Variable spreading factor codes for non-orthogonal multiple access
Aspects of the present disclosure provide techniques for variable spreading factor codes for non-orthogonal multiple access (NOMA). In an exemplary method, a base station assigns, from a first codebook of N short code sequences of length K, a subset of the short code sequences to a number of user equipments (UEs); receives a signal including uplink data or control signals from two or more of the UEs, wherein a first uplink data or control signal is sent using a first subsequence of one of the assigned short code sequences, and a second uplink data or control signal is sent using a second subsequence of one of the assigned short code sequences or using one of the assigned short code sequences; and decodes each uplink data or control signal in the signal based on the assigned short code sequences and subsequences of the assigned the short code sequences.
1. A method of wireless communications performed by a base station (BS), comprising:
assigning, from a first codebook of N short code sequences of length K, a subset of the short code sequences to a number of user equipments (UEs), wherein the number is at least two and at most N;
receiving a signal including uplink data or control signals from two or more of the UEs, wherein:
a first uplink data or control signal is sent using a first subsequence of one of the assigned subset of short code sequences, and
a second uplink data or control signal, different from the first uplink data or control signal, is sent using a second subsequence of one of the assigned subset of short code sequences or using one of the assigned subset of short code sequences; and
decoding each uplink data or control signal in the signal based on the assigned subset of the short code sequences and subsequences of the short code sequences in the assigned subset of the short code sequences.
2. The method of claim 1 , wherein the first codebook of N short code sequences is designed to meet a criterion for low cross correlation across the short code sequences in the first codebook.
3. The method of claim 2 , wherein:
each of the N short code sequences comprises X short code subsequences of length K/X; and
a second codebook of sequences, comprising each short code subsequence of the N short code sequences, is designed to meet the criterion for low cross correlation across the sequences in the second codebook.
4. The method of claim 1 , wherein:
the first uplink data or control signal is sent using a first spreading factor (SF) equal to a length of the first subsequence; and
the second uplink data or control signal is sent using a second SF equal to a length of the second subsequence or the short code sequence used in sending the second uplink data or control signal.
5. The method of claim 1 , wherein decoding the first uplink data or control signal comprises:
despreading the signal using the first subsequence; and
wherein decoding the second uplink data or control signal comprises:
canceling the first uplink data or control signal from the signal to generate a new signal, and
despreading the new signal using the second subsequence.
6. The method of claim 1 , wherein each short code sequence, s, of the N short code sequences, is generated according to a formula:
s
n
(
k
)
=
1
K
exp
(
j
π
(
r
(
k
+
n
+
θ
)
2
N
)
)
,
where:
k is an index of an element within the short code sequence, such that 1≤k≤K,
n is an index of the short code sequence in the N short code sequences, such that 1≤n≤N,
−N≤θ<N, and
1≤r<2N.
7. The method of claim 1 , wherein each short code sequence, s, of the N short code sequences, is generated according to a formula:
s
n
B
(
k
)
=
Δ
1
K
exp
(
j
π
(
r
(
k
+
n
+
θ
)
(
k
+
n
+
θ
+
1
)
N
)
)
,
where:
k is an index of an element within the short code sequence, such that 1≤k≤K,
n is an index of the short code sequence in the N short code sequences, such that 1≤n≤N,
−N≤θ<N, and
1≤r<2N.
8. A method of wireless communications performed by a user equipment (UE), comprising:
obtaining a first codebook of N short code sequences of length K;
receiving, from a base station (BS), an assignment of a first short code sequence in the first codebook; and
transmitting a signal spread using a spreading factor (SF) that is less than K, wherein transmitting the signal comprises transmitting the signal using a subsequence, of length SF, of the assigned first short code sequence.
9. The method of claim 8 , further comprising:
receiving an indication of the SF from the BS.
10. The method of claim 9 , wherein receiving the indication comprises:
receiving an identifier of a group to which the UE is assigned, wherein UEs of the group transmit using the SF.
11. The method of claim 8 , further comprising:
determining, based on a measurement of a downlink signal from the BS, the SF.
12. The method of claim 8 , wherein transmitting the signal comprises transmitting the signal on a set of tones, and the method further comprises:
determining the subsequence based on the set of tones.
13. The method of claim 8 , wherein each short code sequence, s, of the N short code sequences, is generated according to a formula:
s
n
(
k
)
=
1
K
exp
(
j
π
(
r
(
k
+
n
+
θ
)
2
N
)
)
,
where:
k is an index of an element within the short code sequence, such that 1≤k≤K,
n is an index of the short code sequence in the N short code sequences, such that 1≤n≤N,
−N≤θ<N, and
1≤r<2N.
14. The method of claim 8 , wherein each short code sequence, s, of the N short code sequences, is generated according to a formula:
s
n
B
(
k
)
=
Δ
1
K
exp
(
j
π
(
r
(
k
+
n
+
θ
)
(
k
+
n
+
θ
+
1
)
N
)
)
,
where:
k is an index of an element within the short code sequence, such that 1≤k≤K,
n is an index of the short code sequence in the N short code sequences, such that 1≤n≤N,
−N≤θ<N, and
1≤r<2N.
15. An apparatus for wireless communications, comprising:
a processor configured to:
assign, from a first codebook of N short code sequences of length K, a subset of the short code sequences to a number of user equipments (UEs), wherein the number is at least two and at most N;
receive a signal including uplink data or control signals from two or more of the UEs, wherein:
a first uplink data or control signal is sent using a first subsequence of one of the assigned subset of short code sequences, and
a second uplink data or control signal, different from the first uplink data or control signal, is sent using a second subsequence of one of the assigned subset of short code sequences or using one of the assigned subset of short code sequences; and
decode each uplink data or control signal in the signal based on the assigned subset of the short code sequences and subsequences of the short code sequences in the assigned subset of the short code sequences; and
a memory coupled with the processor.
16. The apparatus of claim 15 , wherein the first codebook of N short code sequences is designed to meet a criterion for low cross correlation across the short code sequences in the first codebook.
17. The apparatus of claim 16 , wherein:
each of the N short code sequences comprises X short code subsequences of length K/X; and
a second codebook of sequences, comprising each short code subsequence of the N short code sequences, is designed to meet the criterion for low cross correlation across the sequences in the second codebook.
18. The apparatus of claim 15 , wherein:
the first uplink data or control signal is sent using a first spreading factor (SF) equal to a length of the first subsequence; and
the second uplink data or control signal is sent using a second SF equal to a length of the second subsequence or the short code sequence used in sending the second uplink data or control signal.
19. The apparatus of claim 15 , wherein the processor is configured to decode the first uplink data or control signal by:
despreading the signal using the first subsequence; and
wherein the processor is configured to decode the second uplink data or control signal by:
canceling the first uplink data or control signal from the signal to generate a new signal, and
despreading the new signal using the second subsequence.
20. The apparatus of claim 15 , wherein each short code sequence, s, of the N short code sequences, is generated according to a formula:
s
n
(
k
)
=
1
K
exp
(
j
π
(
r
(
k
+
n
+
θ
)
2
N
)
)
,
where:
k is an index of an element within the short code sequence, such that 1≤k≤K,
n is an index of the short code sequence in the N short code sequences, such that 1≤n≤N,
−N≤θ<N, and
1≤r<2N.
21. The apparatus of claim 15 , wherein each short code sequence, s, of the N short code sequences, is generated according to a formula:
s
n
B
(
k
)
=
Δ
1
K
exp
(
j
π
(
r
(
k
+
n
+
θ
)
(
k
+
n
+
θ
+
1
)
N
)
)
,
where:
k is an index of an element within the short code sequence, such that 1≤k≤K,
n is an index of the short code sequence in the N short code sequences, such that 1≤n≤N,
−N≤θ<N, and
1≤r<2N.
22. An apparatus for wireless communications, comprising:
a processor configured to:
obtain a first codebook of N short code sequences of length K;
receive, from a base station (BS), an assignment of a first short code sequence in the first codebook; and
transmit a signal spread using a spreading factor (SF) that is less than K, wherein transmitting the signal comprises transmitting the signal using a subsequence, of length SF, of the assigned first short code sequence; and
a memory coupled with the processor.
23. The apparatus of claim 22 , wherein the processor is further configured to:
receive an indication of the SF from the BS.
24. The apparatus of claim 23 , wherein the processor is configured to receive the indication by:
receiving an identifier of a group to which the apparatus is assigned, wherein user equipments (UEs) of the group transmit using the SF.
25. The apparatus of claim 22 , wherein the processor is further configured to:
determine, based on a measurement of a downlink signal from the BS, the SF.
26. The apparatus of claim 22 , wherein the processor is configured to transmit the signal by transmitting the signal on a set of tones, and the processor is further configured to:
determine the subsequence based on the set of tones.
27. The apparatus of claim 22 , wherein each short code sequence, s, of the N short code sequences, is generated according to a formula:
s
n
(
k
)
=
1
K
exp
(
j
π
(
r
(
k
+
n
+
θ
)
2
N
)
)
,
where:
k is an index of an element within the short code sequence, such that 1≤k≤K,
n is an index of the short code sequence in the N short code sequences, such that 1≤n≤N,
−N≤θ≤N, and
1≤r<2N.
28. The apparatus of claim 22 , wherein each short code sequence, s, of the N short code sequences, is generated according to a formula:
s
n
B
(
k
)
=
Δ
1
K
exp
(
j
π
(
r
(
k
+
n
+
θ
)
(
k
+
n
+
θ
+
1
)
N
)
)
,
where:
k is an index of an element within the short code sequence, such that 1≤k≤K,
n is an index of the short code sequence in the N short code sequences, such that 1≤n≤N,
−N≤θ≤N, and
1≤r<2N.