S-72333 Postgraduate Course in Radio Counications, Autun 2004 1 RAKE Receiver Toi Heikkilä toiheikkila@teliasoneraco Abstract RAKE receiver is used in CDMA-based (Code Division Multiple Access) systes and can cobine ultipath coponents, which are tie-delayed versions of the original signal transission Cobining is done in order to iprove the signal to noise ration at the receiver RAKE receiver attepts to collect the tie-shifted versions of the original signal by providing a separate correlation receiver for each of the ultipath signals This can be done due to ultipath coponents are practically uncorrelated fro another when their relative propagation delay exceeds a chip period This paper presents the basics of RAKE receiver technique, ipleentation, and design in cellular systes Also the usage of RAKE receiver is introduced in CDMA-based systes such as S-95 and WCDMA (Wideband Code Division Multiple Access) ndex Ters RAKE receiver, CDMA, ultipath, receiver, axial-ratio cobining RAKE RECEVER Due to reflections fro obstacles a radio channel can consist of any copies of originally transitted signals having different aplitudes, phases, and delays f the signal coponents arrive ore than duration of one chip apart fro each other, a RAKE receiver can be used to resolve and cobine the The RAKE receiver uses a ultipath diversity principle t is like a rake that rakes the energy fro the ultipath propagated signal coponents [2] A Multipath Channel Model Multipath can occur in radio channel in various ways such as, reflection and diffraction fro buildings, and scattering fro trees presented in Figure 1 NTRODUCTON n CDMA (Code Division Multiple Access) spread spectru systes, the chip rate is typically uch greater than the flat fading bandwidth of the channel Where as conventional odulation techniques require an equalizer to undo the intersybol interference (S) between adjacent sybols, CDMA spreading codes are designed to provide very low correlation between successive chips Thus, propagation delay spread in the radio channel erely provides ultiple versions of the transitted signal at the receiver f these ultipath coponents are delayed in tie by ore than one chip duration, they appear like uncorrelated noise at a CDMA receiver, and equalization is not required RAKE receiver, used specially in CDMA cellular systes, can cobine ultipath coponents, which are tie-delayed versions of the original signal transission This cobining is done in order to iprove the signal to noise ratio (SNR) at the receiver RAKE receiver attepts to collect the tieshifted versions of the original signal by providing a separate correlation receiver for each of the ultipath signals This can be done due to ultipath coponents are practically uncorrelated fro another when their relative propagation delay exceeds a chip period The basic idea of A RAKE receiver was first proposed by Price and Green These fellows also filed the RAKE receiver patent in 1956 [1] Figure 1 Propagation echaniss An M-ray ultipath odel is shown in Figure 4, which is an extension to the ultipath channel odel presented in [3] Each of the M paths has an independent delay,, and an independent coplex tie-variant gain, G
S-72333 Postgraduate Course in Radio Counications, Autun 2004 2 t(t) Delay 1 Delay 2 Delay M G 1 (t) G 2 (t) G M (t) Figure 2 Multipath channel odel B M-finger RAKE Receiver Additive Gaussian Noise r(t) Multiple Access nterference A RAKE receiver utilizes ultiple correlators to separately detect M strongest ultipath coponents The outputs of each correlator are weighted to provide better estiate of the transitted signal than is provided by a single coponent Deodulation and bit decisions are then based on the weighted outputs of the M correlators [1] r(t) 1 (t) 2 (t) M (t) Correlator 1 Correlator 2 Correlator M Z 1 Z 2 Z M 1 2 Z Z T > ()dt M Figure 3 An M-branch RAKE receiver ipleentation o < (t) Each correlator detects a tie-shifted version of the original CDMA transission, and each finger of the RAKE correlates to a portion of the signal, which is delayed by at least one chip in tie fro the other fingers Assue M correlators are used in a CDMA receiver to capture M strongest ultipath coponents A weighting network is used to provide a linear cobination of the correlator output for bit decision Correlator 1 is synchronized to the strongest ultipath 1 Multipath coponent 2 arrived t 1 later than 1 but has low correlation with 1 The M decision statistics are weighted to for an overall decision statistic as shown in Figure 3 The outputs of the M correlators are denoted as Z 1, Z 2,, and Z M They are weighted by 1, 2,, and M, respectively The weighting coefficients are based on the power or the SNR (Signal-to- Noise Ratio) fro each correlator output f the power or SNR is sall out of a particular correlator, it will be assigned a sall weighting factor, f axial-ratio cobining is used, following equation 1 can be written for Z M Z' Z (1) 1 The weighting coefficients,, are noralized to the output signal power of the correlator in such a way that the coefficients su to unity, as shown in following equation 2 2 Z Z (2) M 2 1 As in the case of adaptive equalizers and diversity cobining, there are any ways to generate the weighting coefficients However, due to Multiple Access nterference (MA), RAKE fingers with strong ultipath aplitudes will not necessarily provide strong output after correlation Choosing weighting coefficients based on the actual outputs of the correlator yields better RAKE perforance [1] C RAKE Receiver Block Diagra When a signal is received in a atched filter over a ultipath channel, the ultiple delays appear at the receiver, as depicted in Figure 4 The RAKE receiver uses several baseband correlators to individually process several signal ultipath coponents The correlator outputs are cobined to achieve iproved counications reliability and perforance [2] Bit decisions based only a single correlation ay produce a large bit error rate as the ultipath coponent processed in that correlator can be corrupted by fading n a RAKE receiver, if the output fro one correlator is corrupted by fading, the others ay not be, and the corrupted signal ay be discounted through the weighting process [1] nput RF signal Correlator Code generators Matched filter Q Tiing and finger allocation Channel estiators Phase rotator Delay equalizer Finger 1 Finger 2 Finger 3 Figure 4 Block diagra of a RAKE receiver Q Cobiner Output pulse response easureents of the ultipath channel profile are executed through a atched filter to ake a successful de-spreading t reveals ultipath channel peaks and gives tiing and RAKE finger allocations to different receiver blocks Later it tracks and onitors these peaks with a easureent rate depending on speeds of obile station and on propagation environent The nuber of available RAKE fingers depends on the channel profile and the chip rate The higher the chip rate, the ore resolvable paths there are, but higher chip rate will cause wider bandwidth To catch all the energy fro the channel ore RAKE fingers are needed A very large nuber of fingers lead to cobining losses and practical ipleentation probles
1 1 6 5 4 3 2 1 0 0 20 4 0 6 0 8 0 1 0 0 1 2 0 14 0 S-72333 Postgraduate Course in Radio Counications, Autun 2004 3 RAKE RECEVER N S-95 SYSTEM n the ipleentation of the S-95 syste, the obile receiver eploys a searcher receiver and three digital data receivers that act as fingers of a RAKE in that they ay be assigned to track and isolate particular ultipath coponents of a single cell site, a single base station in softer handover and ultiple base stations in soft handover The PN chip rate of 1,2288 MHz allows for resolution of ultipaths at tie intervals of 1,2288 x 10-6 s = 0,814 s, which eans that ultipath difference in eters can be around 244 [4] A Downlink The searcher receiver scans the tie doain about the desired signal s expected tie of arrival for ultipath pilot signals fro the sae cell site and pilot signals and their ultipaths fro other cell sites Searching the tie doain on the downlink signals is siplified because the pilot channel perits the coherent detection of signals The search receiver indicates to the obile phone s control processor where, in tie, the strongest replicas of the signal can be found, and their respective signal strengths n turn, the control processor provides tiing and PN code inforation to the tree digital data receivers, enabling each of the to track and deodulate a different signal [4] f a another cell site pilot signal becoes significantly stronger than the current pilot signal, the control processor initiates handover procedures during which the downlinks of both cell sites transit at the sae call data on all their traffic channels When both sites handle the call, additional space diversity or acro diversity is obtained [4] The data fro all three digital receivers are cobined for iproved resistance to fading Different base stations or sectors are distinguished by different short PN code offsets The downlink perfors coherent post-detection cobining after ensuring that the data streas are tie-aligned; perforance is not coproised by using post-detection cobining because the odulation technique is linear Coherent cobining is possible because the pilot signal fro each base station provides a coherent phase reference that can be tracked by the digital data receivers [4] B Uplink On the uplink, the base station receiver uses two antennas for space diversity reception, and there are four digital data receivers available for tracking up to four ultipath coponents of a particular subscriber s signal The searcher receiver at the base station can distinguish the desired obile signal by eans of its unique scrabling long PN code offset, acquired before voice or data transission begins on the link, using s special preable for that purpose [4] During soft handover fro one base station site to another, the voice data that are selected could result fro cobining up to eight ultipath coponents, four at each site The uplink transission, not having a coherent phase reference like the downlink s pilot signal, ust be deodulated and cobined non-coherently; axial-ratio cobining can be done by weighting each path s sybol statistics in proportion to the path s relative power prior to deodulation and decoding decision [4] V RAKE RECEVER N WCDMA SYSTEM A basic ipleentation of RAKE receiver presented in Figure 5 despreads data fro different ultipath coponents, cobines the ultipath coponents, and detects cobined data to soft bits A WCDMA base station RAKE receiver contains the following functions to enable the receiving of CDMA type of ultipath signals [5] 1 Channel delay estiation for ultipath coponents This can also called as pulse Response (R) Measureent 2 RAKE receiver finger allocation based on the channel delay estiation 3 RAKE receiver fingers to perfor the descrabling and despreading operations 4 Adaptive channel estiation 5 Maxial-Ratio Cobining (MRC) ANT A/D pulse Response easureent Matched Filters Chiprate Fin ger Allocation Co rrelator f inger 2 finger n Sybol Rat e Sybol Rate Rate f inger 1 Cob iner C hannel Estiation Figure 5 RAKE receiver in WCDMA A Channel Delay Estiation Soft Decision User Datarate The channel pulse Response Measureent (RM) is perfored by using Matched Filter (MF) type of correlators that correlate the received signal with known reference code sequence such as pilot channel code The MF resources contain shorter filters (length of 64 chips tie period for RACH and 32 chips tie period for DPCCH), which can be concatenated in tie doain to enable the proper delay estiation also in large cells with large delay spreads (eg hilly terrain environents) [5] To iprove the delay estiation perforance and to increase signal to noise ratio the results of MFs are further processed by coherent and non-coherent averaging The length of the coherent R averaging is typically one tie slot while the noncoherent averaging is typically done over radio fraes The length of the averaging operations can be selected by paraetrization The accuracy of the R easureent is ¼ chip (65,1 ns) [5]
S-72333 Postgraduate Course in Radio Counications, Autun 2004 4 B RAKE Receiver Finger Allocation The purpose of the RAKE finger allocation procedure is to define the optial finger delay positions that axiize the receiver perforance The allocation procedure defines the correct delay positions for despreading (in RAKE fingers) the received wideband signal to sybol level inforation n the case of receiver antenna diversity the finger allocation procedure cobines inforation fro separate receiver antennas n softer handover the allocation procedure defines the optial finger delay positions by taking into account the inforation fro all the sectors involved in the handover situation [5] The finger allocation procedure contains algoriths, which eliinate the unnecessary changes in the finger tie positions between successive allocations Thus the despreading of a certain ultipath coponent is kept on the sae RAKE finger as long as possible to axiize the perforance of channel estiation and axial-ratio cobining [5] n the finger allocation procedure also the shape of the channel ipulse response is taken into account when defining the optiu finger delay positions t has been confired that the allocation ust be done differently for the channels where the taps are very close to each others (so called "fat finger") than for channels with clearly separate taps [5] Typically the allocation frequency in noral operation ode is one allocation for a code channel in every 25 s (accuracy of ¼ chip), which is enough for all the practical situations Code tracking with accuracy of 1/8 chip is further used in RAKE fingers to track and copensate sall delay deviations in ultipath coponent tiing The change in the tiing can be caused by the oveent of the UE or by the transission tiing adjustent of the UE [5] C RAKE Receiver Finger Descrabling and Despreading The despreading operation for DPDCH (Dedicated Physical Data Channel) and DPCCH (Dedicated Physical Control Channel) is perfored in RAKE fingers to recover the receiver wideband signal to sybol level inforation - ultiplying of incoing signal by coplex conjugate of scrabling code and channelization code and accuulating the results over sybol periods n the base station receiver 8 fingers are allocated for each code channel (ie 8 ultipath coponents can be despread for a single user) [5] Code tracking is used to track and copensate sall deviations in ultipath coponent delays ie the Code tracking perfors the fine adjustent of the delay used in the despreading The tracking is done for every finger and the accuracy is 1/8 chip Like in the ain finger allocation procedure the shape of the channel ipulse response is taken into account when defining the despreading tiings [5] Typically the delay updating by code tracking is perfored once in each or every second 10 s radio frae [5] D Adaptive Channel Estiation The goal adaptive channel estiation is to estiate the characteristics of the tie-variant channel n WCDMA the solution is Pilot Sybol Aided plus adaptive filtering [5] The channel estiation is used to reove distortion caused by radio channel and it is based on the known pilot sybols on DPCCH The channel estiator filter adapts to the Doppler power spectru (both frequency and the shape of the spectru) The estiation is done for each finger separately The use of adaptive filter ensures good perforance in all kind of propagation conditions The advantage of adaptive filter coefficients copared to use of fixed coefficient is evident since the solution with fixed coefficients would perfor well only in a constricted set of propagation conditions [5] n the case of ultiple receiver antennas the perforance of channel estiation is further iproved by cobining the power spectru inforation available fro different receiver antennas [5] The cobining process is based on axial-ratio cobining, which decreases the effect of additive noise, which can further be decreased by channel decoding[5] E Maxial-Ratio Cobining (MRC) Maxial-Ratio Cobining, first discussed by Brennan, is the optial for of diversity cobining because it yields the axial SNR achievable t requires the exact knowledge of SNRs as well as the phases of the diversity signals [4] Figure 6 Maxial Ratio Cobining in RAKE After despreading the received sybol fro transitter via radio channel the sybols fro allocated fingers are axial-ratio-cobined to construct the cobined sybol The output sybols fro different fingers are ultiplied with coplex conjugate of the channel estiate and the result of ultiplication is sued together into the cobined sybol This is illustrated in Figure 6 and Figure 7 [5]
S-72333 Postgraduate Course in Radio Counications, Autun 2004 5 Finger Allocation Delay estiations 1 2 n [4] Sauel C Yang, CDMA RF Syste Engineering, Norwood MA, USA, Artech House nc, 1998, 280 pp [5] Mikko Järvelä, RAKE training slides by Nokia, 03042001, Oulu RAKE finger bank Finger 1 r 1 MRC cobiner HOMEWORK RX RF ADC Finger 2 r 2 w1 + Output 1 Explain shortly the basic functions in a RAKE receiver Finger n r n w2 w n Channel estiator Figure 7 RAKE receiver using MRC F Practical RAKE Receiver Requireents High bandwidth (5 MHz in WCDMA) and dynaic interference inherent to WCDMA requires that RF and F parts have to operate linearly with large dynaic range n practical RAKE receivers synchronization sets soe requireents Autoatic Gain Control (AGC) loop is needed to keep the receiver at the dynaic range of the A/D converter AGC ust be fast and accurate enough to keep receiver at the linear range Frae-by-frae data range change ay set higher AGC and A/D (Analog-to-Digital) converter requireents The high sapling rates of few tens of MHz and high dynaics of the input signal (80 db) require fast A/D converters and high resolution [5] Autoatic Frequency Control (AFC) loop copensates for drift of the local oscillator and possibly copensates the Doppler shifts Synchronization is required for channel ipulse response easureents and scanning for RAKE finger allocation Also channel delay tracking needs synchronization for fine-adjustent and tracking of ultipath coponents [5] Pilots 2 How any fingers can a Mobile Station RAKE receiver s atched filter or searcher allocate fro a following ultipath tapped delay line channel in WCDMA and S-95 systes? Don t guess! Tap 1 2 3 4 5 6 Avg 0-1,5-6,0-4,5-9,0-15,5 power (db) Relative delay (ns) 0 310 500 1090 2430 2510 V CONCLUSON This paper has introduced the basic operation and requireents of RAKE receiver used in CDMA based systes such as S-95 and WCDMA RAKE receiver attepts to collect the tie-shifted versions of the original signal by providing a separate correlation receiver for each of the ultipath signals The RAKE receiver uses several baseband correlators to individually process several signal ultipath coponents The correlator outputs are cobined to achieve iproved counications reliability and perforance The basic functions of RAKE receiver are Channel delay estiation for ultipath coponents, RAKE receiver finger allocation, descrabling and despreading operations, adaptive channel estiation, and Maxial-Ratio Cobining REFERENCES [1] Rappaport, Wireless Counications Principles and Practice, Prentice Hall, New Jersey, 1996, pp 336-338 [2] Tero Ojanperä, Rajee Prasad, Wideband CDMA for Third Generation Mobile Counications, Norwood MA, USA, Artect House nc, 1998, 439 pp [3] Sion Haykin, Michael Moher: Modern Wireless Counications, Prentice Hall 2005, pp 258-338