U.S. patent application number 10/923501 was filed with the patent office on 2006-02-23 for multiplexing scheme for unicast and broadcast/multicast traffic.
This patent application is currently assigned to Lucent Technologies, Inc.. Invention is credited to Farooq Ullah Khan.
Application Number | 20060039344 10/923501 |
Document ID | / |
Family ID | 35134150 |
Filed Date | 2006-02-23 |
United States Patent
Application |
20060039344 |
Kind Code |
A1 |
Khan; Farooq Ullah |
February 23, 2006 |
Multiplexing scheme for unicast and broadcast/multicast traffic
Abstract
A method is provided for efficiently multiplexing interlaced
broadcast traffic over a wireless network. The multiplexing scheme
organizes unused slots in at least some base stations so that
unicast traffic employing hybrid ARQ may be delivered therein.
Inventors: |
Khan; Farooq Ullah;
(Manalapan, NJ) |
Correspondence
Address: |
WILLIAMS, MORGAN & AMERSON
10333 RICHMOND, SUITE 1100
HOUSTON
TX
77042
US
|
Assignee: |
Lucent Technologies, Inc.
|
Family ID: |
35134150 |
Appl. No.: |
10/923501 |
Filed: |
August 20, 2004 |
Current U.S.
Class: |
370/345 ;
370/498 |
Current CPC
Class: |
H04L 1/0065 20130101;
H04W 72/005 20130101; H04L 1/0083 20130101; H04L 1/0066 20130101;
H04L 1/1816 20130101 |
Class at
Publication: |
370/345 ;
370/498 |
International
Class: |
H04J 3/24 20060101
H04J003/24 |
Claims
1. A method for coordinating transmissions within a first and
second cell using an n-slot interlacing structure, the method
comprising: transforming a first block of broadcast information
into first and second subpackets; transmitting the first subpacket
within the first and second cells during a first time slot;
transmitting the second subpacket within the second cell during a
second time slot; transforming a second block of the broadcast
information into third and fourth subpackets; transmitting the
third subpacket within the first and second cells during an n+1
time slot; and transmitting the fourth subpacket within the second
cell during an n+2 time slot.
2. A method, as set forth in claim 1, further comprising:
transmitting unicast information within the first cell during the
second time slot; and retransmitting the unicast information within
the first cell during the n+2 time slot.
3. A method, as set forth in claim 2, wherein retransmitting the
unicast information within the first cell during the n+2 time slot
occurs in response to receiving a negative acknowledgement signal
with respect transmitting unicast information within the first cell
during the second time slot.
4. A method, as set forth in claim 2, further comprising
terminating retransmission of the unicast information with the
first cell during the n+2 slot in response to receiving an
acknowledgement signal with respect transmitting unicast
information within the first cell during the second time slot.
5. A method for receiving broadcast transmissions from a first and
second cell using an n-slot interlacing structure, the method
comprising: receiving a first subpacket from the first and second
cells during a first time slot; receiving a second subpacket from
the second cell during a second time slot; combining the first
subpackets received from the first and second cells; receiving a
third subpacket from the first and second cells during an n+1 time
slot; receiving a fourth subpacket from the second cell during an
n+2 time slot; combining the third subpackets received from the
first and second cells; receiving a first subpacket of unicast
information from the first cell during the second time slot; and
receiving a retransmission of the first subpacket of unicast
information from the first cell during the n+2 time slot.
6. A method for coordinating transmissions within a first and
second cell using an n-slot interlacing structure, the method
comprising: forming a plurality of first and second subpackets from
a plurality of blocks of broadcast information; periodically
transmitting one of the first subpackets within the first and
second cells during a first common time slot in each n-slot
interlacing structure; periodically transmitting one of the second
subpackets within the second cell during a second common time slot
in each n-slot interlacing structure; transmitting unicast
information in at least a portion of the slots in each n-slot
structure that are free from transmissions of the first and second
subpackets.
7. A method, as set forth in claim 6, wherein transmitting unicast
information in at least a portion of the slots in each n-slot
structure that are free from transmissions of the first and second
subpackets further comprises: forming a plurality of subpackets
from the unicast information; and transmitting the unicast
subpackets within the first cell during the second common time slot
in each n-slot interlacing structure.
8. A method, as set forth in claim 7, wherein transmitting the
unicast subpackets within the first cell during the second common
time slot in each n-slot interlacing structure is terminated in
response to receiving an acknowledgement signal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to telecommunications, and
more particularly, to wireless communications.
[0003] 2. Description of the Related Art
[0004] In the field of wireless data systems, a number of
well-known standards, such as 1x-EV-DO, 1xEV-DV as well as the High
Speed Downlink Packet Access (HSDPA) specification in the Universal
Mobile Telecommunication System (UMTS) standard, have been
employed. Newer technologies such as fast scheduling, adaptive
modulation and coding (AMC) and hybrid ARQ (HARQ) have also been
introduced to improve overall system capacity. However, application
of the above-mentioned techniques has been limited to transmitting
data blocks for unicast traffic, i.e. a data block addressed to a
single mobile station. In general, a scheduler selects a user for
transmission at a given time and adaptive modulation and coding is
used to select an appropriate transport format (modulation and
coding) for current channel conditions seen by the user. Due to
errors in channel quality estimates, a relatively high level of
frame errors may occur in the transmissions performed at a given
rate (transport format). Hybrid ARQ has been employed to recover
from transmission errors without significant loss in
throughput.
[0005] An example of hybrid ARQ operation for the 1xEV-DO system is
shown in FIG. 1. The hybrid ARQ transmissions use a 4-slot
interlacing structure, i.e. the hybrid ARQ retransmissions for an
original transmission in slot n happens in slots (n+4), (n+8), and
so on. A total of 4 interlaces are available for transmission to a
single user or for transmissions to different users. In the example
shown in FIG. 1, a new or first data transmission occurs in slot 2
on interlace 2. In the exemplary scenario illustrated in FIG. 1,
the transmission is unsuccessfully received and the receiver sends
back a negative acknowledgement signal (NACK). The NACK indicates
to the transmitter that the transmission was not properly received,
causing the transmitter to retransmit the same data in slot 6
(again on interlace 2). The receiver combines the retransmitted
data with the previously received first transmission, and based on
the two pieces of data, the transmission is successfully decoded.
Those skilled in the art will appreciate that the process of
retransmitting and combining may be repeated until the data is
successfully received (early termination, as indicated by an ACK)
or a fixed number of attempts have been made. Once the data is
properly received, the receiver sends back an acknowledgement
signal (ACK). The transmitter then starts another new transmission
on interlace 2 in slot 10. Similarly, the transmissions happens in
parallel on other interlaces, such as 1, 3 and 4.
[0006] Unlike unicast traffic, broadcast/multicast data blocks are
addressed to more than one receiver or mobile station. In a
broadcast transmission, the data blocks are addressed to all the
mobiles in the system, whereas in a multicast transmission, the
data blocks are addressed to a subset of mobiles in the system. In
general, no feedback is required from the mobile stations.
Generally, in both multicast and broadcast transmissions, the data
blocks are transmitted on a predetermined number of slots, i.e.
there is no early termination due to hybrid ARQ ACK feedback.
[0007] A stylized representation of a wireless system capable of
broadcast data packet transmission is shown in FIG. 2. The
broadcast data packet contains information from one or more
broadcast streams carrying broadcast programs. In general, two
layers of channel coding are used to provide robustness against
errors. The first layer of coding also called outer code is
performed using well-known Reed-Solomon code. The Reed-Solomon code
adds some redundancy to the data. The Reed-Solomon coded block is
then segmented into smaller data blocks for Turbo coding. A number
of subpackets (e.g., SP1-SP3) from the same data block are created
at the output of the Turbo coding. In general, the data block can
be recovered from any one of the received subpackets (SP1-SP3) as
long as the coding rate is smaller than 1. Table 1 shows data rates
for a 3072 bit data block transmitted within one, two or three
slots (subpackets). A subpacket is transmitted within a slot of
duration 1.67 ms. The received subpackets at the mobile receiver
are used to recover the data block. The data blocks are then
reassembled to form the broadcast packet. TABLE-US-00001 TABLE I
Number of subpackets (slots) transmission Data Rate 1 1843.2 Kb/s 2
921.6 Kb/s 3 614.4 Kb/s
[0008] The broadcast and unicast traffic in the 1xEV-DO system is
multiplexed on an interlace-by-interlace basis. In the example
shown in FIG. 3, interlace 1 is used for broadcast traffic. The
broadcast data block is transmitted in three subpackets (SP1, SP2
and SP3) on three slots i.e. slot # 1, 5 and 9 from the entire
system, i.e., all of the base stations in the system. Therefore,
the mobile station can potentially receive and combine signals from
multiple base stations. The mobile station also combines SP1, SP2
and SP3 transmissions in order to recover the broadcast data block.
The SP2 and SP3 transmissions contain additional redundancy for
broadcast data block recovery.
[0009] The interlace-based multiplexing approach used in the prior
art poses problems when different broadcast data rates are used by
different base stations in different cells in the system. The use
of different data rates in different cells may be the case in a
system deployment where the cell sizes are different. This, for
example, can be the case, for a downtown area surrounded by suburbs
and rural areas. The cell size in densely populated areas is
smaller in order to provide more cell sites to accommodate the
larger amounts of traffic. However, as the population density
decreases in the surrounding suburbs and rural areas, the effective
cell sizes increases. The smaller cells deployments can in general
support higher data rates because of the smaller path loss due to
relatively shorter distance between the base station and the mobile
station. The larger cells have, in general, larger path loss and
therefore cannot support very high data rates. An example of cell
layout showing three sets of cells is stylistically shown in FIG.
4. A set of 7 center cells is labeled as set A. A first and second
ring of cells around set A are labeled as set B and set C,
respectively.
[0010] An example of broadcast transmissions at different data
rates in different sets of cells is shown in FIG. 5. In this
example, set A transmits only SP1 of the broadcast data block,
achieving the highest transmission data rate. Set B transmits both
SP1 and SP2 of the broadcast data block therefore achieving half
the data rate of set A rate. Similarly, set C achieves one-third
rate of set A because the broadcast data block is transmitted in
three subpackets. Note that SP2 and SP3 contain additional
redundancy. Therefore, if a transmission can be decoded using a
smaller number of subpackets, the achieved information data rate is
higher.
[0011] An example of a broadcast transmission over three interlaces
is shown in FIG. 6. Each of the interlaces carries a broadcast data
block consisting of one, two or three subpackets. In this example,
sets A, B and C transmit the broadcast data block in one, two and
three subpackets, respectively. The fourth interlace is used for
the unicast traffic. In set C, slots 1, 2, 3, 5, 6, 7, 9, 10, 11
are used for the broadcast traffic while slots 4, 8 and 12 in
interlace #4 are used for the unicast traffic. In FIG. 6, SPij
denoted the jth subpacket from the ith data block. For example,
SP21 represents the first subpacket from the second data block. The
subpackets transmitted from multiple cells at the same time with
the same subpacket number can potentially be soft combined at the
receiver to assist in decoding the data packet. In FIG. 6, SP11,
SP21 and SP31 are transmitted from all the three sets of cells A-C
at the same time, and, therefore, these subpackets received from
all the cells are combined at the receiver. Similarly, SP12, SP22,
and SP32 are transmitted from cell set B and cell set C. Therefore,
these subpackets are soft combined from cell set B and cell set C.
On the other hand cell set A may potentially be transmitting
unicast traffic during slots 5, 6 and 7 when SP12, SP22 and SP32
are transmitted from cell set B and cell set C. Therefore,
transmissions from cell set A potentially interfere with
transmissions from cell set B and cell set C. SP13, SP23, and SP33
are transmitted from cell set C only. Therefore, these subpackets
potentially get interference from both cell set A and cell set
B.
[0012] In cell set B, slots 9, 10 and 11 are not used for broadcast
traffic because the broadcast data blocks are transmitted in two
subpackets only. Therefore, these free slots can potentially be
considered for transmission of other information, such as unicast
traffic. However, the unicast traffic uses hybrid ARQ and
potentially requires multiple retransmission attempts. For example,
if a unicast data block transmission is started in slot#9, the
retransmission needs to happen in slot#13, but slot#13 belonging to
interlace#1 is reserved for a broadcast data block transmission.
Therefore, a retransmission cannot be performed for unicast
traffic. Similarly, in cell set A, slots 5, 6, 7, 9, 10 and 11
become available, but like slots 9, 10 and 11 in cell set B, these
slots may not be used for unicast traffic due to restrictions on
retransmissions. Thus, these unused slots remain unavailable, and,
therefore, the multiplexing approach used in the prior art poses
serious restrictions on scheduling and results in system
inefficiency.
[0013] The present invention is directed to overcoming, or at least
reducing, the effects of, one or more of the problems set forth
above.
SUMMARY OF THE INVENTION
[0014] In one embodiment of the present invention, a method is
provided for coordinating transmissions within a first and second
cell. The method comprises transforming a first block of broadcast
information into first and second subpackets; transmitting the
first subpacket within the first and second cells during a first
time slot; and transmitting the second subpacket within the second
cell during a second time slot. A second block of the broadcast
information is transformed into third and fourth subpackets, the
third subpacket is transmitted within the first and second cells
during an n+1 time slot, and the fourth subpacket is transmitted
within the second cell during an n+2 time slot.
[0015] In another embodiment of the present invention, a method is
provided for receiving broadcast transmissions from a first and
second cell. The method comprises receiving a first subpacket from
the first and second cells during a first time slot; receiving a
second subpacket from the second cell during a second time slot;
and combining the first subpackets received from the first and
second cells. A third subpacket is received from the first and
second cells during an n+1 time slot. A fourth subpacket is
received from the second cell during an n+2 time slot. The third
subpackets received from the first and second cells are combined. A
first subpacket of unicast information is received from the first
cell during the second time slot, and a retransmission of the first
subpacket of unicast information is received from the first cell
during the n+2 time slot.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The invention may be understood by reference to the
following description taken in conjunction with the accompanying
drawings, in which like reference numerals identify like elements,
and in which:
[0017] FIG. 1 illustrates a stylized representation of hybrid ARQ
operation for a 1xEV-DO system;
[0018] FIG. 2 illustrates a stylized representation of a wireless
system capable of broadcast data packet transmission;
[0019] FIG. 3 illustrates one scheme for multiplexing broadcast and
unicast traffic in the 1xEV-DO system on an interlace-by-interlace
basis;
[0020] FIG. 4 illustrates an examplary cell layout showing three
sets of cells, each transmitting at different rates;
[0021] FIG. 5 illustrates an exemplary embodiment of a multiplexing
scheme for broadcast transmissions at different data rates in
different sets of cells;
[0022] FIG. 6 illustrates an exemplary embodiment of a broadcast
transmission over three interlaces;
[0023] FIG. 7 stylistically illustrates an exemplary embodiment of
a data block transmission according to one aspect of the current
invention; and
[0024] FIG. 8 stylistically illustrates an alternative exemplary
embodiment of a data block transmission according to one aspect of
the current invention.
[0025] While the invention is susceptible to various modifications
and alternative forms, specific embodiments thereof have been shown
by way of example in the drawings and are herein described in
detail. It should be understood, however, that the description
herein of specific embodiments is not intended to limit the
invention to the particular forms disclosed, but on the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0026] Illustrative embodiments of the invention are described
below. In the interest of clarity, not all features of an actual
implementation are described in this specification. It will of
course be appreciated that in the development of any such actual
embodiment, numerous implementation-specific decisions may be made
to achieve the developers' specific goals, such as compliance with
system-related and business-related constraints, which may vary
from one implementation to another. Moreover, it will be
appreciated that such a development effort might be complex and
time-consuming, but would nevertheless be a routine undertaking for
those of ordinary skill in the art having the benefit of this
disclosure.
[0027] The present invention presents a new multiplexing scheme for
unicast and broadcast traffic. Generally, the multiplexing scheme
of the instant invention overcomes the scheduling restrictions in
the prior art to allow use of HARQ transmissions and
retransmissions of unicast data during slots that would otherwise
be unused by broadcast operations.
[0028] One example of data block transmission according to one
aspect of the current invention is illustrated in FIG. 7. In the
illustrated embodiment, it is assumed that cell sets A, B and C use
data rates of 1843.2, 921.6 and 614.4 Kb/s, respectively. As given
in Table 1, these three data rates are achieved by transmitting
one, two and three subpackets (slots) for a data block of size 3072
bits. Therefore, cell sets A, B and C transmit one, two and three
subpackets (slots), respectively, for each data block. In one
embodiment of the instant invention, the subpackets from a given
data block are transmitted contiguously. For example, subpacets
SP11, SP12 and SP13 from data block number 1 are transmitted in
slot 1, 2 and 3 respectively from cell set C. Similarly, SP11 and
SP12 are transmitted from cell set B in slots 1 and 2,
respectively. Cell set A transmits only subpacket SP11 in slot
number 1. A receiver that receives the SP11 subpacket from at least
one cell in more than one of the cell sets A, B and C may combine
the SP11 subpackets to correct for transmission errors. It should
be appreciated that if the receiver receives subpacket SP11 from
all three of the cell sets A, B and C, then all three of the
received subpackets may be combined, whereas if the receiver
receives subpacket SP11 from only two of the cell sets A, B and C,
then the two received subpackets may be combined.
[0029] Similarly, the SP12 subpacket may be combined from cell set
B and cell set C. It should be noted that in the illustrated
embodiment of the instant inention, the same number of soft
combinings of the subpackets can be performed as in the prior art
scheme; however, the slots not used for broadcast in cells using
relatively higher data rates can now be used for unicast traffic
without scheduling restrictions. That is, using the illustrated
multiplexing scheme allows conventional HARQ transmissions and
retransmissions of unicast data during the unused time slots of the
higher speed cells. For example in cell set B, the slots 3, 7 and
11, which belong to interlace 3, are free from broadcast traffic,
and may be used for unicast traffic requiring hybrid ARQ
retransmissions. That is, in cell set B, slots 3, 7 and 11 will
always be free for transmissions and retransmissions. In other
terms, interlace 3 in cell set B will always be available for
unicast traffic.
[0030] Similarly, interlaces 2 and 3 are both free in cell set A.
One advantage of the present invention is it makes complete
interlaces available rather than some slots within an interlace. In
other words, the present invention minimizes the number of
interlaces allocated for broadcast traffic in a given set of cells.
Unicast traffic can be carried over an interlace without any
scheduling and retransmission restrictions because any number of
retransmissions can be performed within that interlace. For
example, assuming that two retransmissions are permitted, then the
first transmission may occur in slot 2, while the two subsequent
retransmissions may occur in slots 6 and 10. Further, assuming that
three retransmissions are permitted, then the first transmission
may occur in slot 2, while the first two subsequent retransmissions
may occur in slots 6 and 10, and the third retransmission may take
place in slot 14. In this manner, any number of retransmissions may
be permitted.
[0031] Those skilled in the art will appreciate that the subpackets
received from multiple cells can be combined in any of a variety of
ways. When an Orthogonal Frequency Division Multiplexing (OFDM)
technique is used for subpacket transmission, an FFT (Fast Fourier
Transform) operation may be performed on the composite signal
received from each of the cells. Therefore, the signal combining
happens as part of the OFDM demodulation. Alternatively, in the
case of CDMA using a RAKE receiver, the received signal from each
of the RAKE fingers may be combined. A RAKE finger may be used to
track and demodulate a signal received from one cell. Therefore,
the number of available RAKE fingers limits the maximum number of
cells from which the subpackets can be combined. An equalizer can
also be employed for decoding signals received from multiple
cells.
[0032] Turning now to FIG. 8, an alternative multiplexing scheme is
stylistically illustrated. In this exemplary embodiment, cell set C
requires only two interlaces for transmission at the lowest data
rate of 614.4 Kb/s. The subpackets SP11 and SP12 are transmitted
contiguously in slots 1 and 2 in cell set C. The third subpacket
from the first block, i.e., SP13 is transmitted subsequently in
slot 6. The first subpacket from the second data block SP21 is
transmitted in slot 5. Therefore, the third subpacket from the
first data block SP13 is transmitted after first subpacket from the
second data block SP21, i.e., an out-of-order transmission of
subpackets. This out-of-order transmission allows combining of SP21
across cell sets A, B and C while requiring only one interlace for
broadcast traffic in cell set A. In prior art multiplexing schemes,
two interlaces would have been blocked by the broadcast traffic in
the whole system (cell set A, B and C). The number of soft
combinings allowed by the present invention is the same as in the
prior art
[0033] Those skilled in the art will appreciate that the various
system layers, routines, or modules illustrated in the various
embodiments herein may be executable control units. The control
units may include a microprocessor, a microcontroller, a digital
signal processor, a processor card (including one or more
microprocessors or controllers), or other control or computing
devices as well as executable instructions contained within one or
more storage devices. The storage devices may include one or more
machine-readable storage media for storing data and instructions.
The storage media may include different forms of memory including
semiconductor memory devices such as dynamic or static random
access memories (DRAMs or SRAMs), erasable and programmable
read-only memories (EPROMs), electrically erasable and programmable
read-only memories (EEPROMs) and flash memories; magnetic disks
such as fixed, floppy, removable disks; other magnetic media
including tape; and optical media such as compact disks (CDs) or
digital video disks (DVDs). Instructions that make up the various
software layers, routines, or modules in the various systems may be
stored in respective storage devices. The instructions, when
executed by a respective control unit, cause the corresponding
system to perform programmed acts.
[0034] The particular embodiments disclosed above are illustrative
only, as the invention may be modified and practiced in different
but equivalent manners apparent to those skilled in the art having
the benefit of the teachings herein. Furthermore, no limitations
are intended to the details of construction or design herein shown,
other than as described in the claims below. It is therefore
evident that the particular embodiments disclosed above may be
altered or modified and all such variations are considered within
the scope and spirit of the invention. Accordingly, the protection
sought herein is as set forth in the claims below.
* * * * *