3- j3-- j-e 4, e a, e `' , e 4, the following transformation is applied n n j-S2(n}-e 4 . e 2 (IS) With this mathematical explanation of how the S(2) family of spreading codes is constructed, reference is now made to a Mobile Call routine (block 80) illustrated in function block format in Fig. 5. lnitially, a mobile station requests a traffic channel (TCH) by sending a traffic channel request over a random access channel (RACH) (block 82). The random access channel has one or more corresponding spreading codes which the mobile station employs to transceive over that random access channel. In response to the mobile's request, the base station sends over the randoni access channel to the mobile station the ntrmber " v" of a spreading code zv (n) from the S(2) spreading code family (block 84) corresponding to an allocated radio channel.
io Zõ(n) is defined in equation (12), and v is defined in equation (13) above.
Using the spreading code number v, the mobile station determines the ordinal numbers r, s, and t which uniquely identify the initial states of shift re~isters used to generate the three component sequences a,. (n), bjn). and c, (i) deiined above in equations (1), (7) and (14), respectively. I'hose thrce component sequcnces arc combined to provide a corresponding S(2) quaternary spreading code z,. (n) in accordance with equation (12) (block 88). "hhc S(2) quatcrnary snreading ccxle is then mappcd to a corresponding quadriphase spreading code (block 90) and used to spread/de-spread (depending upon transmit or receive operation currently being performed in the mobile station) information using the generated quadriphase spreading code (block 92).

Fig. 6 illustrates an example shift register implementation of a code generator 40 for generating S(2) quadriphase spreading (and de-spreading) codes in accordance with one example embodiment of the present invention. Code generator 40 includes three linear, feedback shift registers 100, 102, and 104. Each shift register includes eight memory elements (shift stages) 0-7. At the beginning of each chip interval, the content of each memory element is moved (shifted) to the adjacent, right-hand memory elenient. The outputs of the memory elements are multiplied by the coefficients of the respective recurrence equation and then summed modulo 4 (or 2).

= =

The result of summation is stored in the left-most memory eiement at the beginning of the subsequent chip interval.

Shift register 104 implements the linear recurrence a, (n) defined in equation (1). Shift register 102 generates the bs(n) sequences, and shift register 100 generates the c, (n) sequences in aecordanee with equations (?) and (14). The outputs of shift registers 100 and 102 arc multiplied by two in respective multipliers and 108. Each of the three sequences output by the correspondingthree shi ft -registers is summed in sumnier 1 10 to generate an S(2) quaternary code which is converted to a corresponding S(2) quadriphase spreading code by wav of niapper 112. Of course, the S(2) quaternary code output depends on the actual initial state set in the shi(t registers which are determined in accordance with equations (2), (3), (4), (11). Those initial states may be input into the appropriate shift registers by the transceiver controller 44.
which scts the appropriate values of the adjustable parameters in the transtnitter and receiver, both in the mobile and base station transceivers. Although in a preferred is embodinient, the spreading code generator 40 is implemented using shift registers which generate the necessary S(2) spreading codes as needed, those S(2) spreading codes could be generated in advance, stored in memory, and retrieved using a table lookup function.

Thus, the present invention provides a family of quadriphase, CDMA
spreading codes that provide a maximal number of CDMA spreading codes of a particular length having a minimal cross-correlation. At the sanie time, these spreading codes have a small signaling alphabet which is very convenient for the practical implementation of the sprcader and the despreader.

Although the S(2) faniily of spreading codes may be allocated randonily, a preferred embodiment allocates codes from the S(2) family in a more advantageous fashion. As shown above, the S(I) and S(0) spreading code faniilies are subsets of the S(2) code family and have better cross-correlation properties, and therefore produce less interference between mobile users. Table 1 above shows that the S(I) code family and the S(0) code family have one half and one fourth of the maximum absolute cross-correlation as the S(2) code family, respectively.

5 In this preferred embodiment, the large number of codes provided by the S(2) family is employed by the mobile communications system, but specific subsets of the S(2) codes are allocated to particular base stations or base station sectors.
Consequently, depending on the number of mobile users in a particular area of a CDMA
cellular network, service quality is iniproved, i.e., less interference between niobile to users connected to the same base station or base station sector. For example, the mobile communications system niay use S(2) spreading codes of lengtli L = 255. A
first base station BSO is allocated the subset of S(2) codes defined by the component sequences having indices r = 0, l, 2...,256; s = 0; and t= 0. In other words, BSO is allotted the "pure" S(O) familv of codes. A second neighboring base station BS 1 is allocated - s another subset of S(2) spreading codes defined by the component sequences corresponding to indices r = 0,1,2...,256; .s = L. and t= 0. The second base station codes are very similar to the pure S(0) codes, (the S(0)codes are multiplied chip-by-chip with a common component sequence bi(n)), and have essentially the sanie eharacteristics. As a result of this S(2) code subset allocation, the cross-correlation 20 between those allocated S(2) codes for each base station is the same as for the S(0) code faniily. i.e., less cross-correlation between codes as conipared to that for the S(2) family in general.

Using such an S(2) subsct code allocation strategy, mutual interfcrencc between niobile stations connected to the same base station is minimized, and the interference between the base stations is also bounded according to the properties of the WO 99/66645 PCr/SE99/01040 S(2) codes. The S(2) subset code allocation strategy may be generally defined as follows: each BS (or BS sector) has at least L + 2 spreading codes from the S(2) family defined by three component sequences having an index r = 0,1,2..., L +
I and indices s and 1 that are unique for each base station (or base station sector), i.e.. the indices s and t have different integer values for different base stations.

While this subset code allocation scheme is advantageous in that it reduces cross-correlation between mobile users in a base station/sector as conipared to the general S(2) code family, hand-over situations require some special provision. For the duration of a call, the mobile station keeps the same spreading code allocated at the to beginning of the call by the source base station/sector even if the mobile changes from the original source base station during hand-over to a destination base station. Using the code allocated by the source base station while connected to the destinatiori base station may produce interference greater than that for the S(0) family. But that interference is still no greater than that defined for the S(2) code set.

In the hand-over situation where the source base station allocated a particular spreading code to the mobile station, the source base station is prevented troni allocating that same spreading code before the handed-over mobile station finishes the call to avoid the situation where two mobile stations are assigned the same code. One way of accomplishing this is for the source base station to assign a time-out flag to each 20 available spreading code. The tinie-out flag is set, meaning that the code may be allocated to another mobile only if a predefined time interval has passed since the code was allocated. Alternatively, the flag has a non-zero time-out value only when the mobile is in handover with the time out interval conimencing at the tinie of handover.
Either way, the same code is prevented from being assigned to two mobile stations at 25 the same tinie when the niobiles are connected to the neighboring base stations.

The construction of the S(2) or other family of spreading codes produces spreading codes that each have a length L = 2' - I. Consequently, each code's length is not a power of 2. However, in a CDMA system that supports different data rates over the same physical radio channel depending on which of different services is currently operating, the spreading code length should be expressed as a multiple of each spreading factor existing in the multi-rate CDMA systeni. The spreading factor is the number of chips (plural chips are used to spread one data bit) within the data symbol.
One way of iinplemeriting multiple data rates is to use those data rates that permit the cornesponding spreading factors (SF) to be expressed as io SF(k) = Ll 2~

where the variable k is proportional to the data rate. Moreover, since the number of chips within the data symbol should be an integer, the spreading sequence length should be a power o f 2.

Consequently, spreading code sequences belonging to the S(2) family 1s should be extended with one quaternary symbol for optimal use in a multi-rate CDMA
system. The present invention resolves this need by providing a spreading code extension without increasing the maximum cross-correlation between spreading codes in the spreading code family, with miniiliuni hardware iniplementation complexity.

In a preferred embodiment that tries to reduce hardware implementation 20 complexity, the spreading code synibol is added to the end of the original spreading code in order to extend the code by one symbol. Of course, the length of the original spreading code could be extended by adding a code symbol to other locations in the original code. In other words, extended spreading codes may be obtained by adding an additional spreading codc. symbol. after the L symbols of the original, non-extended 2 s spreading code of length L= 2"' - I.

WO 99/66645 PCr/SE99/01040 In one fixed code extension example embodiment, the additional spreading code symbol is fixed, i.e., the same for all spreading codes. In the case of quaternary codes like the S(2) family of spreading codes, the additional spreading code symbol can have four possible values. The particular code symbol value, i.e., chip s value, may be chosen to minimize the mutual cross-correlation between extended sequences in a set i.e., the S(2) sequences.

An example of the fixed code extension embodiment is shown in Fig. 7 for original S(2) spreading codes of length 255 where like reference numbers refer to like elements from Fig. 6. The code generator 40' in Fig. 7 includes comparator 120 lo connected to the outputs of each memory eiement of the shift register 104 which generates the component sequence a,. (n). In addition, a con: espending register 122 containing the initial state r, of the shift register 104 is connected to the remaining available inputs of the comparator 120. Still further, a switch bfock 124 is connected at one input terminal to the output of summer 110. The other input terminal is connected 15 to the fixed code symbol value x, and the output of the switch is connected to mapper 112. Outputs froni coniparator 120 suspend the shifting operation of all three registers 100, 102, and 104 as well as control the state of the switch 110.

In operation, the comparator 120 detects the end of an original S(2) spreading code by detecting the end of component sequence a,(n). Only the 20 component sequence a,, (n) has the same period as the S(2) spreading code.
The other two component sequences. bS (n) and c, (n), have shorter periods which are contained in the period of an S(2) spreading code, and tlterefore, they are not used to detect the end of S(2) spreading code. The end of component sequence a, (n) is detected by detecting the subsequent periodic occurrence of the same state of the shift register 104 25 which was loaded in register 104 at the initialization of the code generator 40 operation.
During the initialization of the code generator 40, all three shift registers 100, 102.

and 104 are loaded with the corresponding initial states and then released to run in parallel. However, only the internal state of the shift register 104 is monitored by the comparator 120.

When the end of the original spreading code is detected by the comparator 120, the comparator 120 generates a shift suspend operation during the next spreading code symbol cycle. At that time, the extension symbol x, which can be any one of the set of values 0, 1, 2, or 3, is added to the end of the code when the switch 124 is momentarily connected to the x terminal in accordance with an output from the comparator 120. During that time, the internal states of all three shift reaisters remain unchanged. As a result, the S(2) spreading code is extended by one symbol for a total of 256 symbols which is a power of 2, i.e., 2R = 256. After inserted chip interval. thrcc shift registers start shifting from the corresponding initial states without actual re-loading of those initial states.

Fig. 8 shows an example of a periodic code extension embodiment in is which the original spreading code is extended by a single chip whose value is the same as that of the first symbol in the original spreading code. The structure and operation of the code generator 40" shown in Fig. 8 is similar to that described above for the code generator 40" shown in Fig. 7. However, the switch 124 is not employed; nor is there any external source "x" that supplies the extra chip. Instead, when the end of the original spreading code is detected by the comparator 120, the corresponding output of the code generator 40 represents the extended (256 - th) chip value. The shifting in all shift registers is suspended durinc, the next chip cycle-so the same state, equal to the initial state, appears in the first chip cycle of the next spreading code period. After the inserted chip interval, the three shift registers continue shifting from their corresponding initial states without actually re-loading those initial states.

Accordingly, an additional symbol code equal to the first symbol in the original spreading code is inserted after the last symbol in the original spreading code with no added hardware. This same periodic extension may be implemented using a modulo counter, modulo-256 in this example where L = 255, (more generally the modulus of the counter is equal to the period of the extended spreading code), which indicates the end of extended spreading code. In operation, the shift registers are 5 reinitialized as usual at the end of the code period and generate as the next chip output the first chip of the code as determined by the initial states of the shift registers. But after this lirst chip is output, (thereby extending the generated spreading code by onc chip), the counter generates an output that causes the shift registers to re-load their respective initial states so that the extended code generation operation is again re-10 started_ Reference is now made to an extended code routine (block 200) which illustrates an example procedure in accordance with the present invention.
Initially, a family of original spreading codes is generated, each code having a length L

(block 202). I-or each gcneratcd spreading code, the cnd of that original spreading code 15 is detected (block 204). Shifting and linear feedback operations in the code generator are momentarily suspended (block 206). A decision is made in block 208 whether the fixed spreading code extension procedure or the periodic spreading code extension -procedure described in the two example embodiments immediately above is selected.
For periodic spreading code extension, a spreading code symbol equal to the first 20 symbol in that code is added to the end of the spreading code (block 210).
For a fixed spreading code extension, a fixed code symbol is added to the end of the spreading code (block 212). The code extension process is repeated for each generated code (block 214). Of course, once the decision has been made as to the particular type of extension, the decision in block 208 no longer need be made.

25 For the S(2) family of spreading codes, both of the extension procedures described above can be performed easily and with minimal hardware. These extended S(2) codes provide the necessary flexibility to optimize multirate communications while permitting the largest number of users balanced with minimal cross-correlation between extended codes. Because the cross-correlation properties of extended codes is difficult to predict theoretically, the following performance evaluation of extended S(2) spreading codes is done numerically.

The performance of the fixed and periodic spreading code extensions is considered now in conjunction with Fig. 10 and is based on the calculation of the average bit error probability Pe in a multiple access system with K concurrent users.
The bit error probability calculation is implemented relying on a numeTical evaluation of an analytical formula that,includes (K-2)-fold convolution of the code-pair cross-correlation probability density function as follows:

co FN( z 10 ~'e jll(z) l- dz , (19) Eh where ~

Q(z) _ ~e_ tz2 dl, 2ir _ Eb is the data bit (spreading sequence) energy, No is the additive white Gaussian noise power spectral density, and f/ (z) is the multiple access interference probability density function (PDF). The function fl(z) is obtained by (K-2)-fold convolution of the code-pair cross-correlation PDF , fPp;, (z') , i.e., x-i f l 1 Z/ - J P ir (Z I ) * f pi,- W). . . * f,.ir W) = (20) A BPSK data modulation format and time shifts between users corresponding to the integer multiples of the code symbol (chip) period were assumed so that the cross-correlation probability density function can be presumed discrete. The cross-correlation probability density function was obtained by counting all the different values of the real part of even and odd cross-correlations within a given set of spreading codes. Cross-correlation probability density function was evaluated for the extended S(1) spreading codes (which form a subset of S(2)spreading codes) of length L
= 32.

It was found that both the fixed and periodic extension approaches have about the same performance. The average bit error probability Pe for K = 4 concurrent users using periodically extended S(1) sequences of length 32, as well as S(1) sequences extended by a fixed syniboi (equal to 3) is shown in Fig. 10.
Comparing performance between the non-extended and extended S(1) sequences, one would expect, hased on the rr-aximum absoltite period cross-correlation (C,,,(L% ), that the non--o extended spreading codes should have better performance because they have a smaller Cm... r value. If the number of users is K = 4, the periodically extended spreading codes produce a slightly higher average bit error rate. However, when the number of.users increases to K = 6, the extended codes surprisingly produce a lower average bit error rate than non-extended spreading codes. This latter relationship remains valid for all other numbers of users greater than 6 which is another advantage of the extended spreading code embodiments of the present invention. The explanation may lie in the properties of the odd cross-correlation function, which dominantly influences the shape of the code-pair cross-correlation probability density function fP,,;,(z'), both for the non-extended and extended spreading codes. The shape of the multiple access interference probability density function f/ (z) , which directly determines the averagc bit error rate, is influenced both by the shape of the function fP,,;, (z') and by the number of self-convolutions of fP,;,. (z) in equation (20), i.e., by the number of concurrent users.

While the present invention has been described with respect to a particular embodiment. those skilled in the art will recognize that the present invention is not limited to the specific embodinients described and illustrated herein.
Different forniats, WO 99l66645 PCT/SE99/01040 embodiments, and adaptations besides those shown and described as well as many modiFcations, variations, and equivalent arrangements may also be used to implement the invention. Thcrcforc, while the prescnt invcntion has hcen dcscribcd in relation to its preferred embodiments, it is to be understood that this disclosure is only illustrative s and exemplary of the present invention and is merely for the purposes of providing a full and enabling disclosure oFthe invention. Accordingly, it is intended that the invention be limited only by the spirit and scope of the claims appended hereto.

Claims (14)

1. The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
   
   I. In a mobile communications system including plural base stations for communicating with mobile stations and employing spreading codes from a particular spreading code family for radio communications between the mobile stations and the base stations, a method comprising:
   allocating a first subset of the particular spreading code family to a first base station;
   and allocating a second subset of the particular spreading code family to a second base station;
   wherein the spreading codes in the first and second subsets have lower cross-correlation than spreading codes in the particular spreading code family.
2. 2. The method of claim 1, wherein the particular spreading code family corresponds to an S(2) code family and the first and second subsets are associated with one or both of an S(O) code family and an S(1) code family.
3. 3. The method of claim 2, wherein each of the first and second spreading code subsets is defined by three component sequences such that a first component sequence includes an index of r=0, 1, 2..., L+1, where L is the spreading code length, and one or more of indices for the second and third component sequences are different for the first and second base stations.
4. 4. The method of any one of claims 1 to 3, further comprising:
   for a particular call, assigning a mobile station associated with the first base station an assigned code from the first subset of spreading codes;
   associating a flag with the assigned code;
   setting the flag to a first value when the mobile station is involved in the call;
   setting the flag to a second value after a prescribed time expires; and prohibiting assignment of the assigned code to another mobile station until after the prescribed time expires.
5. 5. The method in claim 4, further comprising:
   setting the flag to the first value at the beginning of the call; and measuring the prescribed time from the beginning of the call.
6. 6. The method in claim 4, wherein the mobile station employs the assigned code for the duration of the call even when the call is handedover to the second base station.
7. 7. The method in claim 6, further comprising:
   setting the flag to the first value when the mobile is involved in the call;
   measuring the prescribed time from a time associated with the handover; and if the mobile station has not been in a handover during the call, the flag is set to the second value at the end of call.
8. 8. In a mobile communications system including base station having plural sectors for communicating with mobile stations and employing spreading codes from a particular spreading code family for radio communications between the mobile stations and the base station, a method comprising:
   allocating a first subset of the particular spreading code family to a first base station sector; and allocating a second subset of the particular spreading code family to a second base station sector;
   wherein the spreading codes in the first and second subsets have lower cross-correlation than spreading codes in the particular spreading code family.
9. 9. The method of claim 8, wherein the particular spreading code family corresponds to an S(2) code family and the first and second subsets are associated with one or both of an S(O) code family and an S(1) code family.
10. 10. The method of claim 9, wherein each of the first and second spreading code subsets is defined by three component sequences such that a first component sequence includes an index of r=0, 1, 2..., L+1, where L is the spreading code length, and one or more of indices for the second and third component sequences are different for the first and second base station sectors.
11. 11. The method of any one of claims 8 to 10, further comprising:
    for a particular call, assigning a mobile station associated with the first base station sector an assigned code from the first subset of spreading codes;
    associating a flag with the assigned code;
    setting the flag to a first value when the mobile station is involved in the call;
    setting the flag to a second value after a prescribed time expires; and prohibiting assignment of the assigned code to another mobile station until after the prescribed time expires.
12. 12. The method of claim 11, further comprising:
    setting the flag to the first value at the beginning of the call; and measuring the prescribed time from the beginning of the call.
13. 13. The method of claim 11 or 12, wherein the mobile station employs the assigned code for the duration of the call even when the call is handedover to the second base station sector.
14. 14. The method of claim 13, further comprising:
    setting the flag to the first value when the mobile station is involved in the call;
    measuring the prescribed time from a time associated with the handover; and if the mobile station has not been in a handover during the call, the flag is set to the second value at the end of call.