Sparse Code Multiple Access - SCMA

Sparse code multiple access (SCMA) is another waveform configuration of the flexible new air interface. This non-orthogonal waveform facilitates a new multiple access scheme in which sparse codewords of multiple layers of devices are overlaid in code and power domains and carried over shared time-frequency resources. Typically, the multiplexing of multiple devices may become overloaded if the number of overlaid layers is more than the length of the multiplexed codewords.

However, with SCMA, overloading is tolerable with moderate complexity of detection thanks to the reduced size of the SCMA multi-dimensional constellation and the sparseness of SCMA codewords. In SCMA, coded bits are directly mapped to multi-dimensional sparse codewords selected from layer-specific SCMA codebooks. The complexity of detection is controlled through two major factors. One is the sparseness level of codewords, and the second is the use of multidimensional constellations with a low number of projection points per dimension [3]. An example of device multiplexing with a low projection codebook and the resulting constellation mapping is shown Figure   A device’s encoded bits are first mapped to a codeword from a codebook. In the example, a codeword of length 4 is used. The low projection codebook has a reduced constellation (from 4 points to 3 points). Furthermore, each point (e.g. “00”) has non-zero component only in one tone. A codebook with one non-zero component is a zero-PAPR codebook.

                                        SCMA multiplexing and low projection codebook constellation

Furthermore, a blind multi-device reception technique can be applied to detect device activities and the information carried by them simultaneously. With such blind detection capability, grant-free multiple access can be supported. Grant-free multiple access is a mechanism that eliminates the dynamic request and grant signaling overhead. It is an attractive solution for small packets transmission. SCMA is an enabler for grant-free multiple access. Due to these benefits,SCMA can support massive connectivity, reduce transmission latency and provide energy saving.

5G Spectrum

The growing traffic demand necessitates increasing the amount of spectrum that may be utilised by the

5G systems. High frequency bands in the centimeter wave (cmWave) and millimeter wave (mmWave)

range will be adopted due to their potential for supporting wider channel bandwidths and the consequent

capability to deliver high data rates.

The new spectrum below 6GHz is expected to be allocated for mobile communication at the World Radio

Conference (WRC) 2015, and the band above 6GHz expected to be allocated at WRC 2019, as shown in

Figure.

5G network is a heterogeneous network which enables the cooperation between lower-frequency wide-area coverage network and high-frequency network. The consensus is higher frequency bands are the complementary bands to 5G whereas low frequency bands (<6GHz) are still the primary bands of 5G spectrum.

High frequency also enables unified access and backhaul since the same radio resources is shared. It is expected to use a unified air interface and a hierarchical scheduling for both radio access and backhaul which enables flexible backhauling and low-cost ultra dense networking (UDN).

Future radio access may also employ bands with different levels of access regulation including exclusive licensed, non-exclusive licensed and unlicensed bands. The 5G system treats both the licensed and unlicensed spectrum in a flexible, unified air interface framework.

Wavelength in GSM

There are many different types of electromagnetic waves. These electromagnetic waves can be described by a sinusoidal function, which is characterized by wavelength. Wavelength (l) is the length of one complete oscillation and is measured in meters (m). Frequency and wavelength are related via the speed of propagation, which for radio waves is the speed of light (3 x10^8 m/s or meters per second).

The wavelength of a frequency can be determined by using the following formula:

Wavelength = Speed / Frequency

Thus, for GSM 900 the wavelength is:

Wavelength = 3×10^8 m/s / 900 MHz           

Wavelength = 300,000,000 m/s  / 900,000,000

Wavelength = 0.33 m (or 33 cm)

From this formula it can be determined that the higher the frequency, the shorter the wavelength. Lower frequencies, with longer wavelengths, are better suited to transmission over large distances, because they bounce on the surface of the earth and in the atmosphere. Television and FM radio are examples of applications, which use lower frequencies.

Higher frequencies, with shorter wavelengths, are better suited to transmission over small distances, because they are sensitive to such problems as obstacles in the line of the transmission path. Higher frequencies are suited to small areas of coverage, where the receiver is relatively close to the transmitter.

The frequencies used by mobile systems compromise between the coverage advantages offered by lower frequencies and the closeness-to-the-receiver advantages offered by use of higher frequencies.