US20110194452A1

US20110194452A1 - Transmitting Data with Multiple Priorities as OFDM Symbols

Info

Publication number: US20110194452A1
Application number: US13/092,562
Authority: US
Inventors: Philip Orlik; Raymond Yim; Chunjie Duan; Jinyun Zhang
Original assignee: Mitsubishi Electric Research Laboratories Inc
Current assignee: Mitsubishi Electric Research Laboratories Inc
Priority date: 2006-03-21
Filing date: 2011-04-22
Publication date: 2011-08-11
Also published as: JP5154681B2; JP4762936B2; JP5154680B2; US8451895B2; JP5154679B2; JP4995330B2; JP2007259433A; JP2012016045A; JP2012016046A; US20100322311A1; US7903737B2; JP2011155683A; JP2012016044A; US20070121722A1

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.

Description

FIELD OF THE INVENTION

This invention relates to generally to wireless communications, and more particularly to sending data with multiple priorities as OFDM symbols.

BACKGROUND OF HE INVENTION

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.
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.
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.
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.
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

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

FIG. 1 is schematic of a prior art communication protocol stack;

FIG. 2 is schematic of a communication protocol stack according to embodiments of the invention;

FIGS. 3A and 3B are schematics of channel response and interference as a function of packet priority;

FIG. 4 is a flow diagram of a mapping procedure according to embodiments of the invention;

FIGS. 5A-5C are schematics of probability distribution of channel response of different subcarriers for different priorities.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

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.
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.
The channel coding block sends symbols to a symbol to subcarrier mapping block, which in turn performs OFDM transmission 232.
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.
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.
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.
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.
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.
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.
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.
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.
Mapping Procedure
In general, at the receiver, the interference power at subcarrier i is I_i, and the channel response is H_i.
As shown in FIG. 4, the mapping procedure first determines 410 channel-over-interference ratios ξ_iso that
$ξ_{i} = \frac{H_{i}}{I_{i}} .$
Then, the procedure sorts 420 the ratios in a descending order. The subcarrier index that has the k^thlargest value of ξ_iis z_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 S₁subcarriers, then the data are mapped to subcarriers z₁, z₂, . . . , z_S1.
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.
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_i, i=1, . . . , N. The rate control block allows D_ibits of data to go to PHY for data of priority i. Furthermore, Q_i-QAM (quality quadrature amplitude modulation) is used to send data of priority i.
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_i.
$\begin{matrix} S_{i} = \frac{D_{i}}{{WR}_{i} \log_{2} (Q_{i})} . & (1) \end{matrix}$
The rate control block ensures that S₁+S₂+ . . . +S_N=M.
After the procedure determines the ratio ξ_i, and the sorted subcarrier index z_i, priority 1 data are sent on subcarriers z₁, . . . z_S1, priority 2 data are be sent on subcarriers z_S1+1, . . . , z_S1+S2, and so on.
The channel coding block needs to select the appropriate value for R_iand Q_ito 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.
FIGS. 5A-C shows the probability distribution of channel response of different subcarrier as a function of the channel-over-interference ratio ξ_ifor 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.
Because the channel with a higher channel-over-interference ratio ξ_iis used for higher priority data, more efficient modulation and coding (value for Q_iand R_i) can be used to satisfy stringent quality of service requirement. Hence, the number of subcarrier S_irequired for higher priority data can be reduced.
Rate Control Block
From the perspective of the rate control block, the values Q_i, R_iand 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_i, the number of bits sent to the PHY layer, so that the corresponding S_ivalues satisfy the condition S₁+S₂+ . . . +S_N=M.
We assume that queue i stores B_ibits of data. The rate control block receives B_ifrom the respective queues. As stated previously, Q_i, R_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_ibased on the amount of data in each queue, and the coding information received from the PHY layer.
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 ₁=min(B ₁ ,MWR ₁log₂(Q ₁)).
and in general,
$D_{i} = \min (B_{i}, (M - \sum_{j = 1}^{i - 1} \frac{D_{j}}{{WR}_{j} \log_{2} (Q_{j})}) {WR}_{i} \log_{2} (Q_{i})) for i \geq 2.$
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.

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

ξ_{i} = \frac{H_{i}}{I_{i}},

where I_iis an interference power at a subcarrier i, and H_iis 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} = \frac{D_{i}}{{WR}_{i} \log_{2} (Q_{i})},

where D_iis a number of bits sent to the PHY layer, W is a number of symbols in a frame, R_iis a data rate, and Q_iis a quality of service, and S₁+S₂+ . . . +S_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.