Multiple access schemes are employed by modern radio systems to allow multiple users to share a limited amount of bandwidth, while maintaining acceptable system performance. Common multiple access schemes include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). System performance is also aided by error control codes. Nearly all communications systems rely on some form of error control for managing errors that may occur due to noise and other factors during transmission of information through a communication channel. These communications systems can include satellite systems, fiber-optic systems, cellular systems, and radio and television broadcasting systems. Efficient error control schemes implemented at the transmitting end of these communications systems have the capacity to enable the transmission of data including audio, video, text, etc., with very low error rates within a given signal-to-noise ratio (SNR) environment. Powerful error control schemes also enable a communication system to achieve target error performance rates in environments with very low SNR, such as in satellite and other wireless systems where noise is prevalent and high levels of transmission power are costly, if even feasible.
Interleave Division Multiple Access (IDMA) is a multiple access technique where different users that share the same bandwidth and time slots are separated by user specific interleavers. As the bandwidth and power become scarce to support the ever increasing throughput requirements, more complex but more efficient techniques play more important roles in future communication systems. IDMA is an effective technique that trades extra receiver complexity with bandwidth and power savings. On the other hand, in systems where the number of users is high and the block size is large, storage of a high number of long interleavers may be undesirable. Scrambled Coded Multiple Access (SCMA) addresses this complexity by using a single scrambling sequence with different shift factors for different users without any performance penalty. With SCMA, the user specific interleavers of IDMA are replaced with user specific scrambler sequences. While there is no noticeable performance difference between the two approaches, generation and implementation of scrambler sequences is significantly simpler. In fact, the same scrambler sequence with different rotation factors can be used for different users with no impact on performance, which further reduces receiver complexity. With SCMA, therefore, all of the benefits of IDMA are achieved with reduced complexity.
Similar to IDMA or random waveform Code Division Multiple Access (CDMA), SCMA is a non-orthogonal multiple access technique. While orthogonal multiple access schemes such as Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA) are implicitly too restrictive to achieve theoretical limits in fading channels, non-orthogonal CDMA, IDMA or SCMA have the potential of achieving these limits. Further, as discussed above FEC coding is typically used to improve the performance. The main difference between CDMA and SCMA is that, while in CDMA different users are separated with different signature sequences with a spreading factor greater than one, in SCMA even a spreading factor of one would be enough to detect overlapped users based on user specific scrambler sequences and iterative multiuser cancellation with FEC decoding. As a result, the available bandwidth can be used for very low rate coding which gives SCMA extra coding gain that is not available in CDMA. Actually it is also possible to use SCMA with a spreading factor greater than one. Another benefit of the iterative receiver structure of SCMA is that the system performance actually improves with power variations among the users, which eliminates the need of power control, an important requirement of traditional CDMA.
At the receiver, iterative multiuser detection or interference cancellation followed by decoding is performed to approach maximum likelihood (ML) receiver performance without excessive complexity. But for coded CDMA systems, even this iterative receiver may lead to complicated algorithms especially when the number of users is large. Typically with CDMA, the complexity of multiuser detection or soft interference cancellation algorithms grows in polynomial form with the number of users/user terminals. On the other hand, similar to IDMA, SCMA lends itself to a simple chip by chip detection algorithm whose total complexity grows only linearly with the number of users. Further, uncoded SCMA systems perform at least as well as and usually better than uncoded CDMA, and the performance gap between the two classes of schemes grows bigger for heavily loaded systems.
Further, in conventional burst mode communication systems, a transmitter transmits burst mode signals at a certain frequency, phase and timing, which is received by a receiver through a communication channel. In conventional burst mode communication systems, it is necessary to quickly estimate various parameters of the received bursts as they arrive. These parameters include detection of the presence of a burst (start time), frequency, initial phase, timing and amplitude. In typical burst transmission systems, a unique word is used to facilitate the identification of the beginning of a transmitted burst and the determination of phase offset, by the receiver. The term “Unique Word” (UW) refers to a known, pre-determined pattern (known a priori to the receiver) that is transmitted at the beginning of each burst, whereby the receiver detects the UW and synchronizes with the received bursts (i.e., the receiver estimates the burst parameters based on the detected UW). For classical TDMA systems, the same UW is used by all of the terminals.
While the complexity of SCMA grows only linearly with the number of users, however, with larger systems (e.g., having upwards of tens or hundreds of thousands of user terminals), SCMA system implementations can become relatively complex with each user/user terminal having a distinct scrambling signature. What is needed, therefore, is an approach for an SCMA system that scales more efficiently, and in a relatively less complex manner, to support a relatively large number of users/user terminals.
Some Example Embodiments
Embodiments of the present invention advantageously address the foregoing requirements and needs, as well as others, by providing an approach for an SCMA system that scales more efficiently in a relatively less complex manner, whereby individual terminals utilize respective assigned unique words and the receiver correlates received signal bursts against these UWs, which supports larger numbers of users/user terminals.
Example embodiments of the present invention provide a new SCMA multiple access approach that facilitates random access to a communications channel by a network of terminals in an efficient manner without prior coordination. In accordance with such example embodiments, unique words are respectively assigned to individual terminals, and each terminal utilizes its assigned UW for each transmitted burst. At the receiver side, a receiver correlates the received signal bursts against these UWs to determine whether one or more terminals is accessing the channel and the number of terminals accessing the channel (assuming there is at least one), to identify the scrambling signature or initial vector each such terminal is utilizing to access the channel, and to synchronize with (e.g., determine the timing and phase of) each individual received modulated signal for proper demodulation and decoding. By way of example, a moderately sized set of UWs is assigned to the terminal population, where each different UW is associated with a respective scrambling signature (or, in the case of the use of the same scrambling signature with a different seed or initial vector, each different UW is associated with a respective initial vector) for the scrambler. Accordingly, a receiver separates overlapping transmissions from multiple terminals at the same frequency and the same time slot, based on a UW correlation process employed to detect the transmitted UWs in parallel and thereby identify the number of terminals accessing the channel and the scrambling signature/initial vector of each such terminal, and to synchronize with each individual received modulated signal for proper demodulation and decoding.
In accordance with example embodiments, a communications terminal comprises and encoder, a scrambler and a modulator. The encoder is configured to encode a source digital data signal to generate an encoded signal, wherein the source digital data signal comprises a source bit stream. The scrambler is configured to scramble the encoded signal based on a scrambling signature. The modulator is configured to modulate a received sequence of data frames to generate a transmission signal for transmission via a random access channel of a wireless communications system, wherein each data frame comprises a data payload, which includes a block of the scrambled encoded signal, and a frame header, which includes a start of frame (SOF) sequence associated with the scrambling signature. The use of the SOF sequence for each frame of the sequence of data frames provides a reference for synchronization on frame boundaries and serves to designate use of the associated scrambling signature for descrambling and decoding the respective data payload of the frame. The use of the SOF sequence for each frame of the sequence of data frames serves to distinguish between the data frame and at least one data frame originating from a further communications terminal, transmitted via a common time slot of the random access channel, for which a different scrambling signature was used to scramble a respective encoded signal thereof.
In accordance with further example embodiments, a multiple access communications scheme is provided. A source digital data signal is encodes to generate an encoded signal, wherein the source digital data signal comprises a source bit stream. The encoded signal is scrambled based on a scrambling signature. A received sequence of data frames is modulated to generate a transmission signal for transmission by a communications terminal via a random access channel of a wireless communications system, wherein each data frame comprises a data payload, which includes a block of the scrambled encoded signal, and a frame header, which includes a start of frame (SOF) sequence associated with the scrambling signature. The use of the SOF sequence for each frame of the sequence of data frames provides a reference for synchronization on frame boundaries and serves to designate use of the associated scrambling signature for descrambling and decoding the respective data payload of the frame. The use of the SOF sequence for each frame of the sequence of data frames serves to distinguish between the data frame and at least one data frame originating from a further communications terminal, transmitted via a common time slot of the random access channel, for which a different scrambling signature was used to scramble a respective encoded signal thereof.
In accordance with example embodiments, a further multiple access communications scheme is provided. A transmitted signal is received via a random access channel of a wireless communications network, wherein the transmitted signal originated from a first communications terminal. A first start of frame (SOF) sequence of the transmitted signal is identified, and synchronization is attained on a frame boundary of a first data frame associated with the first SOF sequence. A first scrambling signature is determined based on the identified SOF sequence, and the first data frame is decoded using the determined scrambling signature. The first SOF sequence serves to distinguish between the respective data frame and at least one data frame originating from a further communications terminal, transmitted via a common time slot of the random access channel, for which a different scrambling signature was used to scramble a respective encoded signal thereof.
In accordance with example embodiments, a system comprises a first communications terminal and a second communications terminal. The first communications terminal comprises a first encoder, a first scrambler and a first modulator. The first encoder is configured to encode a first source digital data signal to generate a first encoded signal, wherein the first source digital data signal comprises a first bit stream. The first scrambler is configured to scramble the first encoded signal based on a first scrambling signature. The first modulator is configured to modulate a received first sequence of data frames to generate a first transmission signal for transmission via a random access channel of a wireless communications system, wherein each data frame comprises a data payload, which includes a block of the scrambled first encoded signal, and a frame header, which includes a first start of frame (SOF) sequence associated with the first scrambling signature. The second communications terminal comprises a second encoder, a second scrambler and a second modulator. The second encoder is configured to encode a second source digital data signal to generate a second encoded signal, wherein the second source digital data signal comprises a second bit stream. The second scrambler is configured to scramble the second encoded signal based on a second scrambling signature. The second modulator is configured to modulate a received second sequence of data frames to generate a second transmission signal for transmission via the random access channel of the wireless communications system, wherein each data frame comprises a data payload, which includes a block of the scrambled second encoded signal, and a frame header, which includes a second start of frame (SOF) sequence associated with the second scrambling signature. The use of the first SOF sequence for each frame of the first sequence of data frames provides a reference for synchronization on frame boundaries and serves to designate use of the first scrambling signature for descrambling and decoding the respective data payload of the frame, and the use of the second SOF sequence for each frame of the second sequence of data frames a reference for synchronization on frame boundaries and serves to designate use of the second scrambling signature for descrambling and decoding the respective data payload of the frame, even where at least one frame of the first sequence of data frames and at least one frame of the second sequence of data frames are received in a common time slot of the random access channel.
Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.