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Weightless Physical Layer Transmitter

As described in the OSI protocol stack, physical layer provide services to the MAC layer and upper layers. Moreover it has functionalities such as Forward Error Correction, modulation, scrambling and more to make data transmission reliable from transmit to the receive end. In this article, we will go through weightless physical layer which is used in wireless technology mainly targetted for IoT (Internet Of Things) and M2M(Machine to Machine) communication.

The weightless system consists of two main entities viz. base station and terminals. The transmission from base station to the terminals is referred as downlink and transmissions from terminals to the base station is referred as uplink. There are two physical layer variants defined in the weightless specifications viz. high rate downlink and standard rate downlink. This article explains step by step guide to modules of high rate downlink weightless physical layer transmitter part. In the standard rate downlink physical layer cyclic prefix insertion module is not used.

weightless physical layer transmitter
Fig-1 Weightless Physical Layer Transmitter

As shown transmitter part of weightless physical layer consists of FEC encoding, interleaving, whitening, PSK/QAM modulation types, Spreading, Cyclic Prefix Insertion, Sync Insertion and RRC pulse shaping. Let us understand basics of these modules.

Following table-1 mentions data rate supported for various FEC rates, modulation types and spreading factors in the weightless system. This data rates are achieved assuming 8MHz channel bandwidth and 20% overheads (accounted for synchronisation sequences, cyclic prefixes and cyclic postfixes).


Modulation type FEC rate Spreading factor Physical layer data rate
(after overheads)
16-QAM None 1 16 Mbps
16-QAM 3/4 1 12 Mbps
16-QAM 1/2 1 8 Mbps
π/4 QPSK 3/4 1 6 Mbps
π/4 QPSK 1/2 1 4 Mbps
π/2 BPSK 3/4 1 3 Mbps
π/2 BPSK 1/2 1 2 Mbps
π/2 BPSK 1/2 2 1 Mbps
π/2 BPSK 1/2 4 0.5 Mbps
π/2 BPSK 1/2 8 0.25 Mbps
π/2 BPSK 1/2 16 0.125 Mbps

FEC Encoding:
This module adds redundant bits to the MAC layer data to be transmitted. This extra bits help in error correction at the receiver. Rate-1/2 convolutional encoder generates 2 bits of output data for every 1 input bit. Other rates are also achived by way of technique known as puncturing. Refer Forward Error Correction basics and Convolutional Encoder basics with matlab code.

Interleaving:
Interleaving module may or may not exist depending upon FEC encoder module's availability. Interleaving is done to provide time diversity. It increases robustness in the presence time varying channel along with forward error correction. Refer Interleaver and deinterleaver basics with matlab code used in a OFDM system.

In weightless system, interleaver is a block interleaver which is a matrix of 8 columns and variable rows. The number of rows depend upon spreading factor used. Following table mentions the mapping.


Spreading factor Interleaver rows (Nrow)
1,2,4,8 16
15,31,63 8
127,255 4
511,1023 2

The input bits are written into matrix row by row starting at row 0 and column 0. The final matrix is padded with zero values if not completely filled with the data values. The output of the interleaver is obtained by reading out of the permuted matrix column by column starting from column 0 and row 0.

Whitening:
This module randomizes the bit stream by multiplying with known random sequence. This makes the resulting sequence resemble as white noise. This basically removes long stream of ones and zeros and help in synchronization at the receiver. This is similar to scrambler. Refer white noise vs colored noise for more.

PSK/QAM Modulation:
The modulation block in weightless physical layer basically maps binary bits into complex symbols. 16-QAM, π/4 QPSK and π/2 BPSK modulation types are used in this physical layer. PSK is the Phase Shift Keying modulation technique where in phase of the RF carrier is varied according to the baseband binary information bits to be transmitted.

BPSK is one form of PSK technique. QAM stands for Quadrature Amplitude Modulation. This technique vary both amplitude and phase of the RF carrier before transmission over the air. In BPSK, each symbol represents 1 bit while in QPSK each symbol represents 2 bits. Refer BPSK vs QPSK for comparison between these two modulation techniques. Also refer ASK vs FSK vs PSK to understand basics of these digital modulation techniques.

Spreading:
As mentioned after mapping block, spreading is being applied. This is required to generate chip sequence. Refer Spreading basics. Also in order to reduce the PAPR of the modulated chip sequence, π/2 rotation is applied to BPSK chip sequence and π/4 rotation is applied to QPSK chip sequence.

Rotation is carried out by multiplying the symbol sequence by exp(sqrt(-1)*φ(n)). When spreading module is used, this is done after the spreading of the data packet.

In the spreading module, each data symbol is repeated by the spreading factor. After this result is multiplied with spreading sequence obtained using LFSR. The LFSR in the weightless physical layer is of length 16 m sequence with following polynomial.

X16 + X14 + X13 + X11 + X0

The seed is initialized with the value obtained using bitwise logical OR of 0x8000 with 16 bit BS_ID(Base Station Identifier). This seed is reset at the beginning of each and every burst.

Cyclic Prefix Insertion:
In this module, last few samples of the frame is copied at the start of the frame. This helps convert received time domain frame to frequency domain without any corruption from the effect of multipath transmissions. Refer Cyclic Prefix basics for detailed description.

Transmissions of the base station(i.e. downlink) is partitioned into block of chips with each block having cyclic prefix as well as cyclic postfix. Following table mentions individually these chip values making total chips of about 1184.


Core Block size 1024 chips
Cyclic prefix 144 chips
Cyclic postfix 16 chips

Sync Insertion:
This is similar to the preamble referred as sync pattern. This basically is known patterns of bits used at the receiver for synchronization purpose.

RRC Pulse Shaping:
This module is needed to reduce out of band radiations. The information data bits in the form of square wave is converted into sinusoidal pulse shape for this purpose. RRC stands for Root Raised Cosine. It is basically a digital filter.


weightless system burst format

As shown in the figure-2, weightless data bursts composed of burst synchronisation sequence(variable length) , synchronisation validation word(16 bits except for DL_FCH and 32 bits for DL_FCH) and payload.

FEC encoding, interleaving and whitening modules are not used for Synchronisation valiation word in the weightless physical layer but spreading is applied.

Weightless Physical Layer Receiver

Front end synchronization:
As mentioned in the weightless burst format, burst synchronization sequence is transmitted at the start of each burst. This is used for following Front end synchronization modules in the receiver.

-Coarse Time offset estimation and correction in order to find out start of the burst.

-Fine frequency offset estimation and correction.

-Channel estimation and equalization.

-Timing detection to determine payload starting position.

The burst synchronization sequence consists of 128 chips followed by 3 termination blocks which are formed by negating both I and Q values.

The rest of the modules are reverse of the functions performed in the weightless physical layer transmitter. The receiver consists of front end synchronization as explained above. This is followed by cyclic prefix removal, de-spreading, de-mapping, de-whitening, de-interleaving and FEC decoding.

Front end synchronization related links

Time offset estimation and correction basics
Frequency offset estimation and correction basics
channel estimation and equalization with matlab code
Frequency offset estimation and correction with matlab code
Time offset estimation and correction matlab code

Other Standard Physical Layers

•  Wireless physical layer overview
•  11b physical layer
•  11a physical layer
•  fixed wimax physical layer-OFDM
•  mobile wimax physical layer-OFDMA
•  11n physical layer
•  GSM Physical layer
•  TD-SCDMA Physical layer
•  GPRS physical layer
•  LDACS1 Physical layer
•  10,40,100 Gigabit Ethernet Physical layer
•  Zigbee Physical layer
•  WCDMA Physical layer
•  Bluetooth Physical layer
•  WLAN 802.11ac Physical layer
•  WLAN 802.11ad Physical layer
•  LTE Physical layer

RF and Wireless Terminologies


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