U.S. patent application number 13/092562 was filed with the patent office on 2011-08-11 for transmitting data with multiple priorities as ofdm symbols.
Invention is credited to Chunjie Duan, Philip Orlik, Raymond Yim, Jinyun Zhang.
Application Number | 20110194452 13/092562 |
Document ID | / |
Family ID | 38633130 |
Filed Date | 2011-08-11 |
United States Patent
Application |
20110194452 |
Kind Code |
A1 |
Orlik; Philip ; et
al. |
August 11, 2011 |
Transmitting Data with Multiple Priorities as OFDM Symbols
Abstract
A transmitter transmits data having a set of two or more
priorities on subcarriers using orthogonal frequency division
multiplexing (OFDM) symbols. The transmitter includes a media
access (MAC) layer, wherein the MAC layer further includes a queue
for storing data at each priority, a rate control block connected
to each queue, and a physical (PHY) layer. The PHY layer further
includes a channel coder for each priority, wherein each channel
coder is connected to the corresponding queue to receive data, and
to the rate control block to send coding information.
Inventors: |
Orlik; Philip; (Cambridge,
MA) ; Yim; Raymond; (Cambridge, MA) ; Duan;
Chunjie; (Brookline, MA) ; Zhang; Jinyun;
(Cambridge, MA) |
Family ID: |
38633130 |
Appl. No.: |
13/092562 |
Filed: |
April 22, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11385620 |
Mar 21, 2006 |
7903737 |
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13092562 |
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Current U.S.
Class: |
370/252 ;
370/412 |
Current CPC
Class: |
H04N 19/597 20141101;
H04N 19/13 20141101; H04N 19/63 20141101; H04N 19/46 20141101; H04N
19/105 20141101; H04N 19/61 20141101; H04N 19/615 20141101 |
Class at
Publication: |
370/252 ;
370/412 |
International
Class: |
H04L 12/26 20060101
H04L012/26; H04L 12/56 20060101 H04L012/56 |
Claims
1. A transmitter for transmitting data having a set of two or more
priorities on subcarriers using orthogonal frequency division
multiplexing (OFDM) symbols, comprising: a media access (MAC)
layer, wherein the MAC layer further comprises: a queue for storing
data at each priority; and a rate control block connected to each
queue; a physical (PHY) layer, wherein the PHY layer further
comprises: a channel coder for each priority, wherein each channel
coder is connected to the corresponding queue to receive data, and
to the rate control block to send coding information.
2. The transmitter of claim 1, wherein the PHY layer further
comprises: a symbol to subcarrier mapping block connected to each
channel coding block; and means for transmitting the ODM symbols
connected to the symbol to carrier block; means for determining
channel-over-interference ratios .xi. i = H i I i , ##EQU00004##
where I.sub.i is an interference power at a subcarrier i, and
H.sub.i is a channel response.
3. The transmitter of claim 2, wherein the symbol to subcarrier
mapping is adaptive and depends on an interference location or
channel quality.
4. The method of claim 2, wherein the channel-over-interference
ratios are sorted in a high to low order, and data with high to low
priorities are assigned to the sub-carriers according to the high
to low order of the channel-over-interference ratios.
5. The method of claim 2, wherein the subcarrier mapping block uses
a permutation function to account for channel diversity.
6. The method of claim 4, wherein a number of subcarriers assigned
for each priority is S i = D i WR i log 2 ( Q i ) , ##EQU00005##
where D.sub.i is a number of bits sent to the PHY layer, W is a
number of symbols in a frame, R.sub.i is a data rate, and Q.sub.i
is a quality of service, and S.sub.1+S.sub.2+ . . . +S.sub.N=M,
where M is a number of subcarriers.
7. A method for transmitting data having a set of two or more
priorities on subcarriers using orthogonal frequency division
multiplexing (OFDM) symbols, comprising the steps of storing the
data at each priority in a corresponding queue in a medium (MAC)
layer of a transmitter; sending the data in each queue to a
corresponding coding block in a physical (PHY) layer according to a
rate control in the MAC layer depending on each priority.
Description
FIELD OF THE INVENTION
[0001] This invention relates to generally to wireless
communications, and more particularly to sending data with multiple
priorities as OFDM symbols.
BACKGROUND OF HE INVENTION
[0002] A communication network can carry different types of data
that require different quality of services (QoS) and priority.
Typically, the data are transmitted as of packets, which constitute
bit streams of traffic in the network. Some data can require
transmission to have extremely low probability of error, and other
data can require low latency. Generally, when multiple types of
data are present in a network, high priority data requires
stringent reliability or latency requirement.
[0003] In communication networks, lower priority data can be
delayed at the application, layer, or a medium access control (MAC)
layer of a communication protocol stack. According to the IEEE
802.11e standard, an Enhanced Distributed Coordination Function
(EDCF) deals with data with multiple priorities. In essence,
different back-off parameters are used to control a
contention-based channel access for different priorities, so that
higher priority data have a higher priority access to a channel.
Orthogonal frequency-division multiplexing (OFDM) transmission with
multiple priorities can first allocate wireless resources to
constant bit rate (CBR) data.
[0004] In a physical (PHY) layer of a communication protocol stack,
different channel coding can be used to achieve different level of
error correction. In the prior art, the channel coding is selected
based on a quality of the channel. For example, a better channel
can support a higher data rate. This is achieved by using higher
order modulation and less error correction. When a high level of
reliability is required for data, a new PHY is instantiated with
appropriate parameters so that the reliability requirement is met.
In general, the PHY layer does not consider the priority of the
data.
[0005] FIG. 1 shows a protocol stack with Application 100, MAC 110
and PHY 120 layers. Generally, the Application layer can be, or
include other layers. The MAC layer includes corresponding queues
111-112 for the packets 101-101 received from the application layer
with different priorities, which are then sent 115 to the PHY
layer. The PHY layer performs channel coding 131, symbol to
subcarrier mapping 132, and OFDM transmission 133 independent of
data priorities.
[0006] The frequency response of a wireless channel, as well as the
presence of narrowband interference, can drastically affect the
quality of communication over specific frequency. It is desired to
transmit OFDM symbols with multiple priorities considering the
channel quality.
SUMMARY OF THE INVENTION
[0007] Embodiments of the invention provide a method to transmit
OFDM symbols for data with multiple priorities over wireless
channel in the presence of narrow band interference.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is schematic of a prior art communication protocol
stack;
[0009] FIG. 2 is schematic of a communication protocol stack
according to embodiments of the invention;
[0010] FIGS. 3A and 3B are schematics of channel response and
interference as a function of packet priority;
[0011] FIG. 4 is a flow diagram of a mapping procedure according to
embodiments of the invention;
[0012] FIGS. 5A-5C are schematics of probability distribution of
channel response of different subcarriers for different
priorities.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0013] FIG. 2 shows a protocol stack in a transmitter according to
embodiments of our invention. The protocol stack is used to
transmit data using a set of two or more priorities. For
simplicity, data packets 101-102 with only two priorities are
shown. The generalization to the case with more than two priorities
is straightforward.
[0014] Priority 1 packets are stored in a MAC queue 211, and
priority 2 packets are stored in a MAC queue 2 212. There is one
queue for each priority. MAC queue 1 sends data to a channel coding
block 1, 215, in the PHY layer, and MAC queue 2 sends data to
channel coding block 2, 216.
[0015] The channel coding block sends symbols to a symbol to
subcarrier mapping block, which in turn performs OFDM transmission
232.
[0016] In contrast with the prior art, there is also one channel
coding block for each priority. The channel coding blocks can
provide different level of forward error correction (FEC) for each
priority. After the channel coding, the encoded symbols of all
priorities are mapped to a single symbol using the symbol to
subcarrier block mapping block.
[0017] In the prior art, the symbol to subcarrier mapping block
does not consider the priority of the data. In this invention, the
symbol to subcarrier mapping block does considers the different
priorities. Furthermore, the mapping also depends on an
interference location or channel quality 233. The interference
location and/or channel quality can be estimated directly by the
transmitter. Alternatively, a receiver report 234 the interference
and/or channel quality to the transmitter on an uplink channel.
[0018] Another key difference between this invention and the prior
art is the rate control block to control the rate at which data in
each queue are sent to the PHY layer. Because the rate of incoming
data, cannot be controlled by the MAC layer, the rate can be higher
than the data rate allowed in the transmission, thus some data are
necessarily queued.
[0019] In the prior art, because there is a single interface
between the MAC and PHY layer, only the MAC layer regulates the
amount of data sent to the PHY layer by monitoring a single
interface.
[0020] In this invention, there are multiple MAC queues, and each
queue sends data directly to the corresponding channel coding
blocks of PHY layer. The rate control block ensures that PHY layer
transmits the data at an optimal rate for each priority.
[0021] The rate control block serves two functions. First, the rate
control block determines the data rate supported by PHY layer. In
some networks, parameters of the PHY layer are fixed, and the rate
control block knows exactly how much data can be sent for each
priority. In other networks, adaptive modulation and coding can be
used in the PHY layer. In this case, the rate control block also
receives coding information 217 from the channel coding blocks in
the PHY layer to determine how much data from each queue can be
sent at a given time.
[0022] Second, the rate control block determines queuing
information 218 from and for each of the queues. This enables the
rate control block to control the amount of data sent to the
channel coding blocks. The rate control block has the quality of
service requirements of all data.
[0023] FIG. 3A show the channel response as a function of frequency
for different priorities. FIG. 3B shows the interference power at
the receiver as a function of frequencies for the different
priorities.
[0024] Mapping Procedure
[0025] In general, at the receiver, the interference power at
subcarrier i is I.sub.i, and the channel response is H.sub.i.
[0026] As shown in FIG. 4, the mapping procedure first determines
410 channel-over-interference ratios .xi..sub.i so that
.xi. i = H i I i . ##EQU00001##
[0027] Then, the procedure sorts 420 the ratios in a descending
order. The subcarrier index that has the k.sup.th largest value of
.xi..sub.i is z.sub.k. Then, the procedure assigns 430 with high to
low priorities are assigned to the sub-carriers according to the
high to low order of the channel-over-interference ratios. In other
words, if the highest priority requires data S.sub.1 subcarriers,
then the data are mapped to subcarriers z.sub.1, z.sub.2, . . . ,
z.sub.S1.
[0028] In addition to the subcarrier mapping, the OFDM
transmissions also use a permutation function to account for
channel diversity. Conventional permutation technique also applies
to this invention.
[0029] Consider an OFDM transmission using M subcarriers, and data
with a set of N priorities, where N>1. Each OFDM frame contains
W symbols. Furthermore, the channel coding rate of the respective
priority is R.sub.i, i=1, . . . , N. The rate control block allows
D.sub.i bits of data to go to PHY for data of priority i.
Furthermore, Q.sub.i-QAM (quality quadrature amplitude modulation)
is used to send data of priority i.
[0030] Given this information, we can determine the number of
subcarriers required for data of each priority. We denote the
number of subcarrier for priority i by S.sub.i.
S i = D i WR i log 2 ( Q i ) . ( 1 ) ##EQU00002##
[0031] The rate control block ensures that S.sub.1+S.sub.2+ . . .
+S.sub.N=M.
[0032] After the procedure determines the ratio .xi..sub.i, and the
sorted subcarrier index z.sub.i, priority 1 data are sent on
subcarriers z.sub.1, . . . z.sub.S1, priority 2 data are be sent on
subcarriers z.sub.S1+1, . . . , z.sub.S1+S2, and so on.
[0033] The channel coding block needs to select the appropriate
value for R.sub.i and Q.sub.i to ensure that the reliability of
transmission matches with the quality of service requirement of a
specific priority data. Because the subcarriers corresponding to
better channel response are assigned to high priority data, it is
important to know the receive power for transmitted high priority
data.
[0034] FIGS. 5A-C shows the probability distribution of channel
response of different subcarrier as a function of the
channel-over-interference ratio .xi..sub.i for different
priorities. The distribution in FIG. 5A depends on the wireless
channel. When a better channel is assigned to high priority data,
the resulting probability distribution for high priority channel is
shifted to higher values, see FIG. 5B, and the resulting
probability distribution for low priority channel is shifted to the
lower value, see FIG. 5C. The resulting distribution for high and
low priority data can be obtained from order statistic based on the
original distribution.
[0035] Because the channel with a higher channel-over-interference
ratio .xi..sub.i is used for higher priority data, more efficient
modulation and coding (value for Q.sub.i and R.sub.i) can be used
to satisfy stringent quality of service requirement. Hence, the
number of subcarrier S.sub.i required for higher priority data can
be reduced.
[0036] Rate Control Block
[0037] From the perspective of the rate control block, the values
Q.sub.i, R.sub.i and W are fixed. The rate control block obtains
the values from channel coding blocks, the network, or during
initialization. In view of Eq. 1, the rate control block needs to
determine D.sub.i, the number of bits sent to the PHY layer, so
that the corresponding S.sub.i values satisfy the condition
S.sub.1+S.sub.2+ . . . +S.sub.N=M.
[0038] We assume that queue i stores B.sub.i bits of data. The rate
control block receives B.sub.i from the respective queues. As
stated previously, Q.sub.i, R.sub.i, W and M are known. In general,
the MAC layer does know the rate at which data are stored in the
queues. However, the rate control block can determine D.sub.i based
on the amount of data in each queue, and the coding information
received from the PHY layer.
[0039] In one embodiment, a priority rule is applied, so that
priority 1 data always has priority over all other priority data.
In this case, the priority rate control sets
D.sub.1=min(B.sub.1,MWR.sub.1 log.sub.2(Q.sub.1)).
and in general,
D i = min ( B i , ( M - j = 1 i - 1 D j WR j log 2 ( Q j ) ) WR i
log 2 ( Q i ) ) for i .gtoreq. 2. ##EQU00003##
[0040] Although the invention has been described by way of examples
of preferred embodiments, it is to be understood that various other
adaptations and modifications can be made within the spirit and
scope of the invention. Therefore, it is the object of the appended
claims to cover all such variations and modifications as come
within the true spirit and scope of the invention.
* * * * *