Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Reexamination Certificate
2000-04-21
2004-10-19
Nguyen, Steven (Department: 2739)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
C370S347000, C370S445000
Reexamination Certificate
active
06807146
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to communication systems and networks and is particularly directed to such systems and networks which use multi-carrier protocols such as orthogonal frequency division multiplexing and discrete multi-tone protocols, and to techniques for communicating thereover.
2. Background of the Related Art
Orthogonal frequency division multiplexing (OFDM) and discrete multi-tone (DMT) are two closely related formats which have become popular as communication protocols. Systems of this type take a relatively wide bandwidth communication channel and break it into many smaller frequency sub-channels. The narrower sub-channels are then used simultaneously to transmit data at a high rate. These techniques have advantages when the communication channel has multi-path or narrow band interference.
The following discussion of the prior art and the invention will address OFDM systems; however, it will be understood that the invention is equally applicable to DMT systems (as well as other types of communication systems) with only minor modifications that will be readily apparent to those skilled in the art.
A functional block diagram of a typical OFDM transmitter is shown in FIG.
1
. Here, an incoming stream
10
of N symbols d
0
, d
1
. . . d
N−1
is mapped by a serial-to-parallel converter
20
over N parallel lines
30
, each line corresponding to a particular subcarrier within the overall OFDM channel. An Inverse Fast Fourier Transform circuit
40
accepts these as frequency domain components and generates a set
50
of time domain subcarriers corresponding thereto. These are converted by a parallel-to-serial converter
60
. Due to the characteristics of the inverse Fourier transform, although the frequency spectra of the subcarrier channels overlap, each subcarrier is orthogonal to the others. Thus, the frequency at which each subcarrier in the received signal is evaluated is one at which the contribution from all other signals is zero.
A functional block diagram of the corresponding OFDM receiver is shown in FIG.
2
. Here, an OFDM signal is received and converted into multiple time domain signals
210
by a serial-to-parallel converter
220
. These signals are processed by a Fast Fourier Transform processor
230
before being multiplexed by parallel-to-serial converter
240
to recover the original data stream
250
.
Systems such as OFDM and DMT systems either do not share the main channel with other users at all (e.g., when they are implemented using a telephone modem), or share the channel in time (e.g., when implemented in TDMA and CSMA schemes); thus, their flexibility and ease of use is limited. Sharing the channel in time (i.e., allowing only one user to transmit at a time) has two serious disadvantages. First, to maintain high throughput, all nodes sharing the channel must operate at a high data rate, and therefore be equally complex; thus, no less-complicated processing circuitry which might otherwise be used with low data rate channels can be employed. Second, a user who actually desires a low data rate must send data as very short high speed bursts over the network. In order to overcome propagation loss in the path, such a node must transmit at a high peak power because the transmit power is proportional to the peak data rate. Again, economies inherent in the low data rate processing cannot be exploited.
As a practical example, the IEEE 802.11a communication standard specifies transmission with 52 sub-channel frequencies. This requires substantial signal processing; a high transmit power while active to achieve significant range; a large peak-to-average ratio while actively transmitting; high resolution ADCs and DACs; and very linear transmit and receive chains. While such complicated hardware allows transmission up to 50 Mb/s, this level of performance is overkill for something like a cordless phone, which only requires roughly a 32 kb/s transmission rate.
In connection with the peak-to-average ratio, note that for 52 sub-channels, while transmitting the peak-to-average ratio of the signal is 52
2
/52=52 in power. Therefore, to avoid distortion of the signal, the power amplifier must be substantial enough to provide far more instantaneous power than is required on average. Since the peak-to-average ratio is directly proportional to the number of sub-channels, building a lower capacity unit that uses fewer carriers can substantially decrease the costs of such a device.
In an effort to solve the above shortcomings of the prior art, a system has been proposed which implements frequency communication but allows channel sharing between users in a way that would allow simple nodes such as a 32 kb/s cordless phone to transmit continuously at a low rate while other high speed nodes such as 20 Mb/s video streams communicate at a much higher data rate simultaneously. This can be an OFDM or DMT system in which the simple nodes are allowed to transmit continuously on one or just a few of the frequency sub-channels, while the other nodes avoid putting any signal into those sub-channels.
A system such as the one described above advantageously allows individual carriers to be used by different devices in a multi-carrier communication network such as an OFDM network to allow the simultaneous communication of high data rate devices which use many carriers and low data rate devices which use only one or a few carriers. Devices that use one or a few carriers can be lower in cost and power consumption than their counterparts which use many carriers due to the reduced digital complexity, reduced analog accuracy, reduced required transmit power and reduced peak-to-average power ratios of their communication sections. By allowing the low data rate and high data rate nodes to communicate at the same time, the overall throughput of the network remains high.
There are a number of constraints imposed on the physical layer of almost any communication network in which low-cost implementation and support for multimedia applications are desired. For example, it is more inconvenient, difficult, expensive and time-consuming to construct such communication systems that can simultaneously transmit and receive, particularly in wireless domains. Therefore, it is best to coordinate nodes in communication networks so that no node is required to simultaneously transmit and receive, i.e., to implement half duplex operation.
Further, because of the great attenuation, multi-path propagation and variability of certain communication media such as radio channels, signals transmitted therethrough must live with high error rates. This frequently causes data packets to contain errors. It is important for the device to somehow signal if a packet arrived with an error so that the transmitting node can resend the packet. Also, multimedia traffic requires low latency and low jitter in the arrival of packets. Interactive traffic such as telephony and video telephony are sensitive to both latency and jitter. Streaming applications such as CD-quality audio and digital video are primarily sensitive to the jitter in the time of arrival of subsequent packets of information. Guaranteeing the quality of service by insuring regular access to the medium and preventing collisions is the main goal of the protocol.
In addition to physical layer constraints common to low-cost, multimedia-capable communication systems, there are a number of constraints imposed on the physical layer of the network in order to support overlaid multi-carrier operation. For example, nodes in such overlaid multi-carrier systems preferably precompensate their transmit frequencies. This is because the close carrier spacings used in multi-carrier modulation make it difficult to insure that a given node will transmit its carriers with sufficient frequency accuracy to ensure that adjacent carriers do not bleed over into one another. If frequencies are sufficiently off, two nodes may actually end up transmitting their signals at the same frequency. Ensuring sufficient accur
Atheros Communications Inc.
Nguyen Steven
Pillsbury & Winthrop LLP
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